Erica Synths EDU DIY Dual VCA Eurorack Module Kit - Manual (Page 18)

Two of them show us the base-voltage-to-collector-current-curves for our example
transistors without negative feedback. The other two are what we’d measure with 10k
emitter resistors in place. For better comparison, I used a little trick here: instead of
measuring the actual voltage between base and emitter, I measured the voltage between
base and ground. This way we can integrate the negative feedback directly into the gain
curve.
And as you can see, the rather significant gain disparity between the two
transistors is pretty much eliminated by the negative feedback .
Which is why they’ll perform in a very similar way in our example circuit. But that’s not the
only benefit. As I said before, a transistor’s temperature also has an e
ff ect on its gain.
Normally, that e ff ect is quite strong – but the negative feedback again really mitigates it
here. Because as the temperature rises, the gain attempts to increase, which in turn will
raise the emitter voltage and that dampens the gain in response. Okay, but one final
question remains: why did I pick a 10k resistor and not a bigger one? If the negative
feedback is so beneficial, why not use a ton of it – to negate all di
ff erences between
transistors and eliminate any temperature dependence? Simple: because we’d be
completely crushing the gain –
making our setup basically unusable as an amplifier. So in
the end, choosing a value for the emitter resistor is always a balancing act . How
much gain are we willing to give up for reliability and stability?
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