Oscillator with variable output amplitude

[AlphaRugal79]

Hello,

I am looking for an oscillator circuit that would produce constant frequecy of 10 khz but variable output amplitude. The output amplitude will change in proportion to the voltage feedback to the oscillator. Well, this circuit is for the conductivity sensor, the idea is to change the A.C. generated by the oscillator in proportion to the change in voltage fedback by the difference amp.

 

[Nigel Goodwin]

Feed the oscillator through a VCA (Voltage Controlled Amplifier) and use that to change the amplitude.

 

[AlphaRugal79]

Thanks for the reply,

How would the VCA work if I use 555 timer as the oscillator. I believe that the output of Difference Amp would go the VCA and output of VCA would go to the 555 timer, something like this, but don't know exactly how???? Or which other Oscillator would you suggest instead of 555??

Regards,

 

[Roff]

Thanks for the reply,

How would the VCA work if I use 555 timer as the oscillator. I believe that the output of Difference Amp would go the VCA and output of VCA would go to the 555 timer, something like this, but don't know exactly how???? Or which other Oscillator would you suggest instead of 555??

Regards,

The oscillator is outside your feedback loop. The loop would contain the VCA, the sensor, and the diff amp (and any loop filter you might require).

 

[AlphaRugal79]

Thanks Ron,

Now I am a bit confused. How could the oscillator be out of the feedback loop? The whole idea is to generate A.C. of 10Khz whose amplitude changes with respect to feedback voltage.

 

[Roff]

Thanks Ron,

Now I am a bit confused. How could the oscillator be out of the feedback loop? The whole idea is to generate A.C. of 10Khz whose amplitude changes with respect to feedback voltage.

VCA means "voltage controlled amplifier". You provide it with a signal from your oscillator, and its gain is controllable, meaning it can provide a variable-amplitude signal. This is simpler than making an oscillator with variable output amplitude.

 

[AlphaRugal79]

Thanks,

So which oscillator would you recommend?

 

[Roff]

Thanks,

So which oscillator would you recommend?

That's pretty much your choice. I thought your question would be "which VCA would you recommend?" I don't have any experience with VCAs. Maybe someone else here does. If I were designing this, I would start by Googling "voltage controlled amplifier".

 

[chinsoon]

VCA means "voltage controlled amplifier". You provide it with a signal from your oscillator, and its gain is controllable, meaning it can provide a variable-amplitude signal. This is simpler than making an oscillator with variable output amplitude.

does that mean that you are actually using the output of the oscillator as input to an amplifier whose gain can be varied so that the output of the amplifier can be varied as well?

 

[Roff]

does that mean that you are actually using the output of the oscillator as input to an amplifier whose gain can be varied so that the output of the amplifier can be varied as well?

Yep, that's the idea.

 

[Nigel Goodwin]

Why do you want to do this anyway?, it seems a very strange idea?.

 

[hjames]

An alternative is to use multiplying type elements - monolithic multiplier chips, discrete logarithmic in combintation with anti-log amplifiers, or mulitplying DACs under software control.