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Mosfets and Zeners

More Diode Clipping for Effects

This is a typical connection of mosfets used as clippers for a guitar distortion pedal. You might think that with the gate tied to the source, the gate threashold is the clipping level of the setup but this is not the case. The reverse body diode that is inherent in this type of mosfet has a threshold of around 0.7v. With one of these in each of the mosfets, the body diodes will prevent the gates from ever becoming forward biased. You will only hear clipping from the body diodes and the signal limits will be about +/-0.7v.

The body diode junctions are slower switching and have a high input capacitance. This results in a softer sound than some people like. Any small signal mosfet like the BS170 or 2N7000 can be used for this method of signal limiters.

We can rearrange the mosfets as shown here to get a totally different response. In this configuration, a positive signal on the gate of the upper mosfet (Q1) will turn it on, then the signal passes through the body diode of the lower one. For negative signals it will take the opposite path through the upper body diode and then through the channel of Q2.

In this method, we have brought the gate junctions of the mosfets into play without adding any extra blocking diodes, as is often seen with mosfet clippers. Since the body diodes are used for blocking, you gain additional softness by using them in conjunction with the mosfet gate-bias sound.

An alternate method is to use on a single mosfet as shown to the left. With this clipper, the positive voltages are clipped when the gate signal turns on the mosfet and the negative voltages are limited by the body diode. This version gives asymmetrical clipping that tends to emphasis the even harmonics.

If the gate-source threshold is 2v and the body diode is 0.7v then the signal will be able to swing only +2v and -0.7v. Try this setup for a different and unique sound.

The same technique can be applied if you have p-channel mosfets such as the BS250. This is the equivalent of the second example in this article above.
The clipper shown here is typical of the method used when zener diodes are employed to establish higher clipping thresholds. The voltage limits will be the zener voltage (Vz) of the diode plus a single 0.7v diode drop. The zener rated voltage is conduction from cathode to anode while the normal diode action is from anode to cathode... a zener diode has both of these limits, and when placed in series with another zener as shown here, the Vz of one works with the diode drop of the other.

If we are using 3v zeners in this setup, the clipping will be 3v + 0.7v (3.7v) for each side of the signal... i.e. clipping is symmetrical.

Of course you can switch positions of the diode clippers and get yet another type of sound. With this version the normal diode thresholds are limiting the signal and the zener voltages do not come into effect. This means that the clipping will be +/-0.7v as with a pair of 1N914 or similar silicon diodes.

What do zeners sound like when used as normal diodes? Give it a try and find out!

Let's take it a step further and remove one of the zeners. Now you have a threshold of the Vz rating of the zener for positive swings and 0.7v for the negative excursions. If the zener is rated for 3v then the signal voltage will be clipped at +3v and -0.7v.
Why not combine the zener and mosfet ideas and make a hybrid clipper? Here we have the zener being used for two functions. The forward conduction of the zener limits the positive signals to the mosfet gate threshold and the Vz is added to the body diode threshold. If the zener is rated as 3v and the gate threshold of the mosfet is 2v, then the signal will be limited to +3.7v and -2.7v with a mixture of harmonics generated by the conduction characteristics of the diode materials.
Arrange the pair in parallel and you now have +/-0.7v limits caused by the body diode of the mosfet and the normal diode conduction of the zener. The higher voltages of the zener threshold and the mosfet gate conduction never come into action because the silicon junctions will begin to conduct before the voltages get high enough to forward bias the higher threshold junction materials.
There are plenty of ideas from this article to work with, and lots of new sounds left to be discovered, especially when other types of clipper diodes are brought into play. Here is but one more example of how the palette of tones can be extended by some creative use of the parts that we have available.

If you use any of these clipping ideas, remember to give credit and a link to this site. Thanks!

If you are interested in more information about audio signal limiting and diode clipping methods, be sure to check out the articles in the Lab Notebook section of this site about Tone Clippers and Warp Controls!



Zener Diodes

Interestingly, there are at least two types of voltage reference diodes, which depend on different breakdown mechanisms for their operation. Below about 6v, the voltage reference diodes depend on the "zener effect" while above 6v the "avalanche effect" predominates.

For zener diodes the temperature coefficient is negative and avalanche diodes have a positive temperature coefficient. The low voltage zeners typically are less noisy and fortunately more useful in our effects circuits.

Some zener diodes and their breakdown voltages:

1N5221     2.4v
1N5222     2.5v
1N5223     2.7v
1N5224     2.8v
1N5225     3.0v
1N5226     3.3v
1N5227     3.6v
1N5228     3.9v
1N5229     4.3v
1N5230     4.7v


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