Charge Pump from CMOS Inverter gates? Yes!

About 12 years ago (yikes!), I came across a rather interesting concept: it’s possible to use CMOS inverters to step up input voltage much the same way as dedicated charge pump ICs work. I loved this idea because it would be cheaper (much cheaper) to play with higher supply voltages for pedal circuits while keeping with the standard single-supply +9V input voltages that have ruled the pedal world from the beginning.

So I did some digging and found an old article from Electronics Today International (ETI) originally published in 1983 that laid out several ways to use logic inverters to increase input voltage. Click here for a PDF of the article. I highly recommend you download and read the article, as it has many example circuits and some excellent data on how/why it works.

I tried my hand and setting this up for myself with a 3-stage inverter charge pump using an CD40106 (schmitt trigger inverter). It worked right away, and I was pleased to get around +30v from an input voltage of only +9v. However, that was only when the circuit wasn’t under any kind of load (that is, hooked up to an actual guitar pedal circuit). Under a load, it dropped down to around +16v to +18v, but I considered this pretty good. Granted, this was 12 years ago, and I can’t find my notes, but there was a significant drop in output voltage when put under load. So expect that to happen if you put one together for yourself.

I’ve included my schematic, as it definitely works as drawn. The weak spot of a charge pump configured this way (with inverters) is that it can’t output that much current. So to fix this, you can parallel each inverter with another inverter (to double the output current). This is covered nicely in the ETI article, so definitely have a look at it.

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