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

Which is precisely why the initial idea of a transistor-
based voltage controlled voltage divider does not work.
Since the current going through our transistor is fixed (if
the base voltage does not change and the collector
voltage stays positive), the total current flowing will be
fixed as well.
So in- or decreasing the collector
voltage will only change the potential, not the actual
flow amount
.
This means that the ratio between potential and actual
flow is not fixed anymore –
and that’s why this setup
simply does not work as a voltage divider. Here’s how
this pans out in detail. Let’s assume we’ve set the base
voltage so that our collector-emitter-current is exactly
300 uA. Then the total current going through transistor
and resistor is also 300 uA. Since the 20k resistor could
potentially allow for 600 uA to flow – if we bypassed the
transistor – the ratio between potential and actual flow
is exactly 1/2. So we’d measure 6 V between resistor
and transistor.
Now if we double the voltage up top to 24 V, the current
flowing will stay the same: 300 uA. But since the resistor
could now potentially allow for a flow of 1.2 mA, our
ratio shifts from 1/2 to 1/4, meaning we get 3/4 the
voltage at the transistor’s collector: 18 V.
As we can
see, the increased voltage simply gets added to the
previous value – making this setup unusable for our
purposes
.
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