Creating a variable wien bridge oscillator

[ARNAB MONDAL]

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?





[mod note: reorientated image]

 

[duke37]

To get a variable frequency, you need to change two resistors together. Altering one only will change the gain and so will affect the sine wave. Commercial oscillators use a matched pair of linked resistors or use a two gang capacitor.

Another way of getting a wide audio frequency range is to use two high frequency oscillators, one of which is variable and then mix these two to give the difference frequency. This principle has been used in musical instruments. (Theremin)

 

[davenn]

I had been trying to build a variable wien bridge oscillator . The schematic is shown below -

where did that circuit come from ?
it doesn't look like the normal Wien Osc
pretty much all the Wien Osc's I have seen or build use a thermistor or a small lamp for the feedback control ... .eg.


Some use a FET

Dave

 

[CDRIVE]

where did that circuit come from ?
it doesn't look like the normal Wien Osc
pretty much all the Wien Osc's I have seen or build use a thermistor or a small lamp for the feedback control ... .eg.
View attachment 27712

Some use a FET

Dave

Yes, including this old girl (HP 200) that was extremely popular for many decades after its birth. It touted exceptionally low distortion over its frequency range.

Chris

 

[LvW]

where did that circuit come from ?
it doesn't look like the normal Wien Osc

Of course, it is.
Any oscillator circuit using the WIEN network (as invented by M. Wien) is a so-called "Wien oscillator".
It is another question if and how you stabilize the amplitude (Diodes, FET, light bulb, OTA, thermistor,...).

 

[ARNAB MONDAL]

Stabilised it using diodes. IN4007

 

[ARNAB MONDAL]

Since I do not have an incandesent light bulb to stabilise the amplitude I used the diodes. IN4007

 

[duke37]

I suppose that the LM386 could be called an op amp. However, the LM386 will have more power than the normal op amp and will require fewer components. It is designed to drive a speaker so that is the one to go for.

 

[(*steve*)]

if you go for a lamp stabilized wien bridge oscillator like the hp200, be sure to use a cheap op-amp, not a really good one.

HP have a really good paper describing why distortion is actually required for good stability.

the feedback loop if you use a lamp (and possibly a thermistor) is generally pretty low impedance, so an op-amp that can drive a significant amount of current is required. The 386 is probably a reasonable choice in this respect and since you want a gain of about 3, its more open loop gain is not a big deal.

 

[AnalogKid]

The LM386 is not a general purpose opamp, and will not work in typical opamp circuits. It has a minimum forward gain of 20 and a fixed output DC offset. It will not work as the Wein oscillator component. It will work as a separate speaker amplifier stage.

ak

 

[ARNAB MONDAL]

I had been trying to build an op amp and thereafter i used an lm386 to drive a speaker . but it is not working .

 

[AnalogKid]

What is the overall frequency range you want out of the circuit? Separate from that, what is the frequency range you want with one turn of the adjustment pot?

ak

 

[ARNAB MONDAL]

What is the overall frequency range you want out of the circuit? Separate from that, what is the frequency range you want with one turn of the adjustment pot?

ak

i want the audio range from 100hz-18khz

 

[LvW]

i want the audio range from 100hz-18khz

Don`t forget the slew rate limitations for higher frequencies.
What is the slew rate for your amplifier?

 

[davenn]

i want the audio range from 100hz-18khz

The cct I supplied covers that range ... It's one I have built and it works

 

[LvW]

The cct I supplied covers that range ... It's one I have built and it works

Yes - I believe you, of course. It resembles the classical oscillator principle.
No problems with the slew rate for the LF351 at 150kHz?
To the questioner: I am pretty sure that the circuit will also work if you replace the light bulb by a fixed resistor. The amplitude stabilization can be done using two diodes across the feedback resistor.

 

[AnalogKid]

In the davenn circuit, each capacitor range is 10:1, so covering the span from 100 Hz to 18 kHz would take two full ranges and part of a third. If you want the full 100 Hz - 18 kHz span in a single adjustment range, that is a range of 180:1. I suppose that circuit values can be found to do this, but the adjustability to get to a particular frequency will be very poor. For a normal pot with a 300 degree adjustment range, that's 60 Hz per degree of rotation, and getting the pot to the same position within 1 degree is almost impossible.

So, are multiple ranges with a selector switch (as in post #3) acceptable, or do you want the entire range in one span of the pot?

ak

 

[ARNAB MONDAL]

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 more

 

[BobK]

Use an audio amplifier.

You "emitter follower" circuit is loading the output of the oscillator unduly due to the biasing resistors, which are unnessary in that configuration. But I would still recommend using an audio amp over trying to fix your circuit.

Bob

 

[ARNAB MONDAL]

I tried it but it always distorted