LED flasher circuit question

[zachtheterrible]

Hi, I made the two transistor circuit on a simulation, and it worked great. I made the single transistor circuit on a simulation, and it didn't work. I don't see why it didn't work. Can someone tell me?

 

[Nigel Goodwin]

Hi, I made the two transistor circuit on a simulation, and it worked great. I made the single transistor circuit on a simulation, and it didn't work. I don't see why it didn't work. Can someone tell me?

Because you only had one transistor! - why did you think it might work?.

Any oscillator requires positive feedback, your one transistor design doesn't have any.

 

[zachtheterrible]

Forgive me for being so thickheaded, but i'm not sure what you mean about the positive feedback. Where is the positive feedback in the circuit that works?

 

[Nigel Goodwin]

Forgive me for being so thickheaded, but i'm not sure what you mean about the positive feedback. Where is the positive feedback in the circuit that works?

From the collector of Q2 to the base of Q1, each transistor inverts, so these two points are in phase giving the positive feedback. While the base and emitter of your one transistor circuit are also in phase, there's no voltage gain between them, so it can't oscillate.

 

[zachtheterrible]

Now i get what you are saying. Why is it then, that there are oscillation circuits w/ a single transistor that work? How would I be able to do this? Thank you so much for your help!

 

[Nigel Goodwin]

Now i get what you are saying. Why is it then, that there are oscillation circuits w/ a single transistor that work? How would I be able to do this? Thank you so much for your help!

They all require some kind of 180 degree phase shift - one way is to use a transformer, most RF oscillators do it that way. For audio use a common method is a phase shift oscillator, these have an RC phase shift network from the collector back to the base - it provides 180 degree phase shift at a specific frequency, and causes it to oscillate at that frequency.