BJT Astable multivibrator

Hi,
the load resistor should be beta times smaller than the base one or slightly more. The transistor must be safely saturated, it must not stay in its active zone. The maximum allowed collector current, limited by the load resistor, must not be exceeded in the saturation state. When designing the circuit, you usually start by determining the load current (e.g. through a LED), calculate Rc, then Rb = Rc x beta and finally calculate the C for the timing required. Collector currents of both transistors are usually the same.

BR from Ivan

This might be an interesting article: https://www.electronics-tutorials.ws/waveforms/astable.html

(Maybe you know it already.)

Hi Guys,
I trust these trying times finds you all well.

I have a question regarding the old type flip flops, the types used in flashing 2 LED's with a couple of BC547's. I have no problem calculating the RC values for determining frequency and time however I have always wondered how the load resistor between the transistor's collector and +VCC is calculated. I have searched through numerous websites and tutorials and this fact has for some reason always been missed or I have neglected to see it.

I have always noticed that common designs use of a load resistor with values between 1K or 470R for both transistors and I always assumed it was because of the supply voltage, be it 9 or 12V and naturally using a LED these resistor values are used, could the answer be this simple or is there more to it.

On some designs this isn't always the case for example when you use one flip flop with a slower frequency to drive another flip flop at a higher frequency.

I would appreciate any info on how to calculate these load resistor values?

Kind Regards

Ken