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I have started designing and simulating a simple Jfet amplifier using LTSpice. The circuit I have below seems fine, apart from too much gain for my needs (about 20x to much gain). seems_ok

To fix this, I could make a simple voltage divider on the input, but this just seems far to 'hacky' and not a good approach. The best way it seems to me is to create a negative feedback path, thus reducing distortion and also reducing gain.

However my attempts where unsuccessful, this is my first attempt: try_1

This is the result at 'ac_out' off this circuit:

bad_output

I would guess this is because of the time C4 takes to charge, I try to change the simulation time span to no affect. I also tryed direct coupling between R5 and R10 but couldn't get that to work.

I know this seems to work fine with BJT's and don't know how I've gone wrong.

My second attempt: try_2

At first glance at 'ac_out' it seemed fine, voltage swing halved as expected, but frequency response became WAY off: try_2_resu

I'm sure I'm missing something, and would greatly appreciate some help and understanding of how to implement negative feedback here, ideally without the use of a AC coupling capacitor.

Thankyou :)

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    \$\begingroup\$ Is it just me, or is your choice of curve colour nearly invisible? Deep blue on black is not awesome. \$\endgroup\$
    – Reinderien
    Commented Nov 24, 2020 at 0:35

1 Answer 1

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You are missing some little things .Your first attempt is positive feedback .The second attempt gives a low input impedance which may not be wanted ,This means that the input coupling cap C1 gives far too much low cut .You could make feedback resistor R10 1 meg and place 470K in series with C1 .Expect a gain of about 2 on your sim.If you beat this circuit into shape on the sim and expect consistant performance when built with discrete Jfets you will have to make changes .JFETs have big spreads .

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    \$\begingroup\$ ahh yes, I see R10 will reduce the input impedance, thus creating a highpass filter over C1. thankyou for your help. \$\endgroup\$
    – G Frank
    Commented Nov 24, 2020 at 0:52
  • \$\begingroup\$ @GFrank If the answer is sufficient, can you accept (checkmark) it so it's easier for future users to see and find? Thanks! \$\endgroup\$
    – Ste Kulov
    Commented Nov 26, 2020 at 3:56

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