55:041 Electronic Circuits The University of Iowa Fall Exam 3. Question 1 Unless stated otherwise, each question below is 1 point.

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1 Exam 3 Name: Score /65 Question 1 Unless stated otherwise, each question below is 1 point. 1. An engineer designs a class-ab amplifier to deliver 2 W (sinusoidal) signal power to an resistive load. Ignoring saturation in the output BJTs, what is the required peak-to-peak voltage swing across the load? (2 points) Answer:, so that, so that 2. Consider the following circuit, which is the power output stage of an amplifier. (a) What is the name of the shaded sub-circuit around? (b) Write down one sentence/phrase that describes the purpose of the sub-circuit and constant current source. (2 points) Answers: (a) multiplier (b) Reduction of cross-over distortion 3. An amplifier has gain of 800. After adding negative feedback, the gain is measured as 25. Find the loop gain. (2 points) Answer. so that. Solving for yields the loop gain. 1

2 4. An amplifier has gain of 100,000, and a 20% variation in gain over a certain frequency range. Negative feedback is used to reduce the gain to 10. What is the variation in gain with temperature of the feedback amplifier? (2 points) Answer. The gain is reduced by. The temperature variations are reduced by the same factor, so the feedback amplifier s gain varies by 5. An single-pole op-amp has an open-loop low-frequency gain of and an open loop, 3-dB frequency of 4 Hz. If an inverting amplifier with closed-loop lowfrequency gain of uses this op-amp, determine the closed-loop bandwidth. (2 points). The bandwidth of the closed- Answer. The gain-bandwidth product is loop amplifier is then is. 6. An op-amp has an open-loop gain of 120 db and an input resistance of. An engineer wants to use negative feedback to obtain an amplifier with input resistance of 5. What is the gain (in db) of the feedback amplifier? (2 points) Answer. Negative feedback increases the resistance by (or 40 db) and reduces the gain the same factor, so the feedback amplifier s gain is 80 db. 2

3 Question 2 The multimeter in the figures below has a common-mode rejection specification of 80 db. What possible range of output voltages can the meter indicate? (5 points) Solution The difference voltage is, and the common-mode voltage is A common-mode rejection specification of 80 db means that the multimeter will suppress by a 80 db or by The contribution of the common-mode error is thus. However, the sign is unknown. Thus, the multimeter could display anything in the range 3

4 Question 3 In the circuit below, the FETs have, and Determine the low frequency differential gain, namely. Then draw and fully label (i.e., slopes, intercepts, etc.) the Bode plot of the gain. (15 points) Solution Below is a symmetrical small-signal equivalent for the amplifier. With we will have the same differential gain voltages at nodes K and Z do not change, and we can treat them as virtual grounds. The The resulting half amplifier circuit is then The gain from node a to x, or and the half-amplifier gain is 4

5 is twice the gain of this amplifier or. The 6 pf capacitor between the drain and gate will experience the Miller multiplication. The Miller gain is the gain from node a to x, or. Now The Bode plot for the overall amplifier is 5

6 Question 4 For the circuit above, make reasonable assumptions and then (a) Show that (4 points) (b) Calculate a reasonable estimate for the output resistance (4 points) Solution The three transistors are in a Darlington configuration and we can model it as a pnp transistor with, and most of I Q flows through Q3. Thus,. Part (a) For the composite transistor. Part (b) Use BJT impedance scaling: 6

7 Question 5 The maximum transistor power is in the circuit above is. (a) Determine R L such that maximum power is delivered to the load. (4 points) (b) For, determine average power dissipated in the transistor. (6 points) Do not calculate and neglect the base current when calculating power. Solution Part (a) The transistor will dissipate the maximum power (25 W) when. From this follows that and. Part (b) The gain of the amplifier is, so that the amplitude of the signal output voltage is. The signal power dissipated in the resistor is The average power dissipated in the transistor is 7

8 Question 6 For the class-a amplifier shown, show that the maximum efficiency for a sinusoidal input signal 25%. Clearly state assumptions you make. For example, ignore saturation (6 points) Solution. Assumptions: (a) neglect bias currents, (b) saturation, and (c) for maximum power delivered to the load, the transistor will be biased so that. is the given load, so that and the average supply power is For a sinusoidal input voltage, the voltage across, and the current through the load is The signal power is the rms power dissipated by the load, given by The power conversion efficiency is then Note: many students wrote down a number of loosely connected equations or something such as. This was not sufficient. Rather, students had to show/derive the result in a logical, step-by-step manner, stating assumptions, etc. 8

9 Question 7 Consider the bipolar difference amplifier below. A dc analysis shows that, and. Determine the differential-mode voltage gain. (6 points) You may assume that the output stage ( stage. does not load the differential input Solution Transistor,, form a common-emitter stage with gain. Further, The gain of the differential stage is The overall differential-mode gain is 9

10 Question 8 The figure below is a plot of the open-loop gain function for the LF156 voltage amplifier. An engineer will use the amplifier in a negative feedback configuration to set the midfrequency voltage gain at 100. Use the plot and estimate the bandwidth of the feedback amplifier. Solution. A gain of 100 is equivalent to a gain of. A horizontal line at 40 db intercepts the LF156 gain curve at 100 khz (see above). Thus, the bandwidth ~ 100 khz. 10