55:041 Electronic Circuits The University of Iowa Fall Exam 1 Solution

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Collin Dalton
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Exam 1 Name: Score /60 Question 1 Short takes. For True/False questions, write T, or F in the right-hand column as appropriate. For other questions, provide answers in the space provided. 1. Tue of false: For both the amplifiers below, the voltage gain increases when the drain/collector bias current is increased.

1 Exam 1 Name: Score /60 Question 1 Short takes. For True/False questions, write T, or F in the right-hand column as appropriate. For other questions, provide answers in the space provided. 1. Tue of false: For both the amplifiers below, the voltage gain increases when the drain/collector bias current is increased. (1 point) Answer: True 2. The small-signal output resistance (r o ) of a BJT biased at I C = 1 ma is 100 k. What is r o when the transistor is biased at I C = 10 ma? (2 points) Answer:. Thus, at 10 ma is 3. A MOSFET is biased such that and. If changes with 1 mv, by how much does the drain current change? (2 points) Answer: 1

2 4. Assuming for the amplifiers below. Which is more sensitive to FET parameter variation? (circle one) (1 point) (a) Amplifier 1 (b) Amplifier 2 Answer: (a) 5. Which of the following depicts the correct current direction? Circle one. (1 point) Answer: (a) 6. In the current mirrors below, neglect base currents and, What is? (2 points) Answer: 2

3 7. Write down the load-line equation for the MOSFET in the circuit below. (2 points) Answer: 8. True or false: the of a transistor is a function of temperature, but essentially independent of collector current. (1 point) Answer: False 9. True or false: the voltage gain of the following amplifier is about -5. (1 point) Answer: False 10. The abbreviation/term ESR is often encountered in data sheets of capacitors. What does ESR stand for? (1 point) Answer: Equivalent Series Resistance 3

4 11. What is in the following circuit if = 1.2 V,, and? ( 3 points) Answer: The current through is 1.2/120 = 10 ma, which also flows through. Thus, the output voltage is = 8 V 12. True of false: in the circuit below, is essentially independent of. (1 point) Answer: True 13. True or false: the turn-on voltage for red LEDs is higher than the turn-on voltage of blue LEDs. (1 point) Answer: False 4

5 Question 2 Make a sketch of a representative current-voltage characteristics of a silicon diode. Assume the diode is biased at. Indicate the bias point on the plot. Show that the small-signal or incremental resistance is (6 points) The equation that relates the diode current to the diode voltage is The incremental conductance is Apply the chain rule: At a bias current, the value is The incremental resistance is the inverse of the conductance: 5

6 Question 3 The parameters for the transistor below are = 0.2 ma/v 2 and Determine. Assume the transistor is operating in the saturation region. (5 points) The transistor is operating in the saturation region. Then The correct solution is, since the other solution has, which implies the FET is off. 6

Question 4 C c is a coupling capacitor (a) Write down an expression for. Assume Si transistors. (2 points) (b) Draw a small-signal model for the amplifier above.

7 Question 4 C c is a coupling capacitor (a) Write down an expression for. Assume Si transistors. (2 points) (b) Draw a small-signal model for the amplifier above. Show the output resistances of the transistors. (6 points) (c) Derive an expression for the voltage gain A v = v o /v I. Account for the output resistances of the transistors and (8 points) Part (a) Part (b) R 1 and Q 1 is a current source that sets up a current that is mirror through Q 2. We can replace R 1, Q 1, and Q 2 with a current source with output resistance r o2. Further C C is a coupling capacitor and a short at ac. Thus a small-signal model is then: Note: points are subtracted if you leave out a ground terminal, V O, the signs for V π, r o, etc. Part (c) 7

8 Question 5 Below is the small-signal model for a BJT CC amplifier. Determine. (8 points) Turn off uncontrolled sources, and add a test source and determine the current (see below). The output resistance is then. KCL at E using the convention that currents into the node are positive: Also, using Ohm s law:, so that Thus 8

9 Question 6 For the BJT amplifier below, determine. For the Si transistor,, and assume the transistor is operating in the forward active mode. (6 points) Replace the base bias network with its Thevenin equivalent network as shown below. For a Si transistor,. Now However, so that Solving yields. From this follows 9

Homework Assignment 06

Homework Assignment 06 Question 1 (Short Takes) One point each unless otherwise indicated. 1. Consider the current mirror below, and neglect base currents. What is? Answer: 2. In the current mirrors below,