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# Calculate frequenzy - modified astable

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#### Prototyp_V1.0

Hi. Is there some in here capable to find any expression for the frequenzy of an modified version of an astable multivibrator.

I ask this because I found out that Vbe without diodes can reach a negative voltage that is almost the same value as voltage supply. That means if voltage supply is 12 volts, the npn bjt's probably won't last.

I have tried this amv on a breadboard, and it works the same way as original circuit. It oscilates faster than original circuit.

Anyone has an idea how to find expression for the frequenzy?

#### Audioguru

Connect the diodes in series with the emitters of the transistors to allow the oscillator to work but block the reverse voltage from causing avalanche breakdown of the emitter-base junctions of the transistors.

### Prototyp_V1.0

Points: 2

#### FvM

##### Super Moderator
Staff member
In both cases, the frequency is defined by the timing of capacitor discharge, the initial condition is different, however. You can derive the frequency "formula" by putting in the initial and final condition of the exponential discharge process.

The original expression is simplified, without considering Vbe, finite current gain, saturation voltage. It's simply
Code:
2*(-ln(0.5)) = 1.38
With the diodes, at least Vbe, Vd and also Vcc get relevant. As a rough estimation (assuming Vbe = Vd = 0.6)
Code:
2*(-ln(11.4/12.6)) = 0.2

### Prototyp_V1.0

Points: 2

#### Prototyp_V1.0

Thanks both of you. I didn't think of putting diodes in series with emitter
Then again I guess the frequenzy would be different from the result using the original formula.

@FvM
I'm not sure I really got the hang on the calculation, but it sure must be lower than 1.38 because test shows that frequenzy goes up when using diodes in parallell such as I did.

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