# Circuits that take advantage of tolerances in components

Bit of a wordy question title...

I was trying to simulate an astable multivibrator the other night and the simulation refused to run. I eventually realised that because the simulation assumes perfect components, therefore there is no way for the multivibrator to get going hence the failure.

So this means that the reason the astable multivibrator works as it does is only because of manufacturing defects/tolerances of the components used (You can use different values of resistors etc., but assuming identical components are used).

Which leads to the question, are there any other circuits out there which incorporate the manufacturing tolerances of components in their operation?

Edit:

I've attached an image of an astable multivibrator. Whenever you look up any explanation, that explanation starts "Assume TR1 is OFF" or words to a similar effect.

Now I understand in the real world how this circuit starts and works, this question is not about that.

Assume that the circuit is initially powered up and all components are perfect and our traces have no resistance. This circuit would have no way to 'start'. Therefore, from this we can say this circuit only works they way it does because of imperfections that exist.

What my question is: Are there any other circuits which only function due to imperfections?

• It is a common problem with simulators. Not just "perfect" components, but a lack of noise caused by thermal sources. Yes, the simulation may run at room temperature, but that temperature doesn't generate noise power. Circuits need a "kick" to get going. – glen_geek Nov 1 '18 at 16:46
• Post the circuit? It could also be that in reality there will be noise to supply whatever initial imbalance is needed. – TimWescott Nov 1 '18 at 16:46
• As the 2-transistor's capacitors decay in charge, the transistors will turn on and the VDD NOISE or the thermal noise will causes imbalances. – analogsystemsrf Nov 1 '18 at 16:50
• @analogsystemsrf I understand how the circuit operates using real world components with slight differences, I was just wondering if there were any other circuits which were designed with these imbalances in mind. Because on paper and simulations this circuit shouldn't work, it's only due to these differences that it does and must have been designed with this in mind. – Doodle Nov 5 '18 at 11:58
• There are quite a lot of circuits (a lot of them oscillators) that use inherent imbalances to start; in a simulator it is common to assign an initial condition to actually see what will happen in reality. – Peter Smith Nov 5 '18 at 13:15