Repeat Problem 7.21 if the input signal is a current source of value i_iapplied at the base of Q₁. (That is, i_ireplaces the voltage source Ï…_iand resistance R_S.) The transfer function is then a trans-resistance Ï…_o/i_i.

Data from Prob. 7.21:

A Darlington stage and a common-collector€“common-emitter cascade are shown schematically in Fig. 7.40, where R_S = 100 kΩ and R_L = 3 kΩ.

(a) Calculate the low-frequency small-signal voltage gain Ï…_o/Ï…_i for each circuit.

(b) Use the zero-value time-constant method to calculate the ˆ’3-dB frequency of the gain of each circuit. Data: β = 100, f_T = 500 MHz at I_C = 1 mA, C_μ = 0.4 pF,C_je = 2 pF, C_cs = 1 pF, r_b = 0, r_o = ˆž, I_C1 = 10 µA, and I_C2 = 1mA. (Values of C_μ, C_cs, and C_je are at the bias point.)

Fig. 7.40:

Rg RL Vo V Q2 (a) Rs R1 Q2 Vo (b)