What's the procedure of Wien bridge circuit?

[aomidee]

Hi, Can anybody explain the procedure of "Wien bridge circuit" in brief please? Does it have any input signal? I think it is not anything more than a bandpass filter that is combined from a lowpass and a highpass in series, how does it produce Sine wave?

 

[Audioguru]

Re: Wien bridge

The highpass and lowpass RC sections form a bandpass filter with a loss of 3 times at resonance. It is positive feedback to the opamp. The resistor divider produces negative feedback for a gain of 3 for the opamp. Then noise in the circuit gets amplified until the circuit oscillates at the resonant frequency.

The circuit needs a level stabilizing circuit which could be a light bulb in the negative feedback path or a Jfet transistor. they keep the output level from rising until the opamp clips the signal at its output.

 

[flatulent]

Re: Wien bridge

1. There is positive feedback.

2. The unique RC network produces a quad of singularities. Complex conjugate poles in the left half plane and mirror image zeros in the right half plane.

3. The root locus is then totally horizontal lines between the poles and zeros.

4. When the poles are slightly in the right half plane the circuit oscillates. The signal builds up until there is some form of limiting which reduces the gain.

5. In your circuit, this is the op amp clipping. This is not very good. The limiting device should be across the series RC circuit.

A piece of history. Back in the 1930s this circuit was developed by a Stanford student. The limiting device was a small incandescent light bulb. This student was either Hewlett or Packard. The other was a person with business skills. They formed a company making this oscillator. It was used in the sound effects of the first feature length cartoon movie, Fantasia, by the then new company Walt Disney. The war followed shortly thereafter which increased the market for the HP products.

 

[LvW]

Re: Wien bridge

.........
A piece of history. Back in the 1930s this circuit was developed by a Stanford student. The limiting device was a small incandescent light bulb. This student was either Hewlett or Packard.
.........

Yes, it was W.R. Hewlett in 1939 who invented this circuit in his master thesis.
As opamps were created only more than 20 years later, he did make use of two conventional tubes to create a positive gain of app. three -- stabilized with a tungsten lamp.

 

[flatulent]

Re: Wien bridge

A further piece of history. After the war the US electronics industry was expanding rapidly. The other test equipment makers had the business model of collecting orders and then doing one large production run (to improve efficiency of their company). This made a many month delay in the delivery of the products.

HP had the business model of continuous manufacturing and warehousing the products. Delivery was then made in a week after the order was placed.

You can guess which instrument maker captured the market and the other ones shrank in size.

 

[LvW]

Re: Wien bridge

Hi, Can anybody explain the procedure of "Wien bridge circuit" in brief please? Does it have any input signal? I think it is not anything more than a bandpass filter that is combined from a lowpass and a highpass in series, how does it produce Sine wave?

Back to the original question: As there is some sort of confusion - even in textbooks - on the terminology of this circuit (WIEN oscillator vs. WIEN bridge oscillator) here are some basics:

* The band pass consisting of two RC blocks is known as "WIEN path" (NOT WIEN bridge)
* If properly designed, the resistive negative feedback path forms together with the band pass a balanced bridge with a very small error signal which is amplified by the differential opamp.
* Thus, two interpretations of the circuit are possible: (a) A band pass filtered signal is amplified by a factor of three, or (b) The error signal of some mikrovolts is amplified by a factor in the order of 1E6 (open loop opamp gain).

Perhaps this helps a bit to understand the circuit operation and its terminology.

 

[aomidee]

Wien bridge

thank u all. It helped a lot dear LvW!
But would it produce a sine wave anyway? even when there is a square error signal? in other words I'll ask:can every bandpass (or lowpass) filter render a sine-like wave and reject the noise signals with unwanted frequencies ?!

 

[LvW]

Re: Wien bridge

thank u all. It helped a lot dear LvW!
But would it produce a sine wave anyway? even when there is a square error signal? in other words I'll ask:can every bandpass (or lowpass) filter render a sine-like wave and reject the noise signals with unwanted frequencies ?!

A harmonic oscillator can be built using any filter circuit which is able to produce a phase shift of 180 deg (or 360 deg) at one frequency only (low pass or band pass)!
This filter then is combined with an amplifier which has
- the appropriate gain (depending on the filter loss; in case of WIEN: gain of 3), and
- a phase shift of 180 deg (inverting mode) resp. 0 deg (non-inverting mode).

If both elements are connected in form of a closed loop (which then has a loop gain of app. unity and zero phase shift) you have - in principle - an oscillator. However, some additional circuitry is necessary to stabilize the amplitude; otherwise it will rise and be clipped due to voltage limitations (supply).
Does this answer your question?