Non-Inverting Amplifier

The inverting amp is a useful circuit, allowing us to scale a signal to any voltage range we wish by adjusting the gain accordingly. However, there are two drawbacks to it. First, the signal gets inverted, which can be slightly annoying -- although we can always invert it back with another op-amp.

But the real drawback to the inverting amplifier is the amplifier's input impedance, which is equal to R1.

As we saw with voltage dividers, we need to take a circuit's impedance into account when using it as part of a larger system of circuits. We need each successive circuit stage to have an input impedance at least 10 times greater than the output of the one preceeding it, to prevent loading.

Since the inverting amplifier's input impedance is equal to R1, there may be times we'd be forced to pick unusually large resistors for our feedback loop, which can cause other problems.

The solution to our impedance worries lie in the Non-Inverting Amplifier, also made with an op-amp and negative feedback:

Here, the signal in goes directly into the non-inverting input, which has a nearly infinite input impedance -- perfect for coupling with any previous stage. Also, the output impedance of the op-amp is nearly zero, which is ideal for connecting with whatever comes next in the circuit.

The formula for a non-inverting amplifier's gain is slightly different than the one for the inverting amp. For a non-inverting amp, the gain is:

Gain = 1 + (R2/R1)

Note that while the inverting amp can have a gain less than one for handy signal scaling, the non-inverting amp must have a gain of at least one.

Naturally, we must still power the op-amp with V+ and V- giving enough range to comfortably accomodate our expected signal output.


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