Phys127AL, S22, Quiz 3, Form A
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Phys127AL, S22, Quiz 3, Form A
Name: Solutions
1). (5 pts) Suppose you have two input signals; one is a 10 kHz square wave with an amplitude of 1 V and the other is a 20 kHz sine wave with amplitude of 0.5 V. Draw an op-amp based circuit that will amplify the sine wave by a factor of 2 and add the result to the inverse of the square wave. Remove any DC offsets.
This requires a non-inverting amplifier, with gain = 2, to double the sine wave input and an inverting amplifier, with gain = -1, to invert the square wave. Then we will use a summing amplifier to add them. Finally, we will invert the output of the summing amplifier since it has a negative gain.
To remove any DC offsets, we will use decoupling capacitors on both inputs and DC bias them to ground. I will do that with a follower before the inverted signal just to make things sim- ple. However, you could forgo the follower as long as the inverter’s resistances are sufficiently large. The power supplies for the op-amps need to be both positive and negative to allow full swing without clipping. I’ll use ±15 V supplies.
The circuit below will do the job, where V1 is the input sine wave, and V2 is the input square wave.
Grading guidelines:
+1 for each preamp
+3 for summing
-1 if missing decoupling or no DC biasing
-1 if simple error
-1 if missing negative sign at output
2). (5 pts) For this problem, define Out1 as the signal that is output from the circuit in question 1. Draw a circuit that produces an output signal that is +1 V whenever Out1 is greater than 1.5 V and produces an output signal of -1 V whenever Out1 is less than 1.5 V.
We can do this with a comparator with a threshold set at 1.5 V. However, a comparator will output +VCC or _VEE when the signal is above or below 1.5 V, respectively. We can easily divide that voltage down to ±1.0 V with a voltage divider, but the output impedance of a voltage divider would be too large. We can fix that with a follower.
Another design option would be to use an inverting amplifier, with gain less than one, to reduce the ±15 V of VCC and VEE with appropriate output impedance. That will flip the sign, but we can correct for that by flipping the inputs on the comparator.
These two options are shown below.
Note that you can’t just run the op-amp with power supplies of ±1 V since there is a mini- mum power supply requirement. E.g., it is ±10 V for the 741 that you’ve used in lab.
Grading guidelines:
+2 for threshold
+3 for scaling to desired output
-2 if use low voltage power supplies
-2 if poor output impedance
-1 if simple error
-1 if missing negative sign at output
2023-06-15