(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Suppose you are given an ideal op amp, an ideal voltage source with voltage Vs, 5 resistors with resistance R, and one variable resistor whose resistance can be adjusts continuously between .2R and R. Use these components to design a circuit, assuming ideal op-amp behavior, that provides an output voltage Vo given by Vo = GVo, where G can be adjusted continuously from -.5 to +.5 by adjusting the resistance of the variable resistor over its full range of values. Note that the voltage source can be connected to multiple points in the circuit

2. Relevant equations

V- = V+

I- = I+ = 0

3. The attempt at a solution

Based on the output voltage GVo I assume the op amp would take the model of an inverting amp, with resistors Rs and Rf. Therefore it would theoretically follow the node equation

(Vn - Vs) / (xR) + (Vn - Vo) / (aR) = 0, where xR would be a value of resistor I could create from the extra resistors I have. From here I simplify to (1/(aR) + 1/(xR)) Vn + (-1/(xR))Vs = Vo /(aR). I'm very confused though, because it seems impossible to use a pot/var resistor for negative output if it does not go to 0. I don't understand how using all positive resistors one would be able to invert output when you go across its range.

Because resistance is inversely related to voltage, across the range .2 to 1 I would get coefficients of 1 and 5 in the equation but I don't know how to shift this range 1-5 to -.5 to .5 (shrinking the range is easy but subtracting a constant value, constricted to the materials given? )

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Constructing an op amp circuit of variable gain neg to pos

**Physics Forums | Science Articles, Homework Help, Discussion**