Some Useful Operational Amplifier Circuits
Unity Gain Buffer Amplifier
Summing Amplifier
And, don't forget the original op-amp circuit
The Inverting Amplifier
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        Operational amplifiers are useful devices that can be used for numerous types of operations on signals.  In this lesson you will examine a number of operational amplifier circuits that are useful in many situations.  To predict the behavior of remember these facts about how operational amplifiers perform when they operate out of saturation.
As we encounter different circuits we will use these assumptions to predict circuit behavior.

The Unity Gain Buffer Amplifier

        If the difference between V+ and V- is negligibly small so that V+ = V- we must have:

        This circuit is often used when a voltage source with a high internal impedance is used, and you want to draw more current than the source can deliver.  The solution is to use this circuit to make a copy of the original voltage, Vin, and that copy, Vout, appears at the output of an operational amplifier that can often deliver more current without lowering the output voltage because the internal resistance of the operational amplifier is lower than the internal resistance of the original source.

The Summing Amplifier

          For this circuit, the analysis is very similar to that of the inverting amplifier.  If you want to review that analysis, click here.  We start with the same two assumptions.

        Here's what happens in sequence. The result is that the output is a weighted sum of the two inputs.  If we just wanted to add the two inputs we could choose all of gthe resistors to be equal.  Of course, we get a minus sign, but that's unavoidable.

Q1  If you want to get rid of the minus sign, do you have any options?

        Where would you use a circuit like this?

We're sure that you can - and will - find many other uses for the summing amplifier.

The Integrator

        The integrator does just what the name implies.  It integrates - in the calculus sense - the input signal to produce the output signal.  There is a scaling factor and a minus sign again, but that's pretty much what happens.

Here's the analysis.  We make the usual assumptions:

Then, we have - after we write KCL:
A Circuit with Infinite Input Resistance/Impedance and a Gain Larger Than 1.0

        This circuit is particularly useful when you need to avoid loading the signal circuit, and you also want a gain larger than 1, and you don't want to have to build one of those pesky inverter circuits.

In this circuit, the two resistors form a voltage divider.

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