This problem investigates the use of MOSFETs in the design of wideband amplifiers

Chapter 10, Problem 10.105

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This problem investigates the use of MOSFETs in the design of wideband amplifiers (Steininger, 1990). Such amplifiers can be realized by cascading low-gain stages. (a) Show that for the case Cgd Cgs and the gain of the common-source amplifier is low so that the Miller effect is negligible, the MOSFET can be modeled by the approximate equivalent circuit shown in Fig. P10.105(a), where T is the unity-gain frequency of the MOSFET. (b) Figure P10.105(b) shows an amplifier stage suitable for the realization of low gain and wide bandwidth. Transistors Q1 and Q2 have the same channel length L but different widths W1 and W2. They are biased at the same VGS and have the same fT . Use the MOSFET equivalent circuit of Fig. P10.105(a) to model this amplifier stage, assuming that its output is connected to the input of an identical stage. Show that the voltage gain Vo/Vi is given by Vo Vi = G0 1+ s T / G0 +1 where G0 = gm1 gm2 = W1 W2 (a) (b) Figure P10.105 (c) For L = 0.5 m, W2 = 25 m, fT = 12 GHz, and nCox = 200 A/V2 , design the circuit to obtain a gain of 3 V/V per stage. Bias the MOSFETs at VOV = 0.3 V. Specify the required values of W1 and I. What is the 3-dB frequency achieved?