# Op amp circuit design

I am having trouble with understanding how to design circuits involving op amps.

I need to design a circuit using as less op amps as possible which has the following characteristics:

• high input impedance (greater than 100k)
• low output impedance (smaller than 100)
• can be adjusted with a single 47k potentiometer over the range 0 to -10
• gain should vary linearly with potentiometer rotation

I really don't understand how to complete this...

• We don't just do your homework for you here. Show that you've done some work, and explain exactly what you are stuck on. – Olin Lathrop Jan 14 '15 at 14:33

## 2 Answers

If we can design a bit of a "silly" circuit, then the following would kinda work. (for one opamp) (Gain is not quite linear.) You've got to pick R4 to give a gain a bit more than 20... but I'm too lazy to work out the exact number. simulate this circuit – Schematic created using CircuitLab

Well, if you feed your signal into the pot you get a linear output level from 0 to 1 coming out - that sounds like a start. If you then buffer the signal before the pot you get >100k input impedance and, if you make that op-amp an inverting amplifier with Rin = 110k and Rf at 1.1Mohm you get inverting gain of 10. All that remains is to add a buffer op-amp (unity gain non-inverting) to the pot output and this should work. I'm struggling to see how you would achieve it without two op-amps unless the signal levels were small and you could use a 200 ohm potentiometer feeding the output directly. simulate this circuit – Schematic created using CircuitLab

• It's better to replace R2 by the pot R1 (47 k) connected as a rheostat; then R3 should be 4,7 k... but, unfortunately, the input resistance will decrease:( and a follower will be necessary. – Circuit fantasist Jan 14 '15 at 12:47
• @Circuitfantasist - or use a 2Mohm pot – Andy aka Jan 14 '15 at 12:59
• Another suggestion - connect the right end of R2 to the slider of the potentiometer R1, and use the op-amp output as a circuit output (i.e., implement the negative feedback network as a T-bridge). Thus R3 can be high enough while R2 and R1 - low enough. – Circuit fantasist Jan 14 '15 at 15:33