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

TRANSISTOR SATURATION
Let’s look at our circuit’s behavior using a quick graph. We’ll imagine we steadily increase
the base voltage (V
B
) from -12 V all the way up to +12 V.
In the region between -12 V and around 450 mV, everything works as expected: the
collector voltage (V
C
) drops in sync with the base voltage. But after the base- and
collector voltages meet, something strange happens: the collector voltage starts rising
again. What’s up with that?
Well, we’ve run into a phenomenon called transistor
saturation
. This happens when we try to pull more
current into the collector than is available. Think of it like
this: If we’d remove the transistor from our circuit, we’d
see 600
μ A flow through both resistors. And we can’t
increase that current without bumping the voltage or
reducing the resistance.
So as we push the base voltage above 450 mV, the
transistor tries to blow past that 600
μ A current limit.
But since it can’t get more current out of the top
resistor, it’ll switch to an alternate strategy.
And that strategy is pulling large amounts of current
into the base.
Which will not only cover the missing
amount, but also increasingly substitute the current
flowing into the collector. As a result, less current is
pulled through the top resistor, and the voltage below it
starts to rise. More precisely, for every volt added at the
base, the collector voltage will rise by the same amount.
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