Linear broadband PNP amplifier
A DC-coupled differential amplifier has an output common-mode voltage near the ground reference. A feedback circuit superimposed on a trans-linear loop improves linearity and extends the bandwidth of the basic PNP differential pair.
1. A differential amplifier, comprising:
a differential pair comprising a first transistor and a second transistor, and a first current source and a second current source, the first current source and second current source providing a collector-emitter current of the first transistor and a collector-emitter current of the second transistor, respectively;
a first trans-linear loop and a second trans-linear loop coupled to a base terminal and an emitter terminal of the first transistor and a base terminal and an emitter terminal of the second transistor, respectively, wherein each trans-linear loop comprises:
a third transistor, a fourth transistor and a fifth transistor, connected such that a corresponding one of the first transistor and the second transistor, the third transistor, the fourth transistor and the fifth transistor form a closed loop of base-emitter junctions; and
a third current source, wherein a current in a corresponding one of the first current source and the second current source and a current in the third current source are provided in substantially the same ratio as an area of the emitter terminal of the corresponding one of the first transistor and the second transistor to an area of an emitter terminal of the fifth transistor; and
a first feedback loop and a second feedback loop coupled to the base and the emitter terminals of the first transistor and the base and the emitter terminals of the second transistor, respectively, wherein each feedback loop comprising a sixth transistor having a base terminal connected to a collector terminal of the fifth transistor of the corresponding one of the first trans-linear loop and the second trans-linear loop and a collector terminal connected to an emitter terminal of the fourth transistor of the corresponding one of the first trans-linear loop and the second trans-linear loop.
2. The differential amplifier of claim 1 , wherein the first transistor, the second transistor and the fifth transistor of each of the first trans-linear loop and the second trans-linear loop are PNP transistors.
3. The differential amplifier of claim 2 , wherein the third transistor and the fourth transistor of each of the first trans-linear loop and the second trans-linear loop are NPN transistors.
4. The differential amplifier of claim 3 , wherein the sixth transistor is a NPN transistor.