Consider the BiCMOS amplifier shown in Fig. The BJT has /VBE / = 0.7 V, β = 200, Cμ = 0.8 pF, and fT = 600 MHz. The NMOS transistor has Vt = 1 V, k1nW/L =2 mA/V2, and Cgs = Cgd = 1 pF.
(a) Consider the dc bias circuit. Neglect the base current ofQ2 in determining the current in Q1. Find the dc bias currents in Q1 and Q2, and show that they are approximately 100 μA and 1 mA, respectively.
(b) Evaluate the small-signal parameters ofQ1 and Q2 at their bias points.
(c) Consider the circuit at midband frequencies. First, determine the small-signal voltage gain Vo /Vi . (Note that RG can be neglected in this process.) Then use Miller's theorem on RG to determine the amplifier input resistance Rin . Finally, determine the overall voltage gain Vo /Vsig. Assume ro of both transistors to be very large.
(d) Consider the circuit at low frequencies. Determine the frequency of the poles due to C1 and C2, and hence estimate the lower 3-dB frequency, fL.
(e) Consider the circuit at higher frequencies. Use Miller's theorem to replace RG with a resistance at the input. (The one at the output will be too large to matter.) Use open-circuit time constants to estimate fH.
