How are transistors shorted to be used as clipping diodes?

[fawkes89]

How do you uses transistors as clipping diodes? I am a newbie at pedal boards and thought that this idea was very interesting. To my understanding, this is what they do on the Fulltone OCD drive. I looked up how to use transistors in this way and did not seem to find much. So i turn to you all!  Also it says "instead of connecting the diodes to ground potential, Fulltone uses a floating bias point which is connected to the input of the opamp stage. This creates a clipping threshold proportional to the input signal amplitude, resulting in a strong dependency of the amount of distortion created to the input signal. This is the core idea of the enormous dynamics of the OCD." Have any of you played with this idea at all??? So am I right in thinking this means that unlike the tube screamer, which runs a resistor to ground attached to the soft diode clipping section, this pedal runs the resistor back in line with the input of the op amp as stated?

Here is the link: http://www.pedalarea.com/ocd.htm

Scroll down to the section labeled "Technical Details"

[aron]

You can see this simple mods page which has been around for ages:
http://www.diystompboxes.com/wiki/index.php?title=Simple_mods

I think the diodes maybe just go to the 1/2 VR point, but I'm not sure.

[PRR]

> How are transistors shorted to be used as clipping diodes?

It is in the link you posted: "Fulltone uses NMOS transistors in diode configuration (gate and drain short-circuited)".

> This creates a clipping threshold proportional to the input signal amplitude

What they describe does not do that. If indeed they get "a strong dependency of the amount of distortion created to the input signal", there's something else not mentioned.

[R.G.]

Basics:
1. The diode connection for a bipolar (NPN or PNP) is called that because it makes the device behave like a diode, and in fact more 'ideal' than an actual simple diode in many case. This is with the base shorted to the collector.
2. There is an analogous, but not identical, connection of an (a) enhancement-mode (b) MOSFET  device which connects the gate to drain. This produces a much-different result that is like a diode in some ways but not others. It's called "diode connection" for its similarity to the BJT diode connection.
3. JFETs and depletion-mode MOSFETs aren't used this way.

Discussion:
The diode connection for an enhancement mode MOSFET for clipping has been around for quite some years.

It's claim to fame is that it does not hard-clip a signal to anything like the degree that a real diode or diode-connected BJT does. Rather, it squashes signals progressively as they exceed its gate turn-on threshold voltage. The signal is still in there, but compressed when it exceeds the threshold. Contrast that to a diode that essentially flat-lines the signal over some threshold. It's much less abrupt.

The difficulties with using "diode-connected" MOSFETs are that:
- the signal level needed to make the start clipping is pretty high; generally these devices have conduction thresholds of 2-5V, and it's only over that level that they start clipping.
- so you have to have a pretty big signal, maybe +/-2-3V peak before you get much clipping. This is very close to the 4V or so of signal you can get out of a 9V battery
- the gate threshold is variable, and may be 2V for one and 3V for another of the same part number/type. If you want the thresholds the same, you have to select MOSFETs
- it's hard to get super-squashed mega-metal distortion out of these things; many people say they like soft distortion, but then show up here looking for 'more gain'

"NMOS" is another term for n-channel style MOSFETs; NMOS and PMOS are old technical terms for logic processes made entirely from N-channel and P-channel MOS devices respectively.