# Patent

## Title
Current mirror with low headroom and linear response

## Abstract

A current mirror circuit provided in an emitter follower configuration achieves linearly output over a range of input currents by operating in response to a bias current that is a replica of the input current. The current mirror may include a pair of transistors and a pair of resistors, in which: a first resistor and a base of a first transistor are coupled to a first input terminal for a first input current, an emitter of the first transistor and a base of the second transistor are coupled to a second input terminal for a second input current, the first and second input currents being replicas of each other, an emitter of the second transistor being coupled to the second resistor, a collector of the second transistor being coupled to an output terminal of the current mirror, and a collector of the first transistor and the two resistors are coupled to a common node.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of and claims priority to U.S. patent application Ser. No. 13/042,057 to Sandro Herrera, filed Mar. 7, 2011, which is a divisional of U.S. patent application Ser. No. 12/255,326, filed Oct. 21, 2008, entitled “Current Mirror With Low Headroom And Linear Response,” both of which are herein incorporated by reference in their entireties.


BACKGROUND
The present invention relates to a current mirror circuit that provides linear output with low headroom requirements.
Current mirrors are well known but they suffer from known disadvantages. Two basic current mirrors are shown in FIGS. 1 and 2. The current mirror of FIG. 1 is called a diode current mirror. It includes a pair of transistors Q1 and Q2 in which the input transistor Q1 is connected as a diode. The base and collector of Q1 are connected to the base of Q2. The input current signal is fed at an input node N1.1 which creates an input potential drop of V1.1=VBE1+VRE1. The base-to-emitter voltages of each transistor Q1, Q2 (VBE1 and VBE2) vary together with the input current signal; thus, VBE non-linearities tend to cancel out. The input current IIN generates a corresponding output current IOUT at the same level. Although the diode current mirror provides an output current that has a linear response to a changing input current (IIN=IIN(t)), it imposes an input headroom requirement of VBE1+VRE1. In practice, this can be as high as 1V.
The current mirror of FIG. 2 is called an emitter follower mirror. The circuit also includes a pair of transistors Q1, Q2. This circuit requires a bias current (IBIAS) provided at node N2.2. The input current IIN is fed directly to the resistor RE1, creating a voltage at the input terminal of V2.1=IIN*R (assume RE1=RE2=R). At node N2.2, the input and bias currents create a voltage V2.2=VIN+VBE1. At the emitter of Q2, the current mirror generates a voltage V2.3=VIN+VBE1−VBE2, which results in an output current of IOUT=V2.3/R=1/R*(IIN*R+VBE1−VBE2) if base current errors are ignored. In all known emitter follower mirrors, the bias current IBIAS is provided as a constant current.
The emitter follower mirror possesses a disadvantage because IOUT varies non-linearly with IIN. The input current to the mirror IIN is a time varying signal (IIN=IIN(t)), which causes VBE2 to vary over time (VBE2=VBE2(t)). VBE1 does not vary, due to the constant bias current IBIAS. This configuration generates an output current as follows:

IOUT=V2.3/R=1/R*(IIN*R+VBE1−VBE2(t))

Although the VBE1−VBE2 term in IOUT ideally would cancel out, it does not over most conditions. This leads to the non-linear response of the emitter follower mirror.

By way of example, consider a use case in which the input current IIN doubles over time. The voltage at node N2.1 will double, and the voltage across RE2 will roughly double. As a result, the output current will roughly double which causes a change in VBE2 of about 18 mv. Since IBIAS does not change, VBE1 will not change. This behavior would induce an error in the output current IOUT of about 18 mv/RE2=18 mV/R.
There is no known current mirror circuit that provides a linear output while requiring low input headroom requirements for the input current signal.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a known current mirror.
FIG. 2 is a circuit diagram of another known current mirror.
FIG. 3 is a circuit diagram of a current mirror according to an embodiment of the present invention.
FIG. 4 is a circuit diagram of another current mirror according to an embodiment of the present invention.
FIG. 5 is a diagram of an operational amplifier according to an embodiment of the present invention.
FIG. 6 is a circuit diagram of an operational amplifier according to an embodiment of the present invention.
FIG. 7 is a diagram of a differential amplifier according to an embodiment of the present invention.
FIG. 8 is a circuit diagram of a differential amplifier according to an embodiment of the present invention.



