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So I was reading the chapter on Op Amps in Microelectronic circuits by Sedra, Smith, the topic on Differential Amplifiers to be exact.

Here's the amplifier: https://i.stack.imgur.com/3qWzi.png

To get this to behave as a differential amplifier, it had been proved that R1/R2 = R3/ R4, and the using this condition, the final gain turns our to be R2/R1.

That seemed simple enough, but I didn't get the following statement:

Note that if the amplifier is required to have a large differential gain, then R1 of necessity will be relatively small and the input resistance will be correspondingly low, a drawback of this circuit. Another drawback of the circuit is that it is not easy to vary the differential gain of the amplifier. Both of these drawbacks are overcome in the instrumentation amplifier discussed next.

Here, what exactly do they mean by "it is not easy to vary the differential gain of the amplifier"? It's just R2/R1, so that should be easy enough to manipulate, right?

Thanks in advance.

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  • \$\begingroup\$ Regarding the input resistance of this 1-opamp differential amplidier, it is not the same for both circuit inputs. It is equal to R1 for the inverting input and it is R3 + R4 for the non-inverting input. You can make both input resistances high and equal... and also to adust the gain only by one resistor with a 3-opamp circuit of a more sophisticated differential amplifier known as "instrumentation amplifier". \$\endgroup\$ – Circuit fantasist Aug 14 at 11:00
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what exactly do they mean by "it is not easy to vary the differential gain of the amplifier"

They mean that if you want to change the gain on-the-fly then you have to adjust R1 and R3 simultaneously and that's a lot of bother. The InAmp only has one gain set resistor and is easily changed and doesn't screw with common-mode rejection either.

enter image description here

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