Fig. P5.18 shows a common-gate amplifier augmented with feedback. Assume that the load resistance, . Ideally, all of the source current, , passes through the transistor and none is lost in the source resistance, , resulting in a desired small-signal gain, , of .
a. What is the value of ?
b. For the circuit in Fig. P5.18(a), estimate the dc loop gain, , in terms of the transistor’s small-signal parameters: , , etc. Do so by breaking the loop at the point indicated. Assume the opamp has a voltage gain and infinite input impedance.
c. Combining the results in parts (a) and (b) above, write an expression for the dc gain of the closed-loop circuit, .
d. Rewrite the expression from part (c) for the special case, .
e. Solve for the source resistance, , that causes the closed-loop gain in (c) to equal times the result obtained in part (d). This is a good estimate of the input resistance of the circuit, .
f. Apply the results of parts (b) – (e) to the special case pictured in Fig. P5.18(b) where the opamp is replaced by a simple common-source amplifier.