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If I choose "start external DC supply voltages at 0" from transient simulation settings, the output is centered at 4.55 Volts. The amplification and gain is OK, but the output is not centered at 0.

However, If I don't choose that option "start external DC supply voltages at 0", then output is also centered at 0 volts.

Is it related to charging of C27? But output offset is also not likely to drop towards 0. It continues oscillating around 4.55 volts.

If someone can explain me what is the difference between those two options and how to solve offset problem, it will be nice.

  • \$\begingroup\$ BTW, press "A" to toggle text anchors (those small circles), in case you didn't know. If you did... \$\endgroup\$ – a concerned citizen Dec 13 '19 at 17:46
  • \$\begingroup\$ @aconcernedcitizen those were making me crazy, thanks :) \$\endgroup\$ – muyustan Dec 13 '19 at 17:53

Your "ampout" output is unterminated, so there's no place for any charge on C27 to go. If you start with supply voltages = 0, then when the collector voltage of Q1 goes up (presumably to 4.55V), it'll take "ampout" with it.

You should figure that the voltage of "ampout" at startup without the supplies starting at zero will be at the whim of the tool, and in this case the tool's whim is to have it be 0V. Be thankful it doesn't just give you an error.

If you want things to be sensible, put a sensible resistance to ground -- either whatever you believe the DC impedance to ground will be in the real circuit, or something high enough so that the pole formed by C27 and your resistor is well below any frequency of interest.

  • \$\begingroup\$ Hmmm, which one of the options(start dc supply at 0 or not choosing it) will represent the real behaviour of the circuit? My understanding tells me if the capacitor is uncharged when it is first implemented on the circuit, the real attitude will be similar to "start dc supply voltages at 0" one. Am i right on this? \$\endgroup\$ – muyustan Dec 12 '19 at 23:45
  • \$\begingroup\$ More or less, yes. In reality power supplies usually ramp up, and they're often pretty messy about it, with switch bounce or overshoot or various other bad behaviors. However -- the "start from zero" is probably more representative of reality. \$\endgroup\$ – TimWescott Dec 13 '19 at 0:06

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