The BiCMOS amplifier of Fig. 3.78 is to be used as a low-noise transimpedance amplifier. The input is fed from a current source with a shunt source capacitance of C_S= 1 pF. Assuming that C_Sand C_gs1= 0.5 pF dominate the frequency response, calculate the equivalent input noise current spectral density of the circuit at low frequencies and at the ˆ’3-dB frequency of the transfer function. Use data as in Problem 3.17. Use SPICE to check your result, and also to investigate the effect of adding r_b=200 Ω to the bipolar model.

Data from Prob. 3.17:

A BiCMOS amplifier is shown in Fig. 3.78. Calculate the small-signal voltage gain Ï…_o/Ï…_i. Assume I_S = 10^ˆ’16 A, β_F = 100, r_b = 0, VA †’ ˆž, μ_nC_ox = 200 µA/V², V_t = 0.6 V, and λ = 0. Check your answer with SPICE and then use SPICE to investigate the effects of velocity saturation by including source degeneration in the MOS transistors as shown in Fig. 1.41 using ε_c = 1.5 × 10⁶ V/m.

Fig. 3.78:

Transcribed Image Text:

10 k2 R12 10 k2 Оз 1 kΩ R11 Q1 Q2 M1 300 1 k2 R13 Vi M2 20 30 k2