阅读说明：本技术 用于放大器的跨导增强共源共栅补偿 (Transconductance-enhanced cascode compensation for amplifiers ) 是由 N·古普塔 P·古普塔 于 2020-04-23 设计创作，主要内容包括：本公开的实施例涉及用于放大器的跨导增强共源共栅补偿。差分晶体管对接收输入电压。电流镜晶体管和共源共栅晶体管被耦合至差分晶体管对。差分晶体管对被耦合在共源共栅晶体管与从尾部节点汲取第一偏置电流的尾部晶体管之间,第一偏置电流的幅度等于总偏置电流与小于一的常数的乘积。第一电流源晶体管从差分对和共源共栅晶体管之间的节点汲取第二偏置电流,使得第二偏置电流旁路差分晶体管对中的一个晶体管。第二偏置电流的幅度等于总偏置电流与等于一减去常数的值的乘积。输出级被共源共栅晶体管和电流镜晶体管之间的节点处的输出偏置。(Embodiments of the present disclosure relate to transconductance-enhanced cascode compensation for amplifiers. The differential transistor pair receives an input voltage. The current mirror transistor and the cascode transistor are coupled to a differential transistor pair. The differential transistor pair is coupled between the cascode transistor and a tail transistor that draws a first bias current from a tail node, the first bias current having a magnitude equal to a product of a total bias current and a constant less than one. The first current source transistor draws a second bias current from a node between the differential pair and the cascode transistor such that the second bias current bypasses one transistor of the differential pair. The magnitude of the second bias current is equal to the product of the total bias current and a value equal to one minus a constant. The output stage is biased by an output at a node between the cascode transistor and the current mirror transistor.)

1. An amplifier, comprising:

an amplifier stage, comprising:

A differential transistor pair receiving a first voltage and a second voltage;

a pair of current mirror transistors; and

a cascode transistor pair coupled between the differential transistor pair at first and second nodes and the current mirror transistor pair at third and fourth nodes;

wherein the differential transistor pair is coupled between the cascode transistor pair and a tail node;

a tail transistor drawing a first bias current from the tail node, the first bias current having a magnitude equal to a product of a total bias current and a constant k, the constant k being less than one;

a first current source transistor drawing a second bias current from either the first node or the second node such that the second bias current bypasses one transistor of the differential transistor pair, the second bias current having a magnitude equal to a product of the total bias current and a value equal to one minus k; and

an output stage biased by the third node or the fourth node.

2. The amplifier of claim 1, further comprising:

a second current source transistor coupled in parallel with one transistor of the pair of current mirror transistors; and

A compensation cascode transistor coupled in parallel with one transistor of the cascode transistor pair;

wherein the second current source transistor sources the second bias current to the compensation cascode transistor and the first current source transistor sinks the second bias current from the compensation cascode transistor.

3. The amplifier of claim 1, wherein the differential transistor pair comprises: a first differential input transistor coupled between the first node and the tail node and receiving the first voltage; and a second differential input transistor coupled between the second node and the tail node and receiving the second voltage; wherein the current mirror transistor pair comprises a first current mirror transistor coupled between a supply voltage and the third node and a second current mirror transistor coupled between the supply voltage and the fourth node, the first current mirror transistor and the second current mirror transistor being in a current mirror relationship; wherein the cascode transistor pair includes a first cascode transistor coupled between the first node and the third node, and a second cascode transistor coupled between the second node and the fourth node.

4. The amplifier of claim 3, further comprising:

a second current source transistor coupled in parallel with the second current mirror transistor; and

a compensation cascode transistor coupled in parallel with the second cascode transistor;

wherein the second current source transistor sources the second bias current to the compensation cascode transistor and the first current source transistor sinks the second bias current from the compensation cascode transistor.

5. The amplifier of claim 4, wherein for a given size of the second current mirror transistor equal to k x given _ size, the size of the second current source transistor is (1-k) x given _ size; and wherein for a given size of the second cascode transistor equal to k x given _ size, the size of the compensation cascode transistor is 2(1-k) x given _ size.

6. The amplifier of claim 3, wherein the second current mirror transistor is larger than the first current mirror transistor; and wherein the second cascode transistor is larger than the first cascode transistor.

7. The amplifier of claim 6, wherein for a given size of the first cascode transistor equal to k x given _ size, the size of the second cascode transistor is equal to (2-k) given _ size; and wherein for a given size of the first current mirror transistor equal to k x given _ size, the size of the second current mirror transistor is equal to (2-k) x given _ size.

