Getting back into analysis and need help

I can not take credit for this circuit nor do I know who designed it. I'm trying to get back up to speed on circuit analysis after a long time away and picked this as a simple oscillating circuit to analyze.

I can not understand why removal of the LED and 1ohm resistor have such a large impact, nor can I understand how the base to T1 gets above 3 volts in the time analysis. Any insight into what I am fundamentally missing would be appreciated. The intent of this circuit is to simply be able to flash an LED with low voltage and power drain.

by ajcinquemani
July 09, 2014

Sorry but I think you have chosen a poor example circuit from which to try to get back up speed in circuit analysis and design.

The circuit you have chosen not well designed and only oscillates under certain conditions. It can get stuck with both transistors turned permanently on.

As such it is not worth trying to explain how it is supposed to work.

You would be much better to start with a "standard" well understood and well documented circuit such as these:

If you are struggling to understand why the base of Q1 in your example gets above +3V then you need to go back to some basics about capacitor coupled circuits.

Make yourself some examples like these:

and play with them.

Then see how it behaves in the astable multivibrator.


by signality
July 10, 2014

Well maybe not this one ...

I hadn't spotted the ground connections at Q1 and Q2 collectors shorting them out!

The links to other info are OK though.

You might also like to have a look at:


by signality
July 11, 2014


Your circuit is actually a complementary bjt (PNP/NPN) version of the standard 2 transistor astable multivibrator.

Here's some background to it:

It's described in:

Garner's mysterious oscillator

Garner's oscillator, one last time

To get it to start up reliably in simulation, you need to ramp up the supply using one of the tricks described here:


One of Lou Garner's transistors


I suspect Dr. Covington may have missed the startup trick.

by signality
July 11, 2014

Thanks for the information, reading the first chapter of Horowitz and Hill now. Amazing how much I have forgotten. I'm going to start playing with some of these circuits tonight as well. Circuitlab is a nice tool to have access to.

by ajcinquemani
July 11, 2014

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