In this problem we consider the frequency response of the bipolar cascode amplifier in

Chapter 10, Problem 10.77

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In this problem we consider the frequency response of the bipolar cascode amplifier in the case that ro can be neglected. (a) Refer to the circuit in Fig. 10.30, and note that the total resistance between the collector of Q1 and ground will be equal to re2, which is usually very small. It follows that the pole introduced at this node will typically be at a very high frequency and thus will have negligible effect on fH . It also follows that at the frequencies of interest the gain from the base to the collector of Q1 will be gm1re2 1. Use this to find the capacitance at the input of Q1 and hence show that the pole introduced at the input node will have a frequency fP1 1 2R sig C1 +2C1 Then show that the pole introduced at the output node will have a frequency fP2 1 2RL CL +Ccs2 +C2 (b) Evaluate fP1 and fP2, and use the sum-of-the-squares formula to estimate fH for the amplifier with I = 1 mA, C = 10 pF, C = 2 pF, Ccs = CL = 0, = 100, RL = 2 k, and rx = 0 in the following two cases: (i) Rsig = 1 k (ii) Rsig = 10 k