8. The amplifier of claim 1, wherein the amplifier stage is an error amplifier stage, wherein the first voltage is a feedback voltage representing a current delivered by the output stage to a load, and wherein the second voltage is a reference voltage.

9. An amplifier, comprising:

an amplifier stage, comprising:

a differential transistor pair receiving a first voltage and a second voltage;

a pair of current mirror transistors; and

a cascode transistor pair coupled between the differential transistor pair and the current mirror transistor pair;

a cascode compensation stage comprising:

an output transistor biased by the current mirror transistor pair; and

a compensation cascode transistor biased by the cascode transistor pair; and an output stage coupled to the cascode compensation stage.

10. The amplifier of claim 9, wherein:

the differential transistor pair comprises a first differential input transistor and a second differential input transistor;

the current mirror transistor pair comprises a first current mirror transistor and a second current mirror transistor; and

the cascode transistor pair includes first and second cascode transistors coupled between the first and second differential input transistors and the first and second current mirror transistors, respectively; and is

Wherein:

the output transistor has the same bias as the second current mirror transistor; and

the compensation cascode transistor has the same bias as the second cascode transistor.

11. The amplifier of claim 10, wherein the amplifier stage comprises an error amplifier stage; wherein the first voltage comprises a feedback voltage and the second voltage comprises a reference voltage; and wherein the output stage comprises:

an output transistor biased by an output of the output transistor of the cascode compensation stage and generating a load current; and

a sense resistor configured to generate the second voltage according to the load current.

12. The amplifier of claim 9, wherein the amplifier is,

wherein the differential transistor pair comprises:

a first NMOS transistor having a drain, a source coupled to a tail node, and a gate coupled to receive the second voltage; and

a second NMOS transistor having a drain, a source coupled to the tail node, and a gate coupled to receive the first voltage;

wherein the current mirror transistor pair comprises:

a first PMOS transistor having a source coupled to a power supply node, a drain coupled to a first intermediate node, and a gate coupled to the drain of the first PMOS transistor; and

A second PMOS transistor having a source coupled to the power supply node, a drain coupled to a second intermediate node, and a gate coupled to the gate of the first PMOS transistor;

wherein the cascode transistor pair comprises:

a third NMOS transistor having a drain coupled to the first intermediate node, a source coupled to the drain of the first NMOS transistor, and a gate; and

a fourth NMOS transistor having a drain coupled to the second intermediate node, a source coupled to the drain of the second NMOS transistor, and a gate coupled to the gate of the third NMOS transistor; and is

A tail current source comprising a fifth NMOS transistor having a drain coupled to the tail node, a source coupled to ground, and a gate;

wherein the output transistor of the cascode compensation stage comprises a third PMOS transistor having a source coupled to the power supply node, a drain coupled to the drain of the second PMOS transistor, and a gate coupled to the gate of the first PMOS transistor and the gate of the second PMOS transistor;

Wherein the compensation cascode transistor of the cascode compensation stage comprises a sixth NMOS transistor having a drain coupled to the drain of the third PMOS transistor and a gate coupled to the gate of the third NMOS transistor and the gate of the fourth NMOS transistor; and

wherein the cascode compensation stage comprises a seventh NMOS transistor having a drain coupled to the source of the sixth NMOS transistor, a source coupled to ground, and a gate coupled to the gate of the fifth NMOS transistor.

13. The amplifier of claim 12, wherein the output stage comprises:

a fourth PMOS transistor having a source coupled to the power supply node, a drain coupled to an output node, and a gate coupled to the drain of the second PMOS transistor and the drain of the third PMOS transistor;

a capacitor coupled between the gate of the fourth PMOS transistor and the power supply node;

a compensation capacitor coupled between the source of the sixth NMOS transistor and the output node; and

A voltage divider coupled between the output node and ground, wherein the second voltage is generated at a center tap of the voltage divider, and wherein the center tap of the voltage divider is coupled to the gate of the first NMOS transistor.

14. The amplifier of claim 12, wherein the gate of the fifth NMOS transistor and the gate of the seventh NMOS transistor are coupled to receive a first bias voltage.

15. The amplifier of claim 9, wherein the differential transistor pair comprises a first transistor and a second transistor, a gate of the first transistor being coupled to the second voltage and a gate of the second transistor being coupled to the first voltage; and wherein the transconductance of the first transistor of the differential transistor pair is decoupled from the transconductance of the compensating cascode transistor of the cascode compensation stage.