DETAILED DESCRIPTION
The disadvantages of the prior art are overcome by a current mirror circuit that provides a low headroom input requirement and provides a linear output current. The current mirror is configured as an emitter follower mirror that accepts a first input current signal IIN(t) at an input and also receives a replica of IIN(t) used as a bias current. The replica bias current provides a linear output response of the proposed current mirror.
FIG. 3 is a circuit diagram of a current mirror 300 according to an embodiment of the present invention. The current mirror 300 may include a pair of transistors Q1, Q2 and a pair of resistors RE1, RE2. A first transistor Q1 is coupled to an input current source IIN1(t) at its base which also is connected to the first resistor RE1. An emitter of the first transistor Q1 may be coupled to a replica of the input current IIN2(t).
A base of the second transistor Q2 also may be connected to the replica input current IIN2(t). An emitter of the second transistor Q2 may be coupled to the second resistor RE2. A collector of the second transistor Q2 may be coupled to an output terminal of the current mirror. The first and second resistors RE1, RE2 and a collector of the first transistor Q1 may be connected to a common node, commonly ground or a power rail.
The current mirror 300 of FIG. 3 uses the replica input current IIN2(t) as a bias current to the first transistor Q1. During operation, the input current IIN1(t) generates a voltage at node N3.1 as V3.3=IIN1(t)*RE1. The input headroom limitation is the voltage drop across resistor RE1 which is much lower than the limitations incurred by the current mirror of FIG. 1. The voltage at node N3.2 is IIN1(t)*RE1+VBE1. Finally, the voltage at node N3.3 is IIN1(t)*RE1+VBE1−VBE2. Because the bias current IIN2(t) to this circuit is a replica of the input current IIN1(t), the base-to-emitter voltages of transistors Q1 and Q2 will be equal and will vary together, therefore, cancel each other. Thus, the current mirror achieves linear operation.
The circuit of FIG. 3 finds application in complementary circuits in which it is common to generate input current pairs, in which one current comes from a source and a complementary current comes from a sink. To accommodate this circuit in such designs, the circuit also may include a secondary mirror that is responsive to a sinking input current IIN3(t), which is equal in magnitude to IIN1(t), in this case a sourcing input current. This secondary mirror generates an output current in the same direction as IIN1(t). Thus, in this embodiment, current IIN3(t) generates a second current, IIN2(t), which is equal to and in the same direction as IIN1(t) and the second current IIN2(t) may be input to the primary current mirror of FIG. 3 as a bias current to Q1.
FIG. 4 illustrates a current mirror system according to another embodiment of the present invention. This embodiment finds application in a complementary system where complementary input currents IIN1(t) and IIN2(t) are available. The circuit may include three or, optionally, four copies of the current mirror of FIG. 3.
A first mirror, shown as mirror 1, may include a first pair of transistors Q1, Q2 and a first pair of resistors RE1, RE2 configured as described in FIG. 3. Input current IIN1(t) is the input to the mirror. A bias current IBIAS1(t) is input to the mirror at node N4.2. Mirror 1 may generate an output current IOUT1.1(t).
A second mirror, shown as mirror 2, may include a pair of transistors Q1, Q3 and a pair of resistors RE1, RE3. Transistor Q1 and resistor RE1 are shared among mirrors 1 and 2. Mirror 2 accepts an input current IIN1(t) and a bias current IBIAS1(t), which are shared among mirrors 1 and 2. Mirror 2 may generate an output current IOUT1.2(t).
A third mirror, shown as mirror 3, may include a pair of transistor Q4 and Q6 and a pair of resistors RE4, RE6. Input current IIN2(t) is input to the mirror, which is sink version of IIN1(t). A bias current IBIAS2(t) may be input to the mirror at node N4.6. Mirror 3 may generate an output current IOUT2.1(t).
A fourth mirror, which is optional, may include transistors Q4, Q5 and resistors RE4, RE5. Transistor Q4 and resistor RE4 may be shared among mirrors 3 and 4. Input current IIN2(t) and bias current IBIAS2(t) are shared among mirrors 3 and 4. Mirror 4 may generate an output current IOUT2.2(t).
In the circuit of FIG. 4, mirror 1 may receive an output current from mirror 3 as a bias current (IOUT2.1(t) becomes IBIAS1(t)). This maintains linear operation on mirror 1, ensuring IOUT1.1(t)=IIN1(t). The response of mirror 3 will be kept linear if mirror 3 receives a bias current which also matches the input current. Mirror 3 receives an output current from mirror 2 as a bias current (IOUT1.2(t) becomes IBIAS2(t). This maintains linear operation on mirror 3, ensuring IOUT2.1(t)=IIN2(t). Thus, mirrors 2 and 3 keep each other in balance. They receive the output currents of their counterpart mirror as bias currents and, since these are equal to the input current, a linear response is ensured over the mirror system 400 as a whole.
