Erica Synths EDU DIY Mixer Eurorack Module Kit - Manual (Page 12)

We call this a feedback loop. That feedback loop will ensure that the voltage at the
inverting input is always very close to 0 V. Because if it isn’t, the output voltage will adjust
until it is – the feedback loop forces the whole setup into a stable state.
Here’s how it does that. Whenever the voltage at the inverting input is significantly above
0 V, the result of the subtraction will be negative. Amplify this by the huge gain, and the
output is crashing down to the negative supply voltage. At the same time, if the voltage at
the inverting input is significantly below 0 V, the result of the subtraction will be positive –
so the output jumps up to the positive supply voltage.
This means that whenever the voltage at the inverting input tries to go above 0 V, the
output pushes it back down. And if it tries to go below 0 V, the output pulls it back up.
They’re locked in a stalemate, basically.
Now, because of this, we say that the inverting
input is a virtual ground node – simply because it behaves just like a connection to
ground (by always sitting at 0 V), without actually being one
.
Now, why is this important and/or helpful for our
problem? Well, let’s look at the rest of our example
again. We now know the voltages for these three points:
5 V at input one, 0 V at input two, and 0 V at the
inverting input. Given these three values, we can deduct
what the op amp’s output voltage will have to be.
In order to get the voltage at the inverting input down to
0 V, the output has to neutralize the current flowing
through our two input resistors. Since one signal is
sitting at 0 V though, there’s nothing coming in on that
path anyways.
So we only have to worry about the other one. Here, the relation is really simple: our
output has to pull exactly as much current through the feedback resistor as our
input is pushing through the input resistor
. Since both resistors are of the same value,
both the pushing and pulling forces have to be exactly equal – just inverted. This means
that our output has to sit at -5 V to neutralize the 5 V at input one!
Now, if we ignore the minus sign for a second and check our two input voltages again, we
see that we got exactly what we were looking for: a 5 V output for a combination of a 0 V
and a 5 V input!
So it looks like as if our bu ff er is indeed simply adding the two input
voltages together (while also inverting them, of course) .
To verify this, let’s assume that input two jumps up to 5
V. Then to keep the voltage at the inverting input at 0 V,
the op amp’s output has to also neutralize the additional
current being pushed through the second resistor. Since
the added pushing force is exactly the same as the
other, facing the same resistance, it will simply double
the amount of current flowing. Forcing our op amp’s
output to pull twice as hard.
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