16. The amplifier of claim 9, wherein the pair of current mirror transistors comprises transistors having the same aspect ratio as each other; wherein the cascode transistor pair comprises transistors having the same aspect ratio as each other; and wherein:

The differential transistor pair comprises a first differential input transistor and a second differential input transistor;

the current mirror transistor pair comprises a first current mirror transistor and a second current mirror transistor, wherein the second current mirror transistor is coupled in parallel with the output transistor of the cascode compensation stage; and is

The cascode transistor pair includes a first cascode transistor and a second cascode transistor, the first and second cascode transistors coupled between the first and second differential input transistors and the first and second current mirror transistors, respectively, the second cascode transistor coupled in parallel with the compensation cascode transistor.

17. The amplifier of claim 16, wherein the amplifier stage further comprises a tail current source configured to sink a first current from a tail node; wherein the first differential input transistor is configured to source a second current to the tail node; wherein the second differential input transistor is also configured to source the second current out to the tail node; wherein the first current mirror transistor is configured to source the second current to the first cascode transistor; wherein the second current mirror transistor is configured to source the second current to the second cascode transistor; wherein the second current is equal to half the first current; wherein the output transistor is configured to source a third current to the cascode compensation transistor; and wherein the cascode compensation stage further comprises a current sinking transistor configured to sink the third current from the cascode compensation transistor to cause the third current to bypass the second differential input transistor.

18. The amplifier of claim 9, wherein the amplifier stage further comprises a tail current source configured to sink a first current from a tail node; wherein the current mirror transistor pair are each configured to source a second current to one transistor of the cascode transistor pair, the second current equal to half the first current; wherein the differential transistor pairs are each configured to source the second current to the tail node; wherein the output transistor is configured to source a third current to the cascode compensation transistor; and wherein the cascode compensation stage further comprises a current sink transistor configured to sink the third current from the cascode compensation transistor to cause the third current to bypass the differential transistor pair.

19. An amplifier, comprising:

an amplifier stage, comprising:

a first differential input transistor and a second differential input transistor that receive a first voltage and a second voltage, respectively;

a first current mirror transistor and a second current mirror transistor;

first and second cascode transistors coupled between the first and second differential input transistors and the first and second current mirror transistors;

A tail current source coupled to sink a first current from the first differential input transistor and the second differential input transistor such that the first differential input transistor and the second differential input transistor both source a second current to the tail current source, the second current equal to half the first current;

a current source configured to sink a third current from the second cascode transistor such that the third current bypasses the second differential input transistor; and

an output stage biased by the second current mirror transistor and the second cascode transistor,

wherein the first current mirror transistor is configured to source the second current to the first cascode transistor, but the second current mirror transistor is sized differently than the first current mirror transistor to source a current equal to the sum of the second current and the third current to the second cascode transistor;

wherein the first cascode transistor is configured to source the second current to the first differential input transistor, but the second cascode transistor is sized differently than the first cascode transistor to draw a current from the second current mirror transistor equal to the sum of the second current and the third current such that the transconductance of the second cascode transistor is greater than the transconductance of the first cascode transistor, the second differential input transistor having the same transconductance as the first differential input transistor.

20. The amplifier of claim 19, wherein the second current mirror transistor is larger than the first current mirror transistor; wherein the second cascode transistor is larger than the first cascode transistor; and wherein the first differential input transistor and the second differential input transistor have the same dimensions.

21. The amplifier of claim 20, wherein the tail current source comprises a transistor; and wherein the current source comprises a transistor having the same size as the transistor of the tail current source.

22. The amplifier of claim 19, wherein the amplifier stage comprises an error amplifier stage; wherein the first voltage comprises a feedback voltage and the second voltage comprises a reference voltage; and wherein the output stage comprises:

an output transistor biased by the second current mirror transistor and the second cascode transistor and generating a load current; and

a sense resistor configured to generate the second voltage according to the load current.

