Op-Amps – Do Two Op-Amps Connected to the Same Input Affect Each Other?


I've been given the following circuit to build for an education lab. The values of the resistors aren't accurate; they're only to illustrate that the gains of the two op-amps are different.

The aim is to have two separate op-amps of different gains connected to the same input signals, and then a SPDT switch connects the required output to \$V_{out}\$. The two op-amps will be in the same chip (LM353).

Since the output of AMP1 is fed back, and in some sense is connected to Node2, will the first op-amp effect the gain of the second op-amp and vice-versa? Or will the two op-amps behave as if they are separate and just happen to have the same input signals?


simulate this circuit – Schematic created using CircuitLab

Thank you for the replies. The input will be provided from a MPX2100 thin film pressure sensor which the datasheet indicates should have an output impedance of 1400-3000\$\Omega\$. Is this too large?

If so, perhaps it would be smarter to switch the inputs, or use a buffer op-amp like an instrumentation amplifier? The problem is the PCBs for the circuit were made long ago, and I'm now trying to reverse engineer the circuit from effectively no documentation. So making changes is difficult.

Best Answer

To ensure the two circuits have the correct intended gain, a strong voltage source should feed node 1 and node 2. In other words the output impedance of the voltage source has to be very low or the amplifiers will not have the gain as expected.

Because of this, it's reasonable to say that any loading (due to adding the number of amplifiers in parallel) will have negligible effects.

If the voltage source is not considerably low in its impedance then neither op-amp circuit will work with the gain they are expected to deliver.

Regarding the effect of a feedback resistor, are you aware of what a "virtual" earth means and the spin-off it has on differential amplifiers?