Hello,

I'm analyzing a sawtooth waveform generator below made with discrete components. The circuit does work when the Re resistor is above 5K but doesnt generate a sawtooth when Re is say 2K. My question is why? I noticed that the transistor is in saturation with the 5K and not with the 2K im not sure if that helps?

Thanks

 

Certainly not any kind of expert, but look at the voltage at the base of Q2, it appears to drop below saturation down into the mV range when you change the resistor... Q1 and Q2 appear to function as a latch, providing a path to ground. It looks like your switching it off, hence no ground.

 

Low values of R5 allow sufficient current through Q1,Q2 to keep the latch conducting, so the cap can't charge.

 

Another way to say this is, your scr has enough hold current to stay latched on. You can keep the current low with R5 or lower the quality of the latch with a resistor from base to emitter of Q2.

 

Ok I see it now... I did not suspect the scr made with the 2 transistor be running with such low current.

Thanks!