Top/bottom wavefolder – Erica Synths EDU DIY Wavefolder Eurorack Module Kit User Manual
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TOP/BOTTOM WAVEFOLDER
Now the question is: how do we combine the NPN- and PNP folding circuits into one?
Lucky for us, it’s actually pretty straightforward.
All we have to do is wire them together
like shown here: base connects to base and collector connects to collector
.
If we then send in our triangle wave on the left, we can pick up a top- and bottom-folded
output on the right. It works like this: at 0 V base voltage, without a connection between
the collectors, the NPN’s collector would sit at +700 mV, while the PNP’s collector would
sit at -700 mV.
Wire them together, and we get 0 V – since the two voltages cancel
each other out
.
Now, if we increase the base voltage, the NPN transistor will open up, while the PNP
closes down, leaving us with a current deficit at the collectors. This forces us to pull more
current from the positive rail through the 20k resistor, which, in turn, will cause the voltage
at the collectors to drop. At least until we run into the NPN’s folding threshold. Because
then, that NPN goes into saturation and the collector voltage starts climbing up again.
And the same idea applies in reverse if we decrease the base voltage from the 0 V line.
The NPN will close down, while the PNP opens up – so we pull less current through the
20k resistor and the voltage rises. Until we hit the PNP’s folding threshold, it goes into
saturation and the collector voltage starts dropping again.
This way, we get two folding
thresholds – one slightly above and one slightly below the 0 V-line
You can try this chapter’s circuit in a simulator. I’ve already set it up for you right here:
you can change all values by double clicking on components.
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