23. The amplifier of claim 19, wherein the amplifier is,

wherein the first differential input transistor comprises a first NMOS transistor having a drain, a source coupled to a tail node, and a gate coupled to receive the second voltage;

Wherein the second differential input transistor comprises a second NMOS transistor having a drain, a source coupled to the tail node, and a gate coupled to receive the first voltage;

wherein the first current mirror transistor comprises a first PMOS transistor having a source coupled to a power supply node, a drain coupled to a first intermediate node, and a gate coupled to the drain of the first PMOS transistor;

wherein the second current mirror transistor comprises a second PMOS transistor having a source coupled to the power supply node, a drain coupled to a second intermediate node, and a gate coupled to the gate of the first PMOS transistor;

wherein the first cascode transistor comprises a third NMOS transistor having a drain coupled to the first intermediate node, a source coupled to the drain of the first NMOS transistor, and a gate;

wherein the second cascode transistor comprises a fourth NMOS transistor having a drain coupled to the second intermediate node, a source coupled to the drain of the second NMOS transistor, and a gate coupled to the gate of the third NMOS transistor; and

Wherein the tail current source comprises a fifth NMOS transistor having a drain coupled to the tail node, a source coupled to ground, and a gate coupled to receive a bias voltage; and

wherein the current source comprises a sixth NMOS transistor having a drain coupled to the source of the fourth NMOS transistor, a source coupled to ground, and a gate also coupled to receive the bias voltage.

24. The amplifier of claim 23, wherein the output stage comprises:

a third PMOS transistor having a source coupled to the power supply node, a drain coupled to an output node, and a gate coupled to the drain of the second PMOS transistor;

a capacitor coupled between the drain of the third PMOS transistor and the power supply node;

a compensation capacitor coupled between the source of the fourth NMOS transistor and the output node; and

a voltage divider coupled between the output node and ground, wherein the second voltage is generated at a center tap of the voltage divider, and the center tap of the voltage divider is coupled to the gate of the first NMOS transistor.

25. An amplifier, comprising:

an amplifier stage, comprising:

a first differential input transistor and a second differential input transistor that receive a first voltage and a second voltage, respectively;

a first current mirror transistor and a second current mirror transistor having different sizes from each other;

first and second cascode transistors having different sizes from each other and coupled between the first and second differential input transistors and the first and second current mirror transistors;

wherein the first and second current mirror transistors are coupled between a power supply node and the first and second cascode transistors;

wherein the first and second differential input transistors are coupled between the first and second cascode transistors and a tail node; and

a tail transistor coupled to sink a first current from the tail node;

a current source transistor coupled in parallel with the second current mirror transistor;

a compensation cascode transistor coupled in parallel with the second cascode transistor;

A second tail transistor drawing a second current from the second cascode transistor such that the second current bypasses the second differential input transistor; and

an output transistor coupled between the power supply node and an output node, the output transistor having a control terminal coupled to a drain of the current source transistor and a drain of the compensation cascode transistor.

26. The amplifier of claim 25, wherein for a given size of the second current mirror transistor equal to k x given _ size, the size of the compensation cascode transistor is (1-k) x given _ size.

27. The amplifier of claim 25, wherein for a given size of the second cascode transistor equal to k x given _ size, the size of the compensation cascode transistor is 2(1-k) x given _ size.

28. The amplifier of claim 25, wherein the amplifier stage is an error amplifier stage, wherein the first voltage is a feedback voltage representing a current delivered by the output transistor to a load, and the second voltage is a reference voltage.

Technical Field

The present disclosure relates to the field of operational amplifiers, and in particular, to operational amplifier designs with transconductance enhanced (boost) cascode compensation.

Background

In the design of operational amplifiers, such as for use as error amplifiers, cascode (stacked) transistors are used to increase gain and provide enhanced high frequency power supply rejection ratio. However, this may cause the frequency response of the amplifier to peak, which may lead to instability at higher frequencies. Therefore, it is known to increase the cascode transconductance to obtain higher frequency stability.

This design of amplifier 20 is shown in fig. 1A. Amplifier 20 is comprised of a differential input stage 21 and an output (or gain) stage 22. Differential input stage 21 includes a pair of differential input transistors Tdi1 and Tdi2 having control terminals receiving a reference voltage Vref and a feedback voltage Vfb indicative of an output current Iout generated by output stage 22, respectively. Cascode transistors Tc1 and Tc2 are stacked between a pair of differential input transistors Tdi1 and Tdi2 and load transistors Tm1 and Tm 2. Note that the transistors Tdi1 and Tdi2 have the same size as each other, the transistors Tc1 and Tc2 have the same size as each other, and the transistors Tm1 and Tm2 have the same size as each other.

