An amplifier stage is shown in Fig. 7.41 where bias current I_Bis adjusted so that V_O= 0 V dc. Take V_SUPPLY= 10 V.

(a) Calculate the low-frequency, small-signal trans-resistance Ï…_o/i_i and use the zero-value time-constant method to estimate the ˆ’3-dB frequency. Data: npn: β = 100, f_T = 500 MHz at I_C = 1 mA, C_μ0 = 0.7 pF, C_je = 3 pF (at the bias point), C_cs0 = 2 pF, r_b = 0, and V_A = 120 V. Assume n = 0.5 and ψ₀ = 0.55 V for all junctions. pnp: β = 50, f_T = 4 MHz at I_C = ˆ’0.5 mA, C_μ0 = 1.0 pF, C_je = 3 pF (at the bias point), C_bs0 = 2 pF, r_b = 0, and |V_A| = 50 V. Assume n = 0.5 and ψ₀ = 0.55 V for all junctions.

(b) Repeat (a) if a 20-pF capacitor is connected from collector to base of Q₁.

Fig. 7.41:

VSUPPLY )'s + i; 'a + 30 k2 Q2 Q3 -VSUPPLY