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# Voltage to Frequency Converter....

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#### Rohit Chatterjee

##### New Member
I m supposd to build a voltage to frequency converter using only BJTs,resistors n capacitors....no ICs allowed...any ideas????

Create an oscillator and use a varactor diode as a tuning element. A Varactor is just a diode used while reverse biased, I think some transistor junctions can be used effectively (if not ideally) as varactors.

No actually i cant use varactor diode......just BJTs n normal Rs n Cs....

Use the voltage to control a current source. Use the current to charge a capacitor. Put the capacitor inside an oscillator.

Does the transfer function (Fout/Vin) need to be linear? If so, it will be considerably more difficult.

Does the transfer function (Fout/Vin) need to be linear? If so, it will be considerably more difficult.

NO...not at all....that would be one heck of a job.....

The circuit should produce a voltage wavefrm whose frequency would increase on increasing the magnitude of an applied DC voltage.....n vice versa....the applied DC voltage is not the DC biasing voltage.....
The output need not evn be a proper ac wavefrm.....
The circuit myt be constructed using cross coupled bjts,as in a flip-flop,isnt it????
Maybe by placing capacitors of the correct value at the correct places.....

Ok I was going to ask you if it could be a square wave, because looking through my books all RC oscilators have no real frequency change per voltage change.

BUT a multivibrator does.

Heres how I would try it,

Build a multivibrator, then use a variable resistor in one of the base circuits,
then adjust this resistor up and down to see how the freq, changes, once you establish that, then this variable resistor could be substituded with a transistor amp, so then an input to the amp, would cause a change in base voltage at the multivibrator, thereby changing the freq.

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The simplest example I can think of was hinted at by 1234...
Build a simple two-transistor astable multivibrator. Instead of connecting the timing (base) resistors to Vcc, connect them to the frequency control voltage.

Here is one I use quite often. It is a Hartley oscillator. You can replace the varactor diode with ordinary diodes such as a 1n914. You can increase the ratio of voltage to capacitance by paralleling diodes in place if D1.

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Voltage controled astable circuit,
is do - able

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I just designed one it works fairly good.

Voltage controled astable circuit.

Could use a lot of improvements though....

But it is do,able...

Oh OK, well that's what counts in your situation

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