TABLE OF CONTENTS Up to now, all the circuits we have talked about have used series feedback, where the local feedback element is in series with both the input and output currents. In this chapter, we look at shunt feedback, where some of the output voltages are sampled and shunted back to the input. This is the technique used by almost all op-amp circuits. However, since the same technique can be applied successfully to high-frequency circuits, we need to investigate this important concept.
Moreover, there is another important circuit that does not fit well into the other categories. This is a circuit made up of three transistors that produces the equivalent of a single transistor. This composite circuit has a higher equivalent f t and twice the ? of the devices making up the circuit. Because it can be used almost anywhere a single transistor can be used, it is an extremely useful circuit especially handy when designing integrated circuits, because it is easy to add transistors to an integrated circuit. This circuit provides almost the same bandwidth-improvement factor as series peaking. But because series peaking requires inductors, the composite circuit is much easier to implement on an integrated circuit.
It is often desirable to have higher input impedance than is normally achieved with a standard BJT FETs come by this naturally, but BJTs tend to have a finite ?, which can be quite low. One technique for solving this problem is to use a circuit...
