# Designing the Difference Amplifier

This circuit creates a control signal from the two optodetectors. It finds the difference between the two input voltages and amplifies it to get the ball's position. This stage is often called a comparator.

## Difference Amplifier

The op-amp has a 100K feedback resistor Rf in combination with an 11K input resistor Ri.

This is a standard inverting amplifier with a gain of:
Gain = Rf / Ri = 100K / 11K = 9.

So the output voltage on pin 6 is nine times the difference between the two input signals. Why choose a gain of nine? Because my measurements found the maximum difference between the two signals was a little under two volts, and we want to increase that signal up to about the power supply range of 15 vdc. A larger signal is better; it keeps the rest of the circuit in a comfortable operating region. If the signal were very tiny then it will be harder to work with. A gain of nine is what it takes to increase my "almost two volts" up to about fifteen volts.

If you're following along in your own laboratory, how do you check to see if this circuit is operating properly? Look at my testing pages here.

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