Question

Problem 2. The figure on the right shows a scheme for coupling and amplifying a high- frequency pulse signal. The circuit utilizes two MOSFETS whose bias details are not shown and a 50-02 coaxial cable. Transistor Q1 operates as a CS amplifier, and Q2 as a CG amplifier. For proper operation, transistor Q2 is required to present a 50-Ω resistance to the cable. This situation is known as "proper termination" of the cable and ensures that there will be no signal reflection coming back from the cable. When the cable is properly terminated, its input resistance is 50-2. a) Draw the equivalent small signal circuit by replacing b) Given that the input resistance to Q2 is equal to 50 2. c) IfQ1 is biased at the same point as Q2, what is the d) what is the amplitude of the voltage pulses at the e) What value of Rp is required to provide IV pulses at QI with its π model, and Q2 by its T-model. what must gn be? amplitude of the current pulses in the drain of Q1? 500 ipcoaxial cable the drain of Q2? 5 mV

Answer 1

Since A_2 is CG amplifier, for CG amplifier R_in = 1/g_m R_i2 = 1/g_m2 = 50 Ohm = > g_m2 = 1/50 = 20 mA/v If Q_1 is biased the same as Q_2, then g_m1 = g_m2 i_D1 = g_m1 v_p = 20 ma/v times 5 mv = 100 mu a = > i_D1 = 0.1 mA v_D1 = i_D1 times 50 Ohm = > v_D1 = 0.5 v In order to have v_d2 = v_0 = 1v = > v_0 = i_D R_D = > 1 = 0.1 times 10^-3 R_D = > R_D = 10 k Ohm