Assignment #3
1-9. Design a non-inverting amplifier with a voltage gain of approximately 12 with an input
resistance of 12 kΩ . Use SPICE to confirm the result using a simple model of the OpAmp.
Solution: The non-inverting amplifier has an input resistance of infinity. Therfore, one
alternative is to place a 12k resistor from the positive input terminal of the OpAmp to ground
- this yields an input resistance of 12k || (Large OpAmp Ri) ≈ 12k.
To yield a gain of 12, choose resistor ratio in the branched from the negative terminal of the
OpAmp to have a ratio of 11:1 since the non-inverting gain is: where Rf is the
feedback resistor connecting the output to the negative OpAmp input terminal and RG is the
resistor connecting the negative OpAmp input terminal to ground. Using standard value
resistors, I chose RG = 2k and Rf = 22k.
An alternate scheme could be to include a series resistance connected to vs. In order to meet
the specified input resistance the sum of that series resistance and a resistor from the positive
OpAmp input terminal must equal to Rin. Since this resistor network at the source is a voltage
divider, the values of Rf and Rg must be altered to meet the voltage gain of 12. I chose the
following: Rs1 = 6.04k, Rs2 = 6.04k, Rg = 1k, and Rf = 23.2k. This yields a gain of
approximately 12.
1-12. Design an operational amplifier circuit to meet the following specifications:
• vo = 3.0 vi2 - 5.0 vi1
• Ri1 = 15 kΩ (the input resistance seen by source vi1)
• Ri2 = 25 kΩ (the input resistance seen by source vi2)
Solution:
The basic configuration of the difference amp is used:
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