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Basic Transistor Question

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card9inal

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Hello everyone...

Please forgive me for asking a basic question, but I'm not quite sure I fully grasp what a transistor is capable of, and I'm hoping someone could give me some feedback on this basic question.

Could I pass analog audio through an n-channel JFET that I'd use as a switch?

I'd reverse bias the JFET with 12V, effectively cutting off the audio going through the JFET. To allow the audio to go through, I'd use a pushbutton switch to pull the 12V to ground, opening up the depletion region and allowing the audio to flow through.

Is that possible?
 
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Assuming you want the JFET in series with the audio then it's better to connect the gate to the transistor drain through a resistor (say 10kΩ) rather than connecting the gate to ground to turn it on. That way the gate voltage will follow the audio signal voltage and the transistor will not clip the negative portion of the waveform. Also you should limit the audio voltage to no more than about 1V RMS to minimize distortion.
 
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Ok.

Does this come anywhere close to accurately depicting what you described?

I've never worked with a transistor before, so I truly apologize for seeming so dense.

Thanks for your help!
 
And FYI, a FET is not a transistor.
 
Duh... That was obvious, wasn't it. I'm trying to piece everything together, and it's a struggle obviously. unclejed, I looked over that pdf and the last circuit was a high-frequency switch, so I copied that as attached. Am I any closer?

Mikebits... Huh? A Junction gate Field-Effect Transistor is not a transistor? Please explain.
 
Mikebits... Huh? A Junction gate Field-Effect Transistor is not a transistor? Please explain.

Ya know, your right, my bad... I usually think of transistor as the bipolar type.
 
Below is what I had in mind.

View attachment 66731


Mikebits, if a FET isn't a transistor than what does FET stand for? :confused:
 
You might also look at Analog Switches. They are mosfets in an integrated circuit optimized for switching signals such as audio.

Google "Analog Switch" There are lots of different parts from many manufacturers.
 
crutschow, thanks for the simulation.

Just to make sure I understand this correctly, could you confirm what I posted here?

I thought about analog switches, but I don't want to make this more complicated than it really needs to be. Analog switches could be just as easy... I just don't know. With switches, I'd need to have a 5V logic source (correct?) and I don't know if that would add a lot to the complexity of the circuit.

I'm wondering if instead of using relays, I could use these JFET's to switch audio to an output on and off.

If I'm reading your diagram correctly, when I disconnect the gate from the voltage source and connect the gate to the drain, the depletion region opens up as there's no more biasing. Correct?

If I added another switch that would be "normally open", the gate would be connected to the drain normally, then when I need it to close, I connect it to the 12V supply. How long would it take for the depletion region to be created?
 
Actually, Maxim offers a fine line of analog switches with great performance specs. And these parts require minimal support parts. Have a look here.
**broken link removed**
 
Just to make sure I understand this correctly, could you confirm what I posted here?

I thought about analog switches, but I don't want to make this more complicated than it really needs to be. Analog switches could be just as easy... I just don't know. With switches, I'd need to have a 5V logic source (correct?) and I don't know if that would add a lot to the complexity of the circuit.

I'm wondering if instead of using relays, I could use these JFET's to switch audio to an output on and off.

If I'm reading your diagram correctly, when I disconnect the gate from the voltage source and connect the gate to the drain, the depletion region opens up as there's no more biasing. Correct?

If I added another switch that would be "normally open", the gate would be connected to the drain normally, then when I need it to close, I connect it to the 12V supply. How long would it take for the depletion region to be created?
CMOS analog switches, such as the Maxim devices referenced by Mikebits, work very well as audio switches. They do require a low voltage supply but it's very low power and the voltage value is not critical. A simple series resistor and zener shunt regulator would work.

The JFET should work OK as an audio switch connected as shown. When the gate is connected to the drain through the 10kΩ resistor the JFET is on. It only takes a fraction of a microsecond for the transistor to turn on or off.

Why do you want to add another switch? Do you want a SPDT type of connection?
 
This is the circuit that I'm looking at building, and I don't know if using the JFET as a switch would be a better option than running the audio through the switch, which might cause some issues...

In this circuit, I'm using two relays, but could I get away with that with a two JFETs? Might that be a more stable way of building this circuit?

Thanks!
 
Hello everyone...
Could I pass analog audio through an n-channel JFET that I'd use as a switch?
Is that possible?

A good long time ago circa 1980 I worked on a 10 inch reel-to-reel tape deck that used simple JFETs to switch different tapehead outputs to the preamplifier when the machine auto-reversed direction. The FETs Source to Drain were wired in series with the heads.

So, yes it is possible and it worked quite well.

But I have to add that the reason I serviced it was FET failure. I replaced them and as far as I know, no further problems occured.
 
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