| Electronics > Projects, Designs, and Technical Stuff |
| Please help design discrete JFET preamplifier |
| (1/5) > >> |
| 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. |
| Navigation |
| Message Index |
| Next page |