My Ramp Generator won't oscillate

I built a (physical) ramp generator with two PNP 2N3906 transistors and one NPN 2N3904 transistor. With a supply voltage of +7.9V it oscillates, generating a sawtooth waveform with a frequency of 50.2 Hz. But when I build it in CircuitLab, nothing happens. Any idea what is wrong with my circuit in the simulator?

by davidtro
February 19, 2021

Hi @davidtro,

I made two changes and now it simulates:

  1. I added SW1 a time-controlled switch to turn the power-supply on at V=0. (Many oscillating circuits need something like this because otherwise, the simulator will find a "metastable" DC operating point and just sit there when it's not subjected to real-world random noise.)
  2. I changed Stop Time to 20m and Time Step to 100u. (Yours were set to 2 and 100m respectively.) This tells the simulator to simulate at a higher time resolution, which is necessary because of your 50 Hz operating frequency.

Give it a try here:

by mrobbins
February 20, 2021

Excellent! Thank you @mrobbins. That's exactly what I needed.


by davidtro
February 21, 2021

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