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Creating a variable wien bridge oscillator

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arnab_2014

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I had been trying to build a variable wien bridge oscillator . The schematic is shown below - I wanted to change the frequency by using a pot . But it starts to distort after 12 Khz. Also when i am reducing both the capacitors to 1 nF I find the waveform to be triangular more than sinusoidal. Also if I want to drive 0.5 w 8 ohm ospeaker should i use an op amp or an lm386 ? Also how do i change the frequency with one control pin? IMG_20160705_120430749.jpg
 

At first - changing/tuning the frequency is possible (without violating the oscillation condition) only if you tune both R or both C simultaneously.
If such frequency tuning with a single element is desired you must select another oscillator type.
Secondly, a triangular waveform rather than a sinusoidal is an indication of slew rate limitations.
For example, a 741 type opamp has a large-signal bandwidth of 10 kHz only.
 

The lousy old 741 opamp was designed 48 years ago. Use a modern opamp instead that has a bandwidth of 100kHz or more.
To change the frequency of a Wien Bridge oscillator you must change both resistors at the same time.

My Wien bridge oscillator goes from 10Hz to 100kHz using a TL071 audio opamp that has low noise, low distortion and a bandwidth of 100kHz. Since it has Jfet extremely high impedance inputs I switch decades of frequencies with pairs of resistors that are 22k, 220k, 2.2M and 22M ohms and I change the exact frequency in each decade with a dual 365pF variable capacitor from an old AM radio.
 

Audioguru said:
The lousy old 741 opamp was designed 48 years ago. Use a modern opamp instead that has a bandwidth of 100kHz or more.
To change the frequency of a Wien Bridge oscillator you must change both resistors at the same time.

My Wien bridge oscillator goes from 10Hz to 100kHz using a TL071 audio opamp that has low noise, low distortion and a bandwidth of 100kHz. Since it has Jfet extremely high impedance inputs I switch decades of frequencies with pairs of resistors that are 22k, 220k, 2.2M and 22M ohms and I change the exact frequency in each decade with a dual 365pF variable capacitor from an old AM radio.
Click to expand...

I had built the oscillator and now I want to drive it using a speaker . So i Had used an emitter follower . BUt somehow the speaker is giving a very reduced voice output. How do i increase the power moreWIN_20160706_18_00_43_Pro.jpg
 

arnab_2014 said:
I had built the oscillator and now I want to drive it using a speaker . So i Had used an emitter follower . BUt somehow the speaker is giving a very reduced voice output. How do i increase the power moreView attachment 130485
Click to expand...
Almost all audio amplifiers use a class-AB push-pull output. The NPN emitter follower pulls up the output voltage (a peak of 3V into an 8 ohm speaker is a current that is 3V/8 ohms= 375mA, and the PNP emitter follower pulls down the output voltage with 375mA. Your circuit has the NPN transistor pulling up with a few mA but the 1k emitter resistor pulls down with only a few micro-amps.

Here is a simplified output stage of an audio amplifier. It is missing a voltage gain stage and is missing negative feedback:
 

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