Answer Happy

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. Q4, 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.
Then show that the pole introduced at the output node will have
a frequency
b. Evaluate fP1 and fP2, and use the sum-of-the-squares
formula to estimate fH for the amplifier with I = 1 mA, Cπ = 10 pF,
Cμ =2 pF, Ccs = CL = 0, β = 100, RL = 2 kΩ, and rx = 0 in the
following two cases:
i. Rsig = 1 kΩ
ii. Rsig = 10 kΩ
JP₁ 1 2лR (C₂1 +2C₂₁)

JP₂2 21 1 2π R₁ (C₁+C+C₂2)

Vsig M V₂1 :C₁₂ C₁2 CHI 2₂ C₁ 2₁ Cal Figure Q4 C₂ Cesl -。V₂ C₁+Ccs2