Quick jfet question...

[Colonel_Sanders]

How would I connect 2 different control voltages to a single jfet gate?

Could I run each control voltage through seperate diodes, connected to the gate?

Cheers,
C_S

[jrfrond]

Oops! Edited. I stand corrected. You could use steering diodes from each control voltage leading into the gate, but your logic choice would be limited to "OR".

[Steve Conner]

You never specified what logic you wanted. AND, OR, NAND, XOR, whatever.

[Colonel_Sanders]

You never specified what logic you wanted. AND, OR, NAND, XOR, whatever.

Uhm, I'm not quite sure. All it is is '0V = ON' and '-12V = OFF', with a 40175 chip being used for switching. I guess that is 'OR'?

Thanks,
C_S

[Don Moose]

Uhm, I'm not quite sure. All it is is '0V = ON' and '-12V = OFF', with a 40175 chip being used for switching. I guess that is 'OR'?

The 40175 (at least the one I found a datasheet for) is a quad D-type flip-flop with common clock and reset. Can you post some schematic so we can better understand how it's being used?

[Enzo]

Try this: You have a JFET. You want more than one thing to be able to turn it on, right? If either one OR the other thing would turn it on, that is OR logic. If you want one thing to turn it on, but only if another thing was also on, then that would be AND logic.

In other words in a three channel amp, if you want a gain cap switched in on channel 2 and channel 3 but not channel 1, then you'd wire channel 2 OR channel 3 to turn on the thing.

But if on the lead channel, you wanted something to turn on on channel 3 but only when the boost switch was also on. Then your logic would be turn on the FET when chanel 3 is enabled AND boost switch is on.

You probably want OR.

Since turning on the gate of a JFET is simply applying the voltage there, send each source of that voltage to the gate through a diode. That way the voltage gets to the gate, but cannot wander back up the other control lines to their circuits. I call those isolation diodes.

[Colonel_Sanders]

Hey, theres OR + AND logic in my amp! I feel like a genius. OR is what I'm talking about, thanks for clearing that up Enzo.

Awesome, I'll give the 'isolation diodes' a try.

Cheers for the replies,
C_S

[Enzo]

Here's a simpler way to look at it. A battery and a light bulb. Now add a switch in series to turn the bulb on and off. Now add a second switch.

If the two switches are in parallel, so either one can complete the curcuit, then that is OR logic. Put them in series so both must be on to complete the circuit, then you have AND logic.

Note that in OR, one switch cannot defeat the other. But in AND, either switch can turn the thing off.

[Colonel_Sanders]

Oh, I got it the first time, but I've wired a gain boost as AND logic and even didn't know it, hence the genius feeling

Cheers,
C_S