Designing a multivibrator

Discussion in 'Homework Help' started by Robotics Guy, Nov 30, 2011.

  1. Robotics Guy

    Thread Starter New Member

    Mar 11, 2011
    Hello everyone,

    I'm trying to design an astable multivibrator like the one below:


    I'm still trying to understand exactly how it works, but that's not my question; my question is regarding transistor stability. I'm using BJT NPN transistors which rely on β, and I know that beta changes pretty significantly depending on the temperature of the surrounding environment. This circuit is being designed to work outdoors year round, so the temperature could vary from, say, -20 to 110 degrees Fahrenheit. The extreme change in temperature will definitely change the β of the transistors significantly, right? So how do I ensure that the transistors operate correctly when the β is going to change so much?

    I appreciate your advice :)
  2. Audioguru


    Dec 20, 2007
    Simply make sure that R2 and R3 can supply enough base current when the beta is very low.
    Note that the supply must be low enough (6V) so that the emitter-base of each transistor does not have avalanche breakdown when the capacitor drives the base negative.