Erica Synths EDU DIY Dual VCA Eurorack Module Kit User Manual
Page 22
After you’ve built this, connect an oscillator to the input socket, and your oscilloscope to
the transistor’s collector – which is the amplifier’s output. Then try adjusting the
potentiometer to change the circuit’s gain.
Unfortunately, the results won’t be
particularly useful
. There are two major problems. For one, the output signal’s DC o
ff
set
seems to be tied to the set CV and is moving around wildly. And also, the gain added by
increasing the CV is so minimal it’s actually near impossible to see the oscillation without
zooming in massively. Bummer –
so what’s going on here? Well, two things.
First, let’s talk about the output signal’s DC o
ff
set. Why does it move around so much?
Actually, we already kind of know the answer. What we’re doing by changing the emitter
voltage is moving the operating region further up or down the base-voltage-to-collector-
current-curve. But by doing this, we’re not only changing the amplifier’s gain. We are also
changing the average amount of collector current that’ll be flowing while the amplifier is
operating.
And the more current is flowing on average, the more of our 20k resistor’s
potential is used up on average
.
Meaning that the average voltage at the transistor’s
collector is changing as well
. So e
ff
ectively, the amplifier’s gain is directly tied to the
output signal’s DC o
ff
set. To try and compensate for that, we could of course turn on the
AC coupling
on our oscilloscope. This will remove any DC o
ff
set from the VCA’s output.
Since our oscillation is now semi-fixed to the 0 V line, you can turn up the scaling so you
can actually see what’s going on.
It’ll be a bit hard to tell through the noise floor, but changing the CV really does have an
e
ff
ect on the gain. Unfortunately, the range is definitely way too small. This shouldn’t be
surprising, though. Because if we look at the base-voltage-to-collector-current-curve
again, we can see that the steepness (and therefore the gain level) only changes slightly in
the area between about 400 mV and 450 mV base voltage.
Beyond that, the emitter
resistor is preventing us from increasing the gain at all, since it bends the curve into
a straight line
.
On top of that, the AC coupling is not a perfect solution for removing the DC o
ff
set.
Because as you’re changing the gain, you can see that the signal goes o
ff
-center for a
little while. That’s just how AC coupling works. Because of these two problems, our
simple circuit just doesn’t cut it. Fortunately, we can salvage it by adding in a few extra
components.
If you don’t have an oscilloscope, you can watch me demonstrate this her
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