Behaviour of JFET

[Dr.EM]

Hi. I just tried this out on circuit maker 6, and wondered if this was right for the behaviour of a JFET:

High voltage on gate = low resistance between source and drain
Low voltage on gate = High resistance between source and drain

The one I used was a 2N5457.

 

[audioguru]

Hi Dr. EM,
Not quite.
A JFET is a depletion device and conducts fully when its gate voltage equals its source voltage. An N-channel JFET like a 2N5457 is cutoff and is a very high resistance when its gate voltage is negative with respect to its source voltage, just like a vacuum tube (valve?).

Those darn sim programs never seem to get things straight! :lol:

 

[Dr.EM]

Oh right, i see. Thats very helpful, thanks

So, is it possible to run an AC signal through the source/drain path, and adjust its level via a varying voltage on the gate? Well, I suppose that does depend on all sorts, impedance etc, but basically, can I use this where I might usually have a variable resistor, so that it can be adjusted rapidly?

Valves are groovy, btw 8)

 

[audioguru]

Run the signal through a resistance of about 47K, and use the FET as a shunt at the output of the resistor to ground. You can control the level of the signal by changing the negative voltage on its gate. It will begin distorting when the level across the FET exceeds about 100mVp-p.

 

[zachtheterrible]

audio, how can you specify a resistance when you have no idea what the AC voltage is? Just curious

 

[audioguru]

Hi Zach,
Since the circuit is manipulating audio signals, I assumed they are line level from the output of a preamp or mixer. Circuits usually don't mess with extremely low levels from microphones or extremely high levels from high-power speaker amplifier outputs. :lol:

 

[zachtheterrible]

Oh, i didnt see that part :lol:

 

[Dr.EM]

Ya know what I basically want is a potentiometer that is solid state, and can be adjusted via voltage quite rapidly. I suppose the JFET will work for this in most circumstances?

 

[audioguru]

Hi Dr. EM,
A FET will perform tremolo very well.
For vibrato, have you tried the VCO in a CD4046? :lol:

 

[Dr.EM]

Thats excellent, should be fine for the job then. Ultimately, I am aiming to build my own analogue modular synth, utilising that Moog filter and ring modulator, but this is for part of my custom oscillators. If you look here:

http://www.geocities.com/tpe123/folkurban/fuzz/snippets.html#lofomofo

On the project titled simple square wave shaper made simpler, I intend to use the JFET in series with one of the 500k pots next to the diode. Can its resistance be lowered to absolutely nothing, or very near to, btw?

 

[audioguru]

Hi Dr. EM,
You do like to listen to square-waves! :lol:
The pots in this circuit have a signal at each end and therefore it would be very difficult to bias a JFET to replace one.
An optocoupler can replace a pot in this circuit, like a 4N26. An H11F1 opto-FET might be better. They would go down to about 330 ohms.
Where did they get the names of those circuits? :lol:

 

[Dr.EM]

:lol: well basically, I don't want to limit its range too much. What I really need for it to do if vary its resistance rapidly by about 100k, as I remember, when I set one control about to max, and the other to midway, when i swept the middle control back and forth fast, it sounded nice 8):lol:

I can get a 4N25 over here. I didn't know an optocoupler could be used like this. So, how would I connect it up? If all else fails, it'll have to be a CDS cell and and LED with a bypass switch, though this is slow and not very variable. Thanks for your help so far btw!

 

[audioguru]

Hi Dr. EM,
A 4N25 has only 20% the transfer ratio of a 4N26, so it needs a fair amount of LED current. It should work fine if the modulation input swings a few volts positive, in my modified circuit. I changed the diodes around in the circuit to make it easier to sketch:

 

[Dr.EM]

Excellent, thanks for that! I now need to order the various parts to start testing the design out. Should have CV frequency control, fully variable waveshapes now with modulation, and a hard sync input.