ECE 3110: Engineering Electronics II Fall 2007

Final Exam

Dec. 10, 8:00-10:00am

Name:

(70 points total)

Problem 1: [Small Signal Concepts] Consider the circuit shown in Fig. 1. Thevoltage-controlled current source is nonlinear, with the relationship iOUT = 1 +v2

IN − 0.2v3IN . [10 points]

(a) What is the large signal component of the output voltage (VOUT )? [3]

(b) Using a small signal approximation, find the small signal gain (∆iout

∆vin) of

the voltage-controlled current source. [5]

(c) Using the small signal gain calculated in (b), find the small signal compo-nent of the output voltage (vout). [2]

Figure 1: Nonlinear voltage-controlled current source circuit.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 1 (cont’d)

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ECE 3110: Engineering Electronics II Fall 2007

Problem 2: [Circuit Analysis] Consider the amplifier circuit shown in Fig. 2. Forthe transistor, Vtn = 1 V, µn = 5× 10−2 m2/Vs, Cox = 2× 10−3 F/m2, W

L= 10.

You may neglect channel length modulation effects. [10 points]

(a) What is the large signal component of the output voltage (VOUT )? [3]

(b) Suppose we would like to increase the value of RD while keeping M1 in thesaturation region. What is the maximum allowable value of RD? [3]

(c) What is the small signal gain of this circuit (with the original value of RD)?[4]

Figure 2: Single transistor amplifier circuit.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 2 (cont’d)

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ECE 3110: Engineering Electronics II Fall 2007

Problem 3: [Feedback Concepts] Consider the feedback configuration depictedin 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 β1 = 0.5,β2 = 1, and A1 = A2 � 1? [3]

Figure 3: Signal flow diagram.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 3 (cont’d)

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ECE 3110: Engineering Electronics II Fall 2007

Problem 4: [Digital Logic] Consider the circuit depicted in Fig. 4 that is beingused as an inverter. For the transistor, Vtn = 1 V, µn = 5 × 10−2 m2/Vs,Cox = 2× 10−3 F/m2, and W

L= 10. You may ignore channel length modulation

effects. [10 points]

(a) What are the high and low logic levels at the output (VOH and VOL),assuming the input is being driven by a standard CMOS inverter? [6]

(b) How should the value of RL be modified to improve the noise margins ofthis inverter, and why? [2]

(c) How should the value of RL be modified to reduce the low-to-high propa-gation delay of this inverter, and why? [2]

Figure 4: Inverter circuit.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 4 (cont’d)

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ECE 3110: Engineering Electronics II Fall 2007

Problem 5: [Power Amplifiers] Consider the output stage shown in Fig. 5. TheBJT has β =∞ and VBE = 0.7 V when it is on. [10 points]

(a) Choose RL so that 0.32 W of power is dissipated in the load under thegiven input conditions. [3]

(b) What class of operation is the output stage in under these conditions? [2]

(c) Plot vOUT and iE 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.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 5 (cont’d)

Figure 6: Power amplifier output voltage and emitter current.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 6: [Voltage Rectifiers] Consider the voltage rectifier shown in Fig. 7.The diode is ideal, turning on for VPN = 0.7 V, and the power supplies are ±5V. [10 points]

(a) Plot vOUT for this circuit in the space provided in Fig. 8. [6]

(b) What is the output current of the opamp when vIN = 1 V? [2]

(c) What is the output current of the opamp when vIN = −1 V? [2]

Figure 7: Voltage rectifier circuit.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 6 (cont’d)

Figure 8: Rectifier output voltage.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 7: [Oscillators] Consider the oscillator shown in Fig. 9(a). The voltagetransfer characteristics of the bistable multivibrator are shown in Fig. 9(b), andit has infinite input impedance. [8 points]

(a) Plot vOUT for this circuit in the space provided in Fig. 10, assuming thatvOUT = -5 V at t = 0 s. Remember that for a capacitor, dV

dt= I

C. [6]

(b) How will increasing the size of CINT affect the oscillation frequency, andwhy? [2]

(a) (b)

Figure 9: (a) Oscillator circuit, (b) Bistable multivibrator characteristics.

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ECE 3110: Engineering Electronics II Fall 2007

Problem 7 (cont’d)

Figure 10: Oscillator output voltage.

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ECE 3110: Engineering Electronics II Fall 2007

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!

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