The ac equivalent circuit for a Colpitts oscillator is given in Fig. 1. (a) What is the frequency of ocillation if gm = 10 ms, Bo = 100, RE = 1 kn, L= 5 H, C1 = 20 pF, C2 = 100 pF, C4 = 0.01 F, and C3 = infinity? Assume that the capacitances of the transistor can be neglected (see Prob. 1). (b) A variable capacitor C3 is added to the circuit and has a range of 5-50 pF. What range of frequencies of oscillation can be achieved? (c) What is the minimum transconductance needed to ensure oscillation in part (a)? What is the minimum collector current required in the transistor? Problem 1 The ac equivalent circuit for a Colpitts oscillator is given in Fig. 1. (a) What is the frequency of oscillation if L = 20 H, C1 = 20 pF, C2 = 100 pF, C3 = infinity, C4 = 0.01 F, fT = 500 MHz, rm = = 00, VA = 50 V, rx = 0, RE = 1 kN, Cu = 3 pF, and the transistor is operating at a Q-point of (5 mA, 5 V)? (b) What is the frequency of oscillation if the Q-point current is doubled? Figure 1 C4 L. RE C3