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Please help design discrete JFET preamplifier
Yansi:
Hello,
I am trying to design a discrete preamplifier with JFET to serve as a frontend to a simple measurement jig.
What I am trying to achieve:
Supply voltage: +-5V
Input impedance: 1Mohm
Bandwidth: 20Hz to 100kHz or better
Gain: 11x (21dB)
Input level: 200mV rms
I am kind of lost as how to attack this. For example, how could one adapt the following circuit/s for the symmetrical supply (ground referenced signal input and output)?
I have made a few experiments and this what I came up with seems to be as close as I can get it to do what I want:
Kleinstein:
Reaching some 6 V peak to peak cold be difficult with the amplifiers shown and only a +-5 V supply.
The last circuit is Ok with ground referenced input and output, but I don't think it would reach the amplitude.
For selecting the resistors spice is your friend.
Yansi:
It seems I did not screw up too much, as it indeed does work, even with 2Vrms output. Even the distortion seems to be still very low at 100kHz. But a little compensation would help, testing with square shows fugly overshoots on the edges.
Here's a shot of the circuit above.
I was just curious if there is any better way to do it. :-//
(I did not want to use an opamp, as this will probably be less noisy, than say TL072).
Yansi:
Spent a while tweaking it. Here's the result!
And running a 500kHz square. I call that a success. And mind you, I built the circuit on a breadboard. I think I have pushed the limits. The -3dB point is at about 2.4MHz.
magic:
That's some original way of drawing PNPs ;)
Beware that input impedance is less than 1MΩ at 100kHz.
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