Need help with LTSpice Fet stage

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Printer2

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Joined 2010
#1
Don't know where to get advice but know there are a few smart cookies here. I am just starting to get my toe wet with this circuit but it is driving me crazy. Just a 2N5458 running at 12V. Input 50mV and I am getting a strange output. Seems if I lower the source resistor and increase the drain resistor I get the output signal rising but I get to a point where it decides to go lower. Wondering if someone can take a look at it and tell me where I am going wrong. Files are stored on Microsoft's Onedrive.

https://ufrvka.by3301.livefilestore...tuXRcdi1bxGpOLA1VbA/spice.zip?download&psid=1
 
#2
Try setting the input to be a voltage source rather than a current source. Adjust R4 to give suitable DC conditions for the FET. Your really pushing the 'gain' here with R6 and C4 and as such it will be variable and depend on the FET.

(you can attach all your files directly to the forum 🙂)

FET.JPG
 
#3
Didn't notice the current source, modified an example that was in the library. I was just messing with the resistor values trying to figure out what was going on. Now with a 56k drain and a 22k source and with 50mV in I am getting about a gain of 20. More in line with what I thought I should see. Thanks for your help.
 
#5
🙂 That looks reasonable but do bear in mind that individual FET's may need slightly different biasing (source resistors). Also make sure that you size the coupling caps to give the response you want. 500k and 0.02uF has a -3db point of around 16Hz, and you have 3 such time constants all adding up here.

So look at the response of the circuit overall. Post #14 and 16 here show how to do that if your not sure:

http://www.diyaudio.com/forums/soft...ng-ltspice-beginner-advanced.html#post4029498
 
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