How too Drive j111 from a PCF8574

tom66

Joined May 9, 2009
2,595
Wouldn't a diode suffice for removing the 3.2V bias? From what little I know about JFETs, they shouldn't have a problem with the bias, anyway..
 

Thread Starter

RickH

Joined Dec 10, 2010
21
Ahh yes that might actually work, Like a zener or an LED with a diode could do it. I may try that, Thanks tom66.

I am working with j111's as audio Switches in Existing circuits, I would really like to keep the on voltage too 0VDC if at all possible.
 

SgtWookie

Joined Jul 17, 2007
22,210
If you used a diode to remove the bias, you'd have to ensure that there was a constant current flow through it, or the voltage wouldn't be very stable. It would add to the parts count, too.
 

Thread Starter

RickH

Joined Dec 10, 2010
21
ya, your right I would need to add a resistor to draw current at -15V so add 1 diode and 1 resistor to drop 1 transistor isn't an improvement.
 

tom66

Joined May 9, 2009
2,595
If you used a diode to remove the bias, you'd have to ensure that there was a constant current flow through it, or the voltage wouldn't be very stable. It would add to the parts count, too.
I might be misunderstanding something here, so feel free to correct me. But the problem with your original circuit was that it went from -15V to +3.2V, right? And it needed to go from -15V to 0V. Well, a diode would accomplish that, although it would drop ~0.5V when conducting, but that shouldn't matter too much.
 

Thread Starter

RickH

Joined Dec 10, 2010
21
Its still easier/cheaper/smaller board using quad op amps as switches I think.
3 Resistors 1 Transistor 1 Diode Per switch
2 Resistors 1/4 Op-Amp per switch

Actualy I have seen the diode in series with A jfet gate so it may actually work with little to no current flow, I'll give it a shot this weekend.
 

SgtWookie

Joined Jul 17, 2007
22,210
Well, you may be annoyed with me because I didn't go back and look at my 1st common base simulation. It just hit me that I didn't go back and check the emitter current in the original simulation; which I just did.

It was much too high; 140mA. So, I added resistance between the signal and the emitter, and increased the pull-down resistor to 15k. Wouldn't you know it.... 2 resistors and 1 transistor will get the job done.

 

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RickH

Joined Dec 10, 2010
21
LOL well their it is then, and I just etched my op amp switching board lmao. I'm still gonna breadboard this and see if I like it more, depends on ease of board making and size now as they will basically cost the same I believe.
 
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