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Phys127AL, F19, Final exam

I will not answer any questions during the exam,

other than clarifying the wording for all students.

You may approximate any calculations to within 10%.

Don’t overlook the questions on the back.

1). (10 points) Suppose that this circuit has an input signal

that is given by Vin (t) = 0.1 sin ωt, where ω corresponds

to 100 kHz. Determine time dependent expressions for the

output voltages, Vout1 (t) and Vout2 (t). (Hint: ﬁnd the DC

quiescent values ﬁrst.)

2). (10 points) Determine the input impedance of the circuit in question 1. What could you change to make the circuit have an input impedance that is at least twice as high?

3). (15 points) Design a simple circuit that will turn on a light when you ﬂip a switch on, but when you subsequently ﬂip the switch off, the light will stay on for 10 seconds before turning off. Don’t use a 555 timer for this. For simplicity, use an LED for the light.

4). (15 points) Describe what each of these circuits does. When component values are given, be quantitative about the resulting circuit behavior.

More questions are on the back...

5). (20 points) Electric guitars can use various distortion circuits that modify the sound in interesting ways. One such distortion is to clip the top and bottom of the waveform. You could do that by using an ampliﬁer with such high gain that the output reaches the power supply rails, but then the music would be very loud and the amount of clipping would not be well controlled. Instead of doing that, design a circuit that allows the amount of clipping at the top to be adjustable with a variable resistor, and the amount of clipping at the bottom can be separately adjustable with a different variable resistor. Your circuit should also be able to adjust the volume of the output separately from the amount of clipping. Use +10 and -10 V power supplies.

6). (15 points) The temperature of the circuit components in a computer vary with the rate of calculations that the computer does. For example, you may notice your laptop fan start to spin loudly after you have been running an intense program. Design a circuit that can sense the temperature and vary the speed of a fan as the temperature changes. The speed of the fan can be increased by increasing the voltage that your circuit applies to the fan. You can use thermistors to sense the temperature. A com- mon thermistor approach is to use a ceramic material that has a low resistance, about 10Ω , when it is at or below room temperature, but the resistance increases linearly for temperatures above 20 〇 C, with a slope of about 100Ω per degree above 20 〇 C.