"True" MOSFET clipping at 9V? Green Turtle overdrive and other applications

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"True" MOSFET clipping at 9V? Green Turtle overdrive and other applications

Feb 23, 2023 6 min read

Updated: Mar 10, 2023

MOSFETs, the incredible devices at the core of modern devices, computers, power supplies and… distortion pedals? For some reason, there’s this constant fascination with them: regardless of them being used as diodes, amplifiers, inside op-amps or inverters or even in output stages, the presence of MOSFETs somewhere confers the circuit a powerful charm and the inevitable association with the seemingly unrivaled sound of vacuum tubes.

That said, very few circuits have attempted to use active MOSFETs as clipping devices.

More often than not, the gate is shorted to the source so that the channel can never turn on and only conducts when reverse biased through the body diode, which is just a diode and won’t receive further attention.

One exception seems to be the popular Zendrive, which instead connects the gate to the drain, so that the channel can eventually turn on; at least in those schematics that don’t mistakenly redraw it with the gate again connected to the source. In this configuration, VGS is equal to VDS and the FET will ride on the “knee” of the characteristic curves, before the linear region transitions in the saturation region.

One issue with that is the high voltage required to turn on the MOSFET, which doesn’t pair well with the limited swing of most op-amps at 9v (about 3V peak), if you also add the voltage drop from the series Schottky diodes. The op-amp will run out of headroom before the MOSFET has a chance to work.

Or will it? We might be used to think about MOSFETs turning “on” with about 4-5V of gate voltage when used as switches, the current involved in feedback clipping is tiny (input voltage/grounded feedback resistor), and voltages just above Vth are already enough to fully conduct. This means a G-to-D MOSFET will clip at about 1.6-1.8V by itself, but some series diode must always be added to prevent reverse bias (unless that’s what you’re going for).

These considerations are what brought me to my new applications of active MOSFET limiting. I haven’t found a similar use elsewhere, other than it being a simple application of basic theory.

Biasing the gate

Ignoring the subthreshold region, one can approximate the MOSFET as non-conducting below Vth. This is the same principle behind the G-to-D diode, but what if one wants to lower the clipping threshold? It should be possible to bias the gate a bit positive and shift the whole curve down nearer to 0V, and it turns out that’s the case. Even better, I’ve checked the resulting curve to be the same, except for the shifted voltage (they were perfectly overlapping). Here’s the most representative overview: a log-linear current plot of a 4148 (green), a G-to-D 2N7000 (red), and a 2N7000 with additional 0.9V positive gate bias (blue), up to 10mA.

And a more familiar-looking linear plot, again up to 10mA.

Hard and soft

But wait! Where does the 10mA limi