The current mirror of FIG. 3 finds application in a variety of circuit systems. FIG. 5 illustrates application of the current mirror in an operational amplifier (op amp), according to an embodiment of the present invention. FIG. 5(a) illustrates an op amp symbolically. An op amp is a known circuit that generates an output voltage based on a difference between two input voltages VP, VN as VOUT=A*(VP−VN), where A is a gain factor provided by the op amp.
FIG. 5(b) provides a block diagram for a single stage operational amplifier. In this model, a transconductance cell GM 500 generates a current IOUT1 in response to a difference between the input voltages (IOUT1=GM*(VP−VN)). Signal current mirrors 510 generate an output current IOUT2 corresponding to the current received from the transconductance cell GM 500. The IOUT2 current is passed through a large impedance Z, which creates a voltage GM*Z*(VP−VN). Another amplifier buffer 530 is shown (with a gain of 1) which generates the output voltage VOUT=GM*Z*(VP−VN), where the op amp gain A=GM*Z.
FIG. 5(c) illustrates application of the current mirror to an op amp input stage. A complementary transconductance cell GM 500 generates currents in response to a difference among inputs VP and VN. These transconductance stages 500 are well known; typically, they generate two pairs of differential output currents each having magnitude IOUT/2. A pair of the current mirrors shown in FIG. 3 can be used as mirrors 510.1, 510.2. In response to the respective currents IOUT/2, the current mirrors 510.1, 510.2 may generate corresponding currents IOUT/2 on their outputs, which are summed at an output node to generate IOUT. The output current IOUT of FIG. 5(c) can be used as current IOUT2 of FIG. 5(b).
The second current pair 540.1, 540.2 from the transconductance stage 500 can be shunted to the supply rails of the system (not shown). Alternatively, the second current pair 540.1, 540.2 may be input respectively to the current mirrors 510, 520 as sources for the bias current (shown as IBIAS, in phantom).
FIG. 6 illustrates a circuit diagram of the model shown in FIG. 5(c) according to an embodiment of the present invention. The circuit 600 of FIG. 6 may include six current mirrors. Mirrors 1, 2 generate the main output currents for the stage. Mirrors 3 and 4 generate bias currents for use by mirrors 1 and 2 (also mirrors 5 and 6). Mirrors 5 and 6 generate bias currents for use by mirrors 3 and 4. FIG. 6 identifies the bias currents input to each mirror and the transistors that generate them.
FIG. 7 illustrates application of the current mirror in a differential amplifier (diff amp), according to an embodiment of the present invention. FIG. 7(a) illustrates a differential amplifier symbolically. A differential amplifier is a known circuit that generates a pair of output voltages VOUT+, VOUT− based on a difference between two input voltages VP, VN (VOUT+−VOUT−=A*(VP−VN)). The value A is a gain factor provided by the amplifier, which is quite large.
FIG. 7(b) provides a block diagram for a single stage differential amplifier. In this model, a transconductance cell GM 700 generates differential output currents IOUT1+, IOUT1− in response to a difference between the input voltages (IOUT1+−IOUT1−=GM*(VP−VN)). Signal current mirrors 710 may generate output currents IOUT2+, IOUT2− corresponding to the currents received from the transconductance cell GM 700. The IOUT2+, IOUT2− current may pass through impedance blocks 720.1, 720.2 which create corresponding output voltages having magnitude GM*Z*(VP−VN). Another amplifier buffers 730.1, 730.2 may generate output voltages VOUT+=−VOUT−=GM*Z*(VP−VN).
FIG. 7(c) illustrates application of the current mirror to a diff amp input stage. A complementary transconductance cell GM 700 may generate two pairs of currents each having magnitude IOUT/2 in response to a difference among inputs VP and VN. In the diff amp system, the current mirrors shown in FIG. 3 can be provided as mirrors 710.1-710.4. In response to the respective currents IOUT/2, the current mirrors 710.1-710.4 each may generate corresponding currents IOUT/2. A first pair of output currents may be summed at an output node to generate a first output current IOUT+ and a second pair of output currents, which are oriented opposite to the orientation of the first pair of currents, may be summed to generate the second output current IOUT−. The output currents IOUT+, IOUT− of FIG. 7(c) can be used as currents IOUT2+, IOUT2− of FIG. 7(b).
FIG. 8 illustrates a circuit diagram of the differential amplifier case, in which there are eight current mirrors. Mirrors 1-4 generate the main output currents. Four other mirrors (which are not labeled to retain clarity in the figure) generate bias currents for mirrors 1-4. FIG. 8 identifies the bias currents input to each mirror and the transistors that generates them.
Several embodiments of the present invention are specifically illustrated and described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

