Erica Synths EDU DIY Dual VCA Eurorack Module Kit User Manual
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the negative feedback created by the emitter resistor, which resists the current going
through it.
Since it’s easier to force the right transistor to close down (or open up) than to in- or
decrease the total bias current, the former will happen.
This means that the total bias
current will stay roughly constant, while the two individual collector currents fight
each other for real estate within it
.
Now, the trick here is that if we in- or decrease the total bias current by changing the op
amp’s output voltage, we also change the transistors’ gain levels. This is simply because
the more current is already flowing through a transistor, the more sensitive it becomes to
changes in base-emitter voltage. So as the total bias current increases, the two collector
currents will oscillate more heavily. Causing the collector voltages to oscillate more
heavily as well. This way, we can get as much gain as we need, since we are not relying
on the steepness of the base-voltage-to-collector-current-curve anymore.
Also, if the collector resistors are matched, the collector voltage oscillations should be the
same – just inverted. Additionally,
their DC o
ff
set should be identical, as both
collector currents swing across the exact same mid-point
.
If you connect your oscilloscope to both collectors (on separate channels – or one after
the other) and play with the potentiometer, you should be able to verify our
assumptions. Both signals are moving downwards as their volume increases
significantly. Now if you zoom in a bit, you should see that they indeed are inverted copies
of each other. Okay – but you might say: so what? How is having two signals with their
DC o
ff
sets tied to the gain better than one?
The 20k resistors in this kit have a 0.1% value tolerance – meaning they’re already matched out
of the box.
Again, you can watch me do this if you don’
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