Erica Synths EDU DIY Mixer Eurorack Module Kit - Manual (Page 11)

INVERTING MIXER
How do we achieve that? Simple. Instead of creating the average of our inputs, we need
to add them together – so that oscillation plus silence equals the unaltered oscillation.
To
get there, we have to take a slightly confusing detour, though: turning our regular
bu
ff er into an inverting bu ff er . To do that, we set up our op amp like this.
The jumper between output and inverting input gets replaced with a 100k resistor, while
we tie the non-inverting input to ground. Then, we connect our two input resistors to the
inverting input. What does this do – and why does it help? Well, first of all, it causes our
output signal to be inverted. So the output will be flipped on its head.
In isolation, this seems like a pretty random and useless change. But doing this comes
with a sort of hidden benefit – which becomes apparent once we understand how the
circuit works as a whole. So let’s analyze this setup.
To simplify things, we’ll again assume that our two input
voltages are static, fixed at 5 and 0 V respectively.
Now,
since we’ve tied the op amp’s non-inverting input to
ground (0 V), the output voltage can only stabilize if
the voltage at the inverting input is nearly 0 V as
well
. This is because that output voltage depends on
the two input voltages.
It works like this. An op amp is a di ff erential amplifier – which means that it amplifies the
di ff erence in voltage between its two input terminals. To do that, it subtracts the voltage
measured at its inverting input from the voltage at its non-inverting input. The result gets
multiplied by the op amp’s gain – which is a very large number.
Then, the op amp’s output tries to set its voltage to the result of that calculation. If it’s
outside of the supply voltage range, it will settle for the last possible value. This simple
relation gets complicated by the fact that we connect the output to the inverting input
through a resistor. Because then,
if the output voltage changes, the input voltage will
be a ff ected as well – which in turn a ff ects the output voltage, which again a ff ects
the input voltage, and so on .
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