The emitter resistor – Erica Synths EDU DIY Dual VCA Eurorack Module Kit User Manual
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THE EMITTER RESISTOR
Okay, so we’ll just use a voltage divider to scale our input down even further, right?
Actually, there’s another way to achieve the same result, but with some added benefits:
adding a resistor between the transistor’s emitter and ground
.
How does this work? Well, this is where it gets a bit tricky.
It works by introducing
negative feedback
. As we know, our circuit operates by having a varying amount of
current flowing through the transistor and to ground. Now if there’s no resistance below
the transistor, we have a direct, uncomplicated relation between the base voltage we
apply and our collector current. But by adding in the resistor, we are restricting the current
flow between emitter and ground. And this means that there’s again some unused
potential
here –
which manifests in a voltage at the emitter. How much? That depends on
two factors: the amount of resistance – and the amount of current. Because the
resistance is fixed, we don’t have to worry about it too much. We’ll only have to ask: what
happens when the base voltage –
and thereby the collector current – changes? Simple: if
they increase, then the voltage at the emitter will increase as well. And if they decrease, it
will be mirrored there, too.
Let’s play this out with some actual numbers. We assume the emitter resistor to be a 10k.
So our base voltage is still swinging between 560 mV and 580 mV. But if we probe the
voltage at the transistor’s emitter, we’ll see that it is also oscillating between 87 mV
and
102 mV. Now since for our transistor’s operation, only the voltage di
ff
erence between
emitter and base voltage is what matters, we need to subtract the former from the latter.
Leaving us with an e
ff
ective base voltage swing between 473 mV and 478 mV. And voila:
You can try this chapter’s circuit in a simulator. I’ve already set it up for you right her
you can change all values by double clicking on components.
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