How to replace diodes with MOSFETs?

Question

The circuit shown below allows clips the output to +1 and -1. How can I replace the diodes with MOSFETs that have a similar behavior?

Answer 1

Mosfets have a body diode between drain and source. To use that diode without interference from the channel, you need to keep them off, i.e. set \$V_{\rm GS}=0\$. That's done by shorting the gate and the source. We also need to keep the limit voltages low enough so that the reverse breakdown due to \$V_{\rm GS.MIN}\$ doesn't occur.

I don't see what the point is of using more complex parts in place of the diodes, if all they do is exactly the same, but here you go:

^{simulate this circuit – Schematic created using CircuitLab}

What are the downsides of using the body diode as opposed to real diodes?

They are real diodes. As for their other parameters, such as conductance: you'd have to measure and compare, and first and foremost - decide what's important to you, what sort of performance do you want.

I was under the impression that to get a NMOS to behave like a diode, one needs to connect the gate and DRAIN. Would you mind explaining why such a configuration wouldn't work.

The body diode in a MOSFET transistor is an actual PN junction, and behaves like other PN junctions do: the junction capacitance is modulated by the reverse voltage, the forward voltage is logarithmic in forward current, and so on.

If you connect the gate and drain, you won't have a diode. You'll have a MOSFET that happens to turn on with a couple volts across it one way, and a diode drop the other way - the body diode is still there!

Sure, with low threshold parts, especially ones tweaked to have threshold voltage around 0V, you may get behavior close to a unidirectional ideal diode by using the \$V_{DG}=0\$ connection, as long as you don't expose it to reverse voltages larger than 0.4V or so. But such parts are usually expensive, unless you take off-the-shelf parts and inject enough permanent charge into the gate to move their threshold down to 0V. That's doable, but you'd need to qualify such parts as to longevity etc. It's best to just buy them if you really need them.

The closest you can get to "diode" action with shorted gate and drain would be as below, and even then you still need a reverse isolation diode in series with the channel, so that the body diode doesn't activate. So: an additional diode just to make a mosfet play the role of a diode... doesn't make much sense.

Also note the larger bias voltages, since they have to compensate for the gate-source threshold voltage. The conductance is usually limited by the external diode, except for low-conductance channels typical of CD4007 and older low-signal devices like 2N7000, where the channel itself is limiting. Newer devices have channel conductances that handily beat external diodes.

^{simulate this circuit}

Diode-based limiters don't have enough gain to do stiff limiting. We need differential pairs for that - even if they are very rudimentary and use only resistor loads as opposed to current sources and current mirrors. In practice, this limiter needs an offset adjustment, but it should otherwise work OK and remain fairly accurate as long as the ambient temperature doesn't change much.

^{simulate this circuit}

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Tying the drain to the gate makes the NMOS behave like a resistor having a voltage drop across it equal to V_TH.

This finds uses in IC design where you will purposefully design a MOS device with a channel width and length selected to get the resistance you need, and thus obtain a resistor. This "diode connection" is not a rectifier!, and it doesn't have the logarithmic characteristic of a PN junction either. It is nonlinear (quadratic) and thus usable for setting up current sources/mirrors and such, since those leverage a non-linear current vs. conduction voltage response of the active device.

Answer 2

Here's one with using NMOS:

^{simulate this circuit – Schematic created using CircuitLab}

The diodes shown are body diodes. If you short gate and source of an NMOS, it'll turn into a diode. Remember that the forward voltage may differ from MOSFET to MOSFET. So you may need to adjust the other voltage sources.

It's possible to have the same operation with using PMOS. The connection is the same but the body diodes are reversed.