Erica Synths EDU DIY Dual VCA Eurorack Module Kit - Manual (Page 16)

we’ve reduced it to just 5 mV peak-to-peak. But at the same time something weird
happened to our amplified signal. Instead of 5 V we’re now only getting about 100 mV
peak-to-peak there. Now of course, we’ve e
ff ectively scaled down our input to just 25%
of what we fed into the base before. But 100 mV is not 25% of 5 V. Something else
must’ve changed. And that something is our amplifier’s gain. To understand why, we just
need to have another look at our base-emitter-voltage graph.
Before we added in the 10k emitter resistor, our signal was swinging between the two
points up the curve. But now with the resistor in place, the active range is a much smaller
slice –
that’s also further down the curve. And because it is further down the curve, it is
also noticeably flatter. Which is why our output signal got so much more quiet. By
moving the input oscillation down the curve, we reduced the amplifier’s gain. We’ll
get to the larger implications here in a second, but first, I want to talk about the added
benefits I mentioned earlier. Because at first sight, this seems to be a rather bad deal: we
needlessly cut down our amplifier’s ability to amplify. Right? Yes and no. While we did
lose a significant amount of gain, we also got something important in return: stability and
reliability. That’s because transistors have two severe flaws.
First, if you take two transistors – same type, same label, same manufacturer – and
compare their base-voltage-to-collector-current-curves, you’ll probably notice that they
don’t match up at all. That’s because of small variations in the silicon they’re made of.
And second, those unequal curves are also heavily dependent on the transistor’s
temperature. Heat it up by just a few degrees and your amplifier’s gain shoots up
noticeably. Cool it down, and the output gets more quiet. Which is why all the „actual“
examples and numbers we’ve talked about so far are just rough approximations – your
mileage trying this with a real transistor will vary quite a lot. That is, as long as you don’t
use an emitter resistor!
Why’s that? Simple: because of the negative feedback it introduces. Let’s assume we
build the initial circuit twice, with two di
ff erent transistors. One of them naturally has a lot
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