Multi Vibrator using BC 547

[ansaraet]

I have to design multivibrator using BC 547 to get output of 1 Mhz... I tried a lot but i am not getting the desired output...can anybody tell me what values of registor and capacitor to choose to get 1 Mhz Output Thanks..

 

[hsuang]

The frequency f (Hz) is about f ~ 0.7/ RC, where R is in ohm and C in farad.
To keep the circuit reliable, you have to choose the collector resistor smaller than the base one.
If you have a 5 V supply, and no current drain from outputs, you can start with Rc = 1k5, Rb = 6k8, C 100 pF.

 

[ansaraet]

I am not getting the output of 1 Mhz...... i'm getting only 200kh....
can anybody have another ckt...if i ues crysatl ossillator....will it be Ok

 

[neils_arm_strong]

Check your component values correctly.The frequency should be somewhere near 1Mhz.

 

[BADFLOPPY]

Check your component values correctly.The frequency should be somewhere near 1Mhz.

That is because of stray capacitance of the crystal regions and of course lead capacitance. This adds with your circuit capacitance and reduces the frequency.

 

[Prototyp_V1.0]

I have tested this circuit several times. I often ends up with a maximum f from 30 to 50 kHz. Like mentioned before, stray capacitanses and inductances make the circut unsuitable for higher frequensis. Besides of that, I think your bjt is more of use in audio amplifier circuits.

Also the output pulses will not be rectangular, but rather smooth - like the RC charge curve.

[edit]
I found a circuit in the good old "555 circuits", but I haven't being able to calculate the frequenz (formula).