Erica Synths EDU DIY Dual VCA Eurorack Module Kit - Manual (Page 13)

We’ll simplify things by removing the resistor between voltage source and collector for
now. Here’s how this graph works, then: On the x-axis, we track the voltage applied to the
base, while the y-axis shows us the measured collector-emitter-current. As you can
probably tell,
the resulting plot looks suspiciously exponential . And that’s because it
is . In the area between 0 mV and 600 mV base voltage, we only see a super slight
increase in collector current. But then, suddenly, it starts going through the roof. And at
around 775 mV, we already see a full amp of current flowing through our transistor. Or
rather, we would see – if the transistor weren’t already toast by then.
That’s because most general purpose transistors can only handle a few hundred mA of
collector-emitter-current. And so the actual usable range is somewhere between 0 mV
and 700 mV base voltage. In our amplifier, we’re using just a tiny sliver of this, since the
scaled-down & o
ff set signal swings between 560 mV and 580 mV.
Here’s what that sliver looks like when we zoom in on it. Now, since we are using a
triangle oscillation as our to-be-amplified signal, we are basically moving our base voltage
back and forth along the curve at a constant speed. So e
ff ectively, we are replicating the
oscillation in the amount of collector current going through our transistor. And as we can
see, for a rather small change in voltage we get a pretty significant change in current.
Knowing this, it should should start to make sense why our setup above works as an
amplifier.
Because if the current going through our transistor changes, the voltage at
its collector will also change . That’s because we are converting more – or less – of
the resistor’s potential- into actual current flow .
Let’s break this down in detail. So we know that our 20k resistor could potentially pass
600 uA in our setup. And on the graph above, we saw that our transistor’s collector
current is oscillating between around 250 uA and 550 uA. So the ratio between actual and
potential current flow is also oscillating. At 250 uA collector current, we are using around
42% of the total potential. And at 550 uA, that number rises to about 91%. Which means
that the collector voltage will swing between approximately 7 V and 1 V,
giving us a 6 V
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