Resistors vs. bipolar junction transistors – Erica Synths EDU DIY Dual VCA Eurorack Module Kit User Manual
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RESISTORS VS. BIPOLAR JUNCTION
TRANSISTORS
In my DIY VCO series,
I claimed that you can use NPN
transistors as voltage controlled resistors
were true, then something like this should work, right?
We could simply replace one of the two resistors in our
voltage divider with a transistor. Then, by applying a
voltage to the base, we should be able to control the
amount of resistance between collector and emitter.
Giving us a voltage controlled voltage divider in total.
Unfortunately, if you build and try this, you’ll be severely
disappointed. And that’s because what I said in that
VCO video is at least misleading – if not flat out wrong.
A bipolar junction transistor is not a voltage
controlled resistor
. There is no „resistance“ (in that
sense) between its collector and emitter.
To understand this better, let’s talk about what resistors and transistors have in common –
and what separates them. On the common side, both components allow us to restrict the
amount of current flowing in a circuit. This is why superficially, they seem kind of
interchangeable. Consider this example. We’ll set up two separate, simple circuits.
One is just a resistor between a voltage source and ground. The other is a transistor, with
its collector connected to an identical voltage source, its base connected to another
much weaker voltage source, and its emitter connected to ground. Now if we tune the
base voltage carefully, we can get the exact same amount of current flowing through our
Read more on NPN transistors in the components & concepts appendix (page 42).
You can try this chapter’s circuits in a simulator. I’ve already set them up for you right here:
you can change all values by double clicking on components.
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