25 Khz oscillator

[Kevin Weddle]

Here is something I dredged up in the way of sine wave oscillators. It incorporates a low pass and high pass filter and amplifies noise into a single rate of change.

[audioguru]

Kevin,
I don't see how it can be an oscillator, operate as high as 25KHz nor have a sine-wave output.

The capacitive reactance of its "104" capacitor (0.1uF?) is only 64 ohms at 25KHz so would seriously overload the output of the opamp if it had an output, and the 10M resistor across it is so much higher it doesn't filter anything.

The transistor would also seriously overload the opamp if it had an output.

Why do you post these (same as your 60Hz "oscillator") odd circuits?
Can't you see that the transistor is cutoff all the time because it doesn't have any bias?

[Kevin Weddle]

Oh I made a mistake. The emitter of the transistor is not grounded. It goes to -12 volts.

[audioguru]

Hi Kevin,
Good, you got your transistor doing something, but the whole circuit still has problems:
1) Where are your phase-shifting filters? You have two very low-impedance (at 25KHz) caps in series without any phaseshift.
2) What about the opamp output overloading problem caused by the 64 ohm impedance of the 0.1uF cap?
3) The output of the opamp will also be severely overloaded by the transistor's base, which will act like a rectifier. The coupling cap will be charged and the transistor will get cutoff.
4) The 1uF cap to ground at the transistor's output has an impedance of only 6.4 ohms at 25KHz and would cause a very low, distorted output.


Sorry, but this circuit is not a 25KHz sine-wave oscillator.
Why not just make a tried and proven Wien Bridge oscillator? A standard 3-RC networks phase-shift oscillator works pretty well too. You could also make a simple square-wave oscillator and filter its harmonics away.