A tail current source transistor Tt controlled by a bias voltage Vb provides a bias current I to the differential input stage 21; therefore, note that when Vref equals Vfb, the current through Tdi1 and Tdi2 will be I/2. The output stage 22 generates an output current Iout for the load according to the drain voltage of the load transistor Tm 2.

The frequency response of amplifier 20 with respect to gain is shown in fig. 1B. A gain peak can be observed. As the output current Iout increases, the gain peak will deteriorate, possibly causing high frequency instability. To reduce the gain peaks, it is known to increase the transconductance of the cascode transistors Tc1 and Tc2 by increasing the bias current I. However, this has the effect of increasing the transconductance of the pair of differential input transistors Tdi1 and Tdi2 and thereby increasing the unity gain bandwidth, which actually reduces stability. Therefore, this design is ineffective in some cases, as it would be preferable to increase the transconductance of the cascode transistors Tc1 and Tc2 independently of the pair of differential input transistors Tdi1 and Tdi 2.

An amplifier design 20' that achieves the transconductance increase of the cascode transistors Tc1 and Tc2 independently of the pair of differential input transistors Tdi1 and Tdi2 is shown in fig. 1C (note that all details of the design can be found in the publication "An improved frequency compensation technique for CMOS operational amplifiers" (IEEE Journal of Solid State Circuits, volume 18, No. 6, p. 629, p. 12 1983), which is incorporated herein by reference, of ahuja, b.k.ahuja). Here, a common gate stage 23 is interposed between the differential input stage 21 and the output stage 22. Common gate stage 23 is comprised of cascode transistor Tcg2 stacked between transistor Tcg1 and transistor Tcgt. Transistors Tcg1 and Tcg2 are biased by bias voltages Vbcg1 and Vbcg2, respectively, while transistor Tcgt is biased by the same bias voltage Vb as the tail transistor Tt. Note that, therefore, current Icg passes through cascode transistor Tcg 2. This increases the transconductance of the amplifier 20 at the cost of additional power consumption due to the increased current Icg in the common gate stage 23.

This additional power consumption is undesirable in some applications. Therefore, further development is required.

Disclosure of Invention

An amplifier is disclosed herein that includes a differential amplifier stage and an output stage. The amplifier stage includes: a differential transistor pair receiving a first voltage and a second voltage, a current mirror transistor pair, and a cascode transistor pair coupled between the differential pair at the first and second nodes and the current mirror pair at the third and fourth nodes. A differential pair is coupled between the cascode pair and the tail node. The tail transistor draws a first bias current from the tail node, the first bias current having a magnitude equal to a product of the total bias current and a constant k, where k is less than 1. The first current source transistor draws a second bias current from the second node such that the second bias current bypasses a second one of the differential pair, the second bias current having a magnitude that is the same as a product of the total bias current and a value equal to 1 minus k. The output stage is biased by the fourth node.

The differential pair includes a first differential input transistor coupled between the first node and the tail node, the first differential input transistor receiving a first voltage. The differential pair also includes a second differential input transistor coupled between the second node and the tail node and receiving a second voltage. The current mirror pair includes: a first current mirror transistor coupled between the supply voltage and a third node, and a second current mirror transistor coupled between the supply voltage and a fourth node. The cascode pair includes: a first cascode transistor coupled between the first and third nodes, and a second cascode transistor coupled between the second and fourth nodes.

For a given size of the first cascode transistor equal to k × given _ size, the size of the second cascode transistor is equal to (2-k) × given _ size. Furthermore, for a given size of the first current mirror transistor equal to K × given _ size, the size of the second current mirror transistor is equal to (2-K) × given _ size, K being smaller than 1.

Drawings

Fig. 1A is a schematic diagram of a prior art amplifier with cascode compensation.

Fig. 1B is a plot of the frequency response of the amplifier of fig. 1A with respect to gain.

Fig. 1C is a schematic diagram of another prior art amplifier with cascode compensation.

Fig. 2 is a schematic diagram of an amplifier with cascode compensation according to the present disclosure.

Fig. 3 is a graph of gain versus frequency for the design of fig. 2 versus a prior art design.

Fig. 4 is a plot of Power Supply Rejection Ratio (PSRR) versus frequency for the design of fig. 2 versus a prior art design.

Fig. 5 is another plot of Power Supply Rejection Ratio (PSRR) versus frequency for the design of fig. 2 versus a prior art design.

Fig. 6 is a schematic diagram of another amplifier with cascode compensation according to the present disclosure.

