Final Exam Dec. 10, 8:00-10:00am Name: (70 points total) Problem 1: [Small Signal Concepts] Consider the circuit shown in Fig. 1. The voltage-controlled current source is nonlinear, with the relationship i OUT = 1 + v 2 IN 0.2v 3 IN. [10 points] (a) What is the large signal component of the output voltage (V OUT )? [3] (b) Using a small signal approximation, find the small signal gain ( iout v in ) of the voltage-controlled current source. [5] (c) Using the small signal gain calculated in (b), find the small signal component of the output voltage (v out ). [2] Figure 1: Nonlinear voltage-controlled current source circuit. 1
Problem 1 (cont d) 2
Problem 2: [Circuit Analysis] Consider the amplifier circuit shown in Fig. 2. For the transistor, V tn = 1 V, µ n = 5 10 2 m 2 /Vs, C ox = 2 10 3 F/m 2, W L = 10. You may neglect channel length modulation effects. [10 points] (a) What is the large signal component of the output voltage (V OUT )? [3] (b) Suppose we would like to increase the value of R D while keeping M 1 in the saturation region. What is the maximum allowable value of R D? [3] (c) What is the small signal gain of this circuit (with the original value of R D )? [4] Figure 2: Single transistor amplifier circuit. 3
Problem 2 (cont d) 4
Problem 3: [Feedback Concepts] Consider the feedback configuration depicted in Fig. 3. [8 points] (a) What is the closed-loop gain of this feedback configuration? [5] (b) Provide an approximate expression for the closed-loop gain when β 1 = 0.5, β 2 = 1, and A 1 = A 2 1? [3] Figure 3: Signal flow diagram. 5
Problem 3 (cont d) 6
Problem 4: [Digital Logic] Consider the circuit depicted in Fig. 4 that is being used as an inverter. For the transistor, V tn = 1 V, µ n = 5 10 2 m 2 /Vs, C ox = 2 10 3 F/m 2, and W = 10. You may ignore channel length modulation L effects. [10 points] (a) What are the high and low logic levels at the output (V OH and V OL ), assuming the input is being driven by a standard CMOS inverter? [6] (b) How should the value of R L be modified to improve the noise margins of this inverter, and why? [2] (c) How should the value of R L be modified to reduce the low-to-high propagation delay of this inverter, and why? [2] Figure 4: Inverter circuit. 7
Problem 4 (cont d) 8
Problem 5: [Power Amplifiers] Consider the output stage shown in Fig. 5. The BJT has β = and V BE = 0.7 V when it is on. [10 points] (a) Choose R L so that 0.32 W of power is dissipated in the load under the given input conditions. [3] (b) What class of operation is the output stage in under these conditions? [2] (c) Plot v OUT and i E in the space provided in Fig. 6. [4] (d) What is the efficiency of the output stage under these conditions? [3] Figure 5: BJT output stage. 9
Problem 5 (cont d) Figure 6: Power amplifier output voltage and emitter current. 10
Problem 6: [Voltage Rectifiers] Consider the voltage rectifier shown in Fig. 7. The diode is ideal, turning on for V P N = 0.7 V, and the power supplies are ±5 V. [10 points] (a) Plot v OUT for this circuit in the space provided in Fig. 8. [6] (b) What is the output current of the opamp when v IN = 1 V? [2] (c) What is the output current of the opamp when v IN = 1 V? [2] Figure 7: Voltage rectifier circuit. 11
Problem 6 (cont d) Figure 8: Rectifier output voltage. 12
Problem 7: [Oscillators] Consider the oscillator shown in Fig. 9(a). The voltage transfer characteristics of the bistable multivibrator are shown in Fig. 9(b), and it has infinite input impedance. [8 points] (a) Plot v OUT for this circuit in the space provided in Fig. 10, assuming that v OUT = -5 V at t = 0 s. Remember that for a capacitor, dv = I. [6] dt C (b) How will increasing the size of C INT affect the oscillation frequency, and why? [2] (a) (b) Figure 9: (a) Oscillator circuit, (b) Bistable multivibrator characteristics. 13
Problem 7 (cont d) Figure 10: Oscillator output voltage. 14
Problem 8: [Martial Arts Legends] How does Chuck Norris obtain butter? [2 points] (a) He buys it at the store. (b) He makes it using a butter churn. (c) He roundhouse kicks a cow and the butter comes straight out. Figure 11: Outer space exists because it s afraid to be on the same planet with this guy! 15