Optimizing astable multivibrator for minimum power consumption

Wendy

Joined Mar 24, 2008
21,906
First of all, I would loose the transistors. If you must have them raise the resistors as high as possible, with the collector resistors being 1/10 the value of the base resistors. The base formula's for frequency still apply, you adjust the values of the capacitors to match.

Having said that, have you considered CMOS gates? A CMOS inverting Schmitt Trigger would be very low current, and have good drive characteristics.

If you haven't studies Hysteric Oscillators before here is a good example.

555 Hysteretic Oscillator

You could even use a CMOS 555 oscillator for good results.
 

Thread Starter

aeftimia

Joined Sep 16, 2011
18
Thank you for your reply. I suppose getting rid of the transistors would mean using an integrated circuit--which is perfectly logical (pun!)--however I am trying to make due with just transistors, capacitors and resistors right now. I am trying to get a feel for how these things work without buying a black box with 6 pins--at least until I understand the circuitry well enough that these ICs are no longer black boxes to me.

PS: Why should the ratio between the collector resistors and the base resistors be 1/10? That seems rather arbitrary. Could that be optimized?
 

SgtWookie

Joined Jul 17, 2007
22,210
The 1/10 rule involves transistor saturation characteristics. In this circuit, the transistors are being used as saturated switches. For most transistors, the base current necessary to saturate the transistor is 1/10 the desired collector current.

By the way, "saturated" means that a further increase of the base current will not result in a corresponding further reduction in Vce (voltage on the collector with respect to the emitter).

There are some high-gain transistors that will saturate pretty well with less than the 1/10 rule. However, they are the exception and not the rule. Those transistors are also usually quite a bit more expensive than standard versions.
 

Ron H

Joined Apr 14, 2005
7,014
Bear in mind that if your supply voltage is higher than the transistor base-emitter breakdown voltage, the oscillation frequency will be higher than the calculated frequency, and the transistors may eventually be damaaged.
 
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