Fig. 7 is a schematic diagram of the differential amplifier of fig. 2 illustrating small signal analysis.

Detailed Description

The following disclosure enables one of ordinary skill in the art to make and use the subject matter disclosed herein. The general principles described herein may be applied to embodiments and applications other than those detailed above without departing from the spirit and scope of the present disclosure. The present disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed or suggested herein.

Described now with reference to fig. 2 is an amplifier 100 (e.g., operational amplifier) with cascode compensation, the transconductance of the cascode transistors being enhanced in a manner that provides enhanced stability and reduced power consumption as compared to prior art designs.

The amplifier 100 includes a differential amplifier stage 110 (e.g., an error amplifier stage) and an output stage 120, and a load 125 is connected to the output stage 120.

The differential amplifier stage 110 includes a pair of differential input transistors MN1 and MN2, cascode transistors MN3 and MN4, and current mirror (or load) transistors MP1 and MP2 coupled to a tail current source transistor MN 5.

In more detail, the differential input transistor pair is composed of NMOS transistors MN1 and MN 2. The source of the NMOS transistor MN1 is connected to the tail node Ntail, the drain thereof is connected to the first node N1, and the gate thereof is connected to receive the feedback voltage Vfb. The source of the NMOS transistor MN2 is connected to the tail node Ntail, the drain thereof is connected to the second node N2, and the gate thereof is connected to receive the reference voltage Vref. Note that the tail transistor MN5 is an NMOS transistor having its drain connected to the tail node Ntail, its source connected to ground, and its gate connected to receive the first bias voltage Vb 1.

The cascode transistor MN3 is an NMOS transistor having a drain connected to the first node N1, a source connected to the third node N3, and a gate connected to the second bias voltage Vb 2. The cascode transistor MN4 is an NMOS transistor, whose drain is connected to the second node N2, whose source is connected to the fourth node N4, and whose gate is connected to the gate of the NMOS transistor MN3 and thus to the second bias voltage Vb 2. Cascode transistors MN3 and MN4 have the same size.

The current mirror transistor MP1 is a PMOS transistor whose source is connected to the power supply voltage Vsup, whose drain is connected to the third node N3, and whose gate is connected to its drain at the third node N3. The current mirror transistor MP2 is a PMOS transistor whose source is connected to the power supply voltage Vsup, whose drain is connected to the fourth node N4, and whose gate is connected to the gate of the PMOS transistor MP1 and thus to the third node N3. The PMOS transistors MP1 and MP2 have the same size.

The compensated output transistor MP3 is a PMOS transistor whose source is connected to the supply voltage Vsup and thus to the source of the transistor MP2, whose drain is connected to the fourth node N4 and thus to the drain of the transistor MP2, and whose gate is connected to the gates of the PMOS transistors MP1 and MP 2. The cascode transistor MN6 is an NMOS transistor having a drain connected to the node N4 and thus to the drain of the NMOS transistor MN4, a source connected to the second node N2 and thus to the source of the transistor MN4, and a gate connected to the gates of the NMOS transistors MN3 and MN4 and thus to the second bias voltage Vb 2. The current source transistor MN7 is an NMOS transistor having a drain connected to the second node N2 and thus to the sources of the NMOS transistors MN6 and MN4, a source connected to ground, and a gate connected to the gate of the NMOS transistor MN5 and thus to the first bias voltage Vb 1.

Note that the dimensions of the transistors in fig. 2 are scaled relative to the dimensions of the transistors in fig. 1A on the same scale as the proportion of the current flowing through them.

Thus, note that the size of MN5 is k × (size of Tt), the size of MN1 is k × (size of Tdi 1), the size of MN2 is k × (size of Tdi 2), the size of MN3 is k × (size of Tc 1), the size of MN4 is k × (size of Tc 2), the size of MP1 is k × (size of Tm 1), and the size of MP2 is k × (size of Tm 2). Therefore, it is also noted that MP3 is of size 2(1-k) × (size of Tm1 or Tm 2), MN6 is of size 2(1-k) × (size of Tc1 or Tc 2), and MN7 is of size (1-k) × (size of MN 5).

The output stage 120 includes a PMOS transistor MP4 having a source connected to the supply voltage Vsup, a drain connected to the output node Nout, and a gate connected to the fourth node N4 and thus to the drains of the PMOS transistors MP2 and MP 3. The capacitor C1 represents the gate capacitance due to the PMOS transistor MP4 and the parasitic capacitance at the node N4. The compensation capacitor Cc is connected between the second node N2 and the output node Nout.

