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I tried to simulate an RC lowpass circuit, but MultiSim doesn't quite provide the expected result.

enter image description here

Q2 being npn-type, and forward-biased, it should immediately turn on, becoming low-impedance on its collector-emitter path, so C3 should see almost GND on its lower terminal. On scope channel 'B', I have ~0,76V as expected, but channel 'A' carries a constant 5V where I would like to see a nice exponential.

p.s. There is no change in things if I pass an initial condition to the capacitor (IC=0V).

Any clues to what I did wrong?

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  • \$\begingroup\$ There's no power connected to the transistor. Also several other circuit errors. \$\endgroup\$ – Chu May 29 '17 at 12:43
  • \$\begingroup\$ Given the schematic you have drawn you CANNOT have 5 volts on channel A - more like 3.2 volts. \$\endgroup\$ – Andy aka May 29 '17 at 13:32
  • \$\begingroup\$ @Andy aka R3 was connected to the +ve terminal of V1 originally, but since that did not change a thing, I wanted to try something else. Thing is, I also tried a voltage controlled switch (SPST), but still no exp(). What is going on here? I remember the venerable Orcad 9.2 student edition where I used that SPST switch a lot... \$\endgroup\$ – user2286339 May 29 '17 at 14:27
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When the transistor initially turns on, it will immediately turn off because C3 nearly simultaneously appears as a short to the collector due to its discharged state. The condition is nearly transient in nature. The SIM may not recognize the repeated infinitesimal charging of C3.

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Success!

The solution is in the way Multisim goes about analyzing the circuit, see http://digital.ni.com/public.nsf/allkb/030CDBEF18515D6486257199006AEF56

To force the sim use the uncharged cap, go to Simulate -> Interactive Simulation Settings -> Initial Conditions -> Set to Zero.

NB With the above settings in place you don't have to set an initial condition on the capacitor, though it doesn't change the outcome:

enter image description here

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  • \$\begingroup\$ Thank people by upvoting their answers and/or comments and please no personal tags. (Site etiquette) \$\endgroup\$ – Adam Lawrence Jun 2 '17 at 17:03

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