Erica Synths EDU DIY Dual VCA Eurorack Module Kit - Manual (Page 10)

resistor and our transistor – about 600 uA. This might lead you to believe that for this
base voltage, the path between collector and emitter acts just like a 20k resistor .
If that were true, then it would need to exhibit the same two properties that resistors do:
linearity and symmetry. Let’s check for linearity first. Doubling the voltage applied to the
resistor will give us twice the amount of current: 1.2 mA. But for the transistor, something
odd happens: nothing.
We’ll still see the same 600 uA flowing through this circuit. Why is that? Because the
only thing determining the amount of current that can flow between collector and
emitter is the amount of current flowing into the base
. And the latter only depends on
the voltage at the base – not the voltage at the collector. Now while this would already
disqualify the transistor, let’s also check for the second property: symmetry. What
happens when the collector voltage goes negative?
Interestingly, our transistor would theoretically still work roughly in the same way – we’ll
see a current flow between emitter and collector. But it won’t be the same current, just
inverted. Instead, it will be a ridiculously strong current that’ll wreck our transistor. This is
simply because the huge negative voltage between base and collector pulls a much too
large amount of current into the base, disqualifying this use case without further
alterations to the circuit.
So it’s o ffi cial: transistors are not voltage controlled
resistors!
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