In operation, differential pair MN1 and MN2 take the difference between Vfb and Vref and multiply the difference by a gain. Cascode transistors MN4 and MN6 provide compensation through capacitor Cc, while the current mirror formed by transistors MP1, MP2, and MP3 forms the active load for differential amplifier 110 and provides the output of differential amplifier 110 at node N4. The output transistor MP4 is biased by the node N4 and generates an output current Iout. The feedback voltage Vfb is generated at the center tap of the sense resistors R2 and R1 and represents the output current Iout.

The total bias current I of the differential amplifier stage 110 is split into two-a current I1 through the tail transistor MN5 and a current I3 through the tail transistor MN 7. Note that the current I1 drawn by tail transistor MN5 is equal to k I, and when Vref is equal to Vfb, the current I2 drawn through each leg is I2 k I/2. The current I3 emitted by the transistor MP3 and absorbed by the transistor MN7 is I3 ═ 1-k × I; therefore, note that I-I1 + I3-k I + (1-k) I. This division of the total bias current I means that adding the cascode compensation transistor MN6 to the differential amplifier stage does not consume additional power compared to prior art designs with bias current I (such as fig. 1A), which provides increased cascode transconductance, as will be explained. Note that the value of k used in this example is less than 1, and is the same as the value of k for the transistor dimensions listed above.

The addition of the cascode compensation transistor MN6 enables the transconductance of the cascode transistor MN4 (labeled kgm3) to be effectively enhanced by the transconductance of MN6 (labeled 2(1-k) gm3) to help compensate while leaving the overall transconductance of the differential amplifier 110 unchanged. Thus, note that instead of the cascode MN4 transconductance of gm3 (such as the prior art design of fig. 1A), the effective transconductance of the cascode becomes:

kgm3+2(1-k)gm3＝kgm3+2gm3-2kgm3＝2gm3-kgm3

Enhancement of the transconductance of cascode transistor MN6 is achieved by feeding current I3 to the drain of cascode transistor MN6 using transistor MP3 and taking current I3 from the source of cascode transistor MN6 using tail transistor MN7, and using the same bias for transistors MP2 and MP3 so that the overall transconductance of differential amplifier 110 is unchanged.

Note that the overall transconductance of cascode transistors MN4 and MN6 is increased by a factor of (2-k) while keeping the overall transconductance of differential amplifier 110 the same.

For example, the improved performance of the design of amplifier 100 over prior art designs can be seen from the graph of fig. 3, which shows a DC gain reduction of 4.86dB (e.g., by a factor of 1/(2-k) for k ═ 0.25), unchanged high frequency performance, and reduced peak values. It should also be noted that the graph of fig. 4 shows a DC supply rejection ratio (PSRR) reduction of 4.86dB (e.g., by 1/(2-k) times for k 0.25), a constant intermediate frequency PSRR, and improved high frequency and peak PSRR due to the transconductance increase provided by cascode compensation transistor MN 6.

If one were to attempt to achieve the same increase in transconductance of amplifier 100 using the design of fig. 1A, for example, current Icg would need to be equal to I (1-0.5 k). Thus, the design of amplifier 100 reduces current consumption by I x (1-0.5k) compared to this prior art design. This also means that the transconductance can be improved by a factor of (2-0.5k) considering that the equivalent current consumption is acceptable, which in turn will further improve the high frequency PSRR, bearing in mind that the value of Cap2 needs to be increased accordingly Multiple to maintain the same phase margin. Thus, note that in the graph of fig. 5, for k 0.25, the low frequency PSRR remains unchanged, while the high frequency PSRR improves by 5.4dB, bearing in mind that for the example of fig. 5, the overall bias current I increases by the amount of current saved over the prior art.

Those skilled in the art will appreciate that in the design of fig. 2, instead of using transistor MP3 coupled in parallel with transistor MP2 and instead of using transistor MN6 coupled in parallel with transistor MN4, the size of transistors MP2 and MP4 may be increased in reverse (while keeping added transistor MN7 which draws current I3 from node N2).

Such an embodiment is shown in fig. 6. In the present embodiment, the amplifier 100 'is composed of a differential amplifier stage 110' and an output stage 120. The output stage 120 remains as described above with reference to fig. 2.

