Transistor astable multivibrator calculations

[boylesg]

I know how to calculate the frequency of these and I know how to calculate the base resistor and collector current for a transistor in switch mode.

But in an astable multivibrator you also have a collector resistor.

How do you go about incorporating a collector resistor into the calculations for a transistor in switching mode?

If you calculate the base resistor for CE saturation for one of the trannies, the collector resistor is going to limit the current such that it is not what your calculations say it should be.

**broken link removed**

 

[BradtheRad]

There is a necessary proportion between the inner and outer resistors...

such that the inner ones provide just the right current to keep bias levels in a particular range.

Try a simulation, and experiment with different values.

Check out my Youtube video, portraying the inner workings of this circuit at:

www.youtube.com/watch?v=HjSgdcgYnFk

You'll see that the bias volt levels go negative for a time, due to the capacitors.

 

[boylesg]

There is a necessary proportion between the inner and outer resistors...

such that the inner ones provide just the right current to keep bias levels in a particular range.

Try a simulation, and experiment with different values.

Check out my Youtube video, portraying the inner workings of this circuit at:

www.youtube.com/watch?v=HjSgdcgYnFk

You'll see that the bias volt levels go negative for a time, due to the capacitors.

Is the ratio always 1:10 or there abouts?

 

[BradtheRad]

1:10 is a pretty reliable combination. And it's easy to remember.
The outer resistors are often shown as a few hundred ohms, which makes a convenient load of a few mA.

The inner resistors are usually between 1k and 10k, which are typical values for biasing a transistor.

From experimenting with simulations, I find the inner resistors can be anywhere from 1.1 to 500 times the outer resistors.

The idea is for capacitor action to cause the majority of voltage swing at the transistor bias.

Notice the ratio cannot be 1:1. This would set the ends of the capacitors at the same volt level. There must be a differential across the capacitors, in order for action to start.

There also must be some asymmetry of component values, so the circuit will not stagnate when powered up.

 

[boylesg]

1:10 is a pretty reliable combination. And it's easy to remember.

There also must be some asymmetry of component values, so the circuit will not stagnate when powered up.

I thought that was highly unlikely since discrete transistors are never exactly the same.

 

[D.A.(Tony)Stewart]

Vce sat gain varies considerable but for reasonable saturation.
20:1 for better switch transistors to 5:1 for high power loads

 

[srizbf]

I know how to calculate the frequency of these and I know how to calculate the base resistor and collector current for a transistor in switch mode.

But in an astable multivibrator you also have a collector resistor.
.......

for transistor based astable design in this route:

assuming 50% duty cycle,
0.7RC=1/(2*f)

now select the R and C combination.

for the selected R , find Ic=Vcc/R
(R is the collector resistor.)

and Ic=Vcc/R

find Ib=Ic/beta(minm)

calculate the base resistor Rb as
Rb=(Vcc-0.7)/(10*Ib)