I'm trying to simulate a simple amplifier for an electret microphone, but having problems regarding the output level and virtual ground.

The circuit in Multisim is as follows: Circuit Diagram

What I'm having trouble understanding is:

  • Why is the virtual ground at 3v and not 2.5v?
  • Why is the output swing 3x larger than the input swing and not ~100x?

Also, here's a scope trace which may be useful: (input - orange (ch.A), output - red (ch.B)) Scope Trace

Edit: Here are the currents in the feedback network:

Current in feedback resistors

  • \$\begingroup\$ Where is the DC current through R1 flowing? Also the current in R5 & R6? \$\endgroup\$ – John D Aug 14 '14 at 16:43
  • \$\begingroup\$ Also what's the reactance of 1uF at 1kHz? \$\endgroup\$ – John D Aug 14 '14 at 16:57
  • \$\begingroup\$ I would suggest replacing your virtual ground with a real voltage source in your simulation, then when you get everything working decide what you want to do about a virtual ground. \$\endgroup\$ – John D Aug 14 '14 at 16:58
  • \$\begingroup\$ Replacing VGND with a voltage source solves the alignment problem, I'll have a look at the currents in a second. \$\endgroup\$ – tehwalris Aug 14 '14 at 17:29
  • \$\begingroup\$ DC current through R1 flows through the AC source to ground. I've added a picture showing the currents in R5 & R6. \$\endgroup\$ – tehwalris Aug 14 '14 at 17:38

The DC currents through R1 and R5 are flowing through R3, causing the virtual ground to increase in voltage. According to the OP the gain is working correctly now.

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