In this application, the differential amplifier stage 110' includes: a differential input transistor pair MN1 and MN2, cascode transistors MN3 and MN4 ', and current mirror (or load) transistors MP1 and MP 2' coupled to the tail current source transistor MN 5.

In more detail, the differential input transistor pair is composed of NMOS transistors MN1 and MN 2. The source of the NMOS transistor MN1 is connected to the tail node Ntail, the drain thereof is connected to the first node N1, and the gate thereof is connected to receive the feedback voltage Vfb. The source of the NMOS transistor MN2 is connected to the tail node Ntail, the drain thereof is connected to the second node N2, and the gate thereof is connected to receive the reference voltage Vref. Note that the tail transistor MN5 is an NMOS transistor having its drain connected to the tail node Ntail, its source connected to ground, and its gate connected to receive the first bias voltage Vb 1.

The cascode transistor MN3 is an NMOS transistor having a drain connected to the first node N1, a source connected to the third node N3, and a gate connected to the second bias voltage Vb 2. The cascode transistor MN 4' is an NMOS transistor having a drain connected to the second node N2, a source connected to the fourth node N4, and a gate connected to the gate of the NMOS transistor MN3 and thus to the second bias voltage Vb 2.

The current mirror transistor MP1 is a PMOS transistor whose source is connected to the supply voltage Vsup, whose drain is connected to the third node N3, and whose gate is connected to its drain at the third node N3. The current mirror transistor MP 2' is a PMOS transistor whose source is connected to the supply voltage Vsup, whose drain is connected to the fourth node N4, and whose gate is connected to the gate of the PMOS transistor MP1 and thus to the third node N3.

The drain of NMOS transistor MN7 is coupled to node N2 and capacitor Cc, its source is coupled to ground, and its gate is coupled to bias voltage Vb 1.

The sizes of transistors MP1, MN1, MN2, MN3, and MN5 are k times the size of transistors Tm1, Tid1, Tid2, Tc1, and Tt, respectively, of fig. 1A. The size of transistors MP2 'and MN 4' is (2-k) multiplied by the size of transistors Tm2 and Tc2 of fig. 1A, respectively.

In the operation of the differential amplifier 100, the differential pair MN1 and MN2 takes the difference between Vfb and Vref and multiplies the difference by the gain. Cascode transistors MN3 and MN4 'provide compensation, while the current mirror formed by transistors MP1 and MP 2' forms the active load of the differential amplifier stage 110 and provides the output of the differential amplifier stage 110 at node N4. The output transistor MP4 is biased by the node N4 and generates an output current Iout. The feedback voltage Vfb is generated at the center tap of the sense resistors R2 and R1 and represents the output current Iout.

Here, note that: the bias current I1 sunk by transistor MN5 is equal to kI; when Vref equals Vfb, the current I2 emitted by transistor MP1 equals kI/2; and the current I3 sunk by the transistor MN7 is equal to (1-k) I, while when Vref is equal to Vfb, the current issued by the transistor MP 2' is I2+ I3 ═ kI/2+ (1-k) I. As a result, the increased current I2+ I3 issued by the current mirror transistor MP2 ' to the cascode transistor MN4 ' causes an increase in the transconductance of MN4 ' without increasing the transconductance of transistor MN2 (because MN2 sinks current I2, because current I3 bypasses MN 2). Thus, the transconductance of transistor MN2 remains kgm1, while the transconductance of cascode transistor MN 4' increases to:

kgm3+2(1-k)gm3＝2gm3-kgm3

The overall transconductance of the differential amplifier 110' remains gm 1.

This may be most easily seen with reference to the small signal diagram of the differential amplifier 110' shown in fig. 7. Thus, it can be easily observed that the overall transconductance of the differential amplifier 110' is gmin gm1 and thus remains unchanged. As described above, the transconductance of the cascode transistor MN 4' is gmcas (2-k) gm3, and thus is increased by a factor of 2-k over the prior art. Thus, it will be appreciated that by the design of the differential amplifier 110' (and differential amplifier 110), the overall cascode device transconductance increases by a factor of 2-k, but the input transconductance remains the same for the same total bias current I. Thus, power is saved.

The performance of the amplifier 100' of fig. 6 remains unchanged compared to fig. 2, so the previously presented curves are applicable.

While the present disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the disclosure as disclosed herein. Accordingly, the scope of the disclosure is limited only by the following claims.