Eliminating transistor mismatch – Erica Synths EDU DIY Dual VCA Eurorack Module Kit User Manual

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ELIMINATING TRANSISTOR
MISMATCH

Here’s how it works. Instead of connecting our second transistor’s base directly to
ground, we’ll give it a very small, constant, non-zero voltage via a precision trimmer set
up as a variable voltage divider.

By doing that, it’s as if we’re changing the

transistor’s natural gain level – a positive voltage simulates more gain, while a
negative one simulates less

. And it’s this natural gain level that would ideally be

identical between our two transistors. Because then, their collector voltage swings would
have the exact same DC o

ﬀ

set – and subtracting them from each other would give us our

precious properly centered output signal.

You can try this chapter’s circuit in a simulator. I’ve already set it up for you right here: https://

tinyurl.com/y7kh5c72 –

you can change all values by double clicking on components.