## Claims

1. A complementary current mirror system, comprising:
three current mirrors connected in an emitter follower configuration,
wherein
each current mirror includes a pair of transistors and a pair of resistors,
a first and a second current mirrors of the three current mirrors share a transistor and a resistor, and
output currents generated by the second current mirror and a third current mirror provide bias currents to the third and first current mirrors respectively.

2. The current mirror circuit of claim 1, wherein the three current mirrors include:
a first resistor and a base of a first transistor coupled to a first input terminal for a first input current,
an emitter of the first transistor and a base of the second transistor coupled to a second input terminal for a second input current that serves as bias current,
an emitter of the second transistor coupled to the second resistor,
a collector of the second transistor coupled to an output terminal, and
a collector of the first transistor and the first and second resistors are coupled to a common node.

3. The current mirror circuit of claim 2, wherein the first resistor and first transistor are shared with the second current mirror.

4. The current mirror circuit of claim 2, wherein an output current generated at the output terminal varies linearly with a first input current received at the first input terminal.

5. The current mirror circuit of claim 2, wherein a voltage at the first input terminal is characterized by low headroom.

6. The current mirror circuit of claim 1, wherein a collector of the second transistor of the second current mirror is coupled to an output terminal of the second current mirror and provides a bias current to the third current mirror.

7. The current mirror circuit of claim 6, wherein the first transistor of the third current mirror is biased by the output current from the second current mirror.

8. The current mirror circuit of claim 7, wherein a collector of the second transistor of the third current mirror is coupled to an output terminal of the third current mirror and provides a bias current to the first and second current mirrors.

9. The current mirror circuit of claim 8, wherein a second input current is coupled to an input terminal of the third current mirror.

10. The current mirror circuit of claim 9, wherein the first transistor of the first and second current mirrors is biased by the output current from the third current mirror.

11. The current mirror circuit of claim 1, wherein an output of the first current mirror is an overall output of the current mirror circuit, and wherein the first and second current mirror include a first input and the third current mirror includes a second input.

12. A method for generating an output current, comprising:
connecting three current mirrors in an emitter follower configuration, and
applying output currents generated by second and third current mirrors of the three current mirrors as bias currents to third and first current mirrors respectively,
wherein
each current mirror includes a pair of transistors and a pair of resistors, and
the first and second current mirrors of the three current mirrors share a transistor and a resistor.

13. The method of claim 12, wherein the first current mirror includes:
a first resistor and a base of a first transistor coupled to a first input terminal,
an emitter of the first transistor and a base of the second transistor coupled to a second input terminal,
an emitter of the second transistor coupled to the second resistor,
a collector of the second transistor coupled to an output terminal of the first current mirror, and
a collector of the first transistor and the first and second resistors are coupled to a common node.

14. The method of claim 13, wherein the first resistor and first transistor are shared with the second current mirror.

15. The method of claim 13, wherein an output current generated at the output terminal varies linearly with a first input current received at the first input terminal.

16. The method of claim 13, wherein a voltage at the first input terminal is characterized by low headroom.

17. The method of claim 12, wherein a collector of the second transistor of the second current mirror is coupled to an output terminal of the second current mirror and provides a bias current to the third current mirror.

18. The method of claim 17, wherein the first transistor of the third current mirror is biased by the output current from the second current mirror.

19. The method of claim 18, wherein a collector of the second transistor of the third current mirror is coupled to an output terminal of the third current mirror and provides a bias current to the first current mirror.

20. The method of claim 12, wherein an output of the first current mirror is an overall output of the current mirror circuit, and wherein the first and second current mirror include a first input and the third current mirror includes a second input.