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Help with FET's
Help with FET's
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Old 25th March 2003, 01:47 PM   #1
RichardJones is offline RichardJones
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Default Help with FET's

I need simple help and recommendations.

I want a simple FET stage... not a DA or anything... that has a voltage gain of about 10 and that can be run from a +40v supply.

It will have an unbypassed source resistor to lower gain and lower distortion.

Should I look for FETs that have a lower inherent gain, or just get one and increase my source resistor to lower gain?

Recommendations on which FET's to use (ya I know.. the 2SK170)..

Running anywhere from 1-5mA is good. Will be driving a NPN follower with a current gain of 400. BC549.

Thanks for ANY specific recommendations.

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Old 25th March 2003, 02:32 PM   #2
Jocko Homo is offline Jocko Homo  United States
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Default Why 40 volts????

Most good JFETs don't like that much voltage across them. Sounds like time for one of them cascode things.............

Jocko
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Old 25th March 2003, 02:34 PM   #3
RichardJones is offline RichardJones
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Default Yeah...

Cascode... was another idea too....

Ideas for good types? And should I bias the cascode up to over 5v or so?

Thanks!
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Old 25th March 2003, 02:39 PM   #4
Jocko Homo is offline Jocko Homo  United States
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I would run the JFET with at least 5 V, probably 10 V. Too low, and they sound like crud. Too high, and they get hot and, in theory, noisy.

BTW..................

If I get one more member ask me via e-mail why cascodes have more gain, I'm gonna rant.

They don't have more gain, just more bandwidth. And for all practical (engineering) purposes, they are a single gain element; as their response is very close to that of a single pole stage.

Jocko
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Old 25th March 2003, 02:45 PM   #5
RichardJones is offline RichardJones
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Default Where to connect?

So for 12v... connect between upper FET gate and lower FET source?

Could I use zeners or diodes?

Should I just lower the supply voltage to 28v?

What FETS should I use.. and should I really cascode them?

TXS!
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Old 25th March 2003, 05:30 PM   #6
SY is offline SY  United States
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Help with FET's
Important question: what's the voltage level you want to amplify? I'd give you a different answer for 1 volt than 1 millivolt.
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Old 25th March 2003, 05:37 PM   #7
RichardJones is offline RichardJones
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Default Line Stage

Its for a pre-amp.

So... 1v is what I want to amplify.

50K volume pot b4 FET.

So... yeah it does limit my choices....

Looking for low distortion, simple 1 gain stage, voltage gain of 10, running from 24v to 36v power supply (SLA batteries).

Cascode or not.... doesn't matter really...

BC549 emitter follower on output.

does that help?
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Old 25th March 2003, 05:51 PM   #8
SY is offline SY  United States
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Help with FET's
Then a plain vanilla 2N5566 or the like should work just fine. You could cascode, but you probably don't need to; since it's a matched pair, you can parallel the two devices. Run it with 1.5 ma or so in cascode, 3 ma or so in parallel.
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Old 25th March 2003, 06:02 PM   #9
RichardJones is offline RichardJones
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Default temp drift?

Looking at those FETs....

there is wide variation in the graphs when temp is changed.

The best place to run them at looks like 4mA each.

Also noise level seems high.....
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