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I'm new to electronics and discovered the falstad.com simulator. I wanted to model this audio pre-amplifier circuit:

enter image description here

So I went into falstad and drew this:

My pre-amplifier circuit on falstad

The output voltage is showing a flat line at about 6V. I was hoping for an amplified signal. So I can't tell if there's something wrong with the circuit I was attempting to model of if I'm just not using falstad correctly?

John
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    increase the input signal amplitude. – G36 Feb 03 '19 at 14:54
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    And 100nF capacitor value. Because now the C1 together with 1k from a high pass filter. Fc = 0.16/Rc =1.6kHz. Hence C = 0.16/(8Hz * 1k) = 22uF – G36 Feb 03 '19 at 14:58
  • http://tinyurl.com/y6wltxjm – G36 Feb 03 '19 at 15:02
  • oooh! Ok this is great, the capacitors made a filter that attenuated the AC signal. You connected a 100k resistor from output to ground to center the wave form around 0V again. – John Feb 03 '19 at 15:10

1 Answers1

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Two things are going on here:

  • First, your input coupling capacitor is way too small for that frequency. Use at least 10 µF in order to prevent attenuation at 80 Hz.

  • Second, and this is peculiar to the simulator, you have no DC path to the output pin, which means that it "floats" up to 6V. You're trying to look at a 100 mV output signal (assuming you fix the first problem) superimposed on that 6V DC bias, and the simulator display simply doesn't have the resolution to show that. Add a 10k resistor from the output node to ground.

With these issues fixed, the simulator shows an output signal that's close to 100× the input signal.

Dave Tweed
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