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Wein Bridge Oscillators - Diode Stabilization

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dzafar

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Hello there,

I cam across Wien Bridge Oscillators (attached below) but am having a hard time understanding why both the diodes are OFF at low signals?

My understanding says: when signal is low (i.e. negative), D1 will be ON and D2 OFF, and vice versa. Hence, the effective resistance will always be 22.1k || 100k.

Screen Shot 2018-08-01 at 12.52.33 PM.png


Thanks,
dzafar
 

Both diodes are off at low level output signals. Then the gain is higher and the circuit will increase the output level until the diodes conduct and cause a little distortion.
 

Voltage across 22.1kOhm can change from -0.6V to 0.6V about, between these values both diodes are "OFF". Beyond these limits the dynamic resistance of the diodes starts to drop exponentially, and with the 100kOhm resistor they start to shunt the 22.1kOhm resistor. It decreases the loop-gain to 1 and keeps the oscillation amplitude on constant.

So low signals mean the amplitude of the signal is low, not its momentary value. Doesn't matter the sign of its momentary value, e.g. negative or positive.

Simulate it to see how it works, easiest way.
 

Ohh!! So you mean, when the amplitude of Vo < 0.6 both diodes are OFF?
 

Ohh!! So you mean, when the amplitude of Vo < 0.6 both diodes are OFF?
Not exactly. The datasheet for a diode shows that it conducts a little with 0.6V across it and when cold. It conducts more when it has more voltage across it or when warm.
Different diodes have different amount of conduction at a certain voltage.
Here is from the datasheet of a 1N4148:
 

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  • 1N4148 diode conduction.png
    1N4148 diode conduction.png
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Ohh!! So you mean, when the amplitude of Vo < 0.6 both diodes are OFF?
Almost, but you didn't calculate with the feedback divider and the characteristic of the diode.
Under 0.6V forward bias voltage even silicon diodes conduct the current, temperature doesn't matter. The current exponential function of the voltage, and numerous current only can flow over ~0.6V...0.8V for silicon diodes.
When the output voltage amplitude is 0.6V, on the diodes the amplitude is only 0.4V because of the resistive feedback divider. So the output minimum amplitude should be Vo>0.9V if we say that numerous current only can flow through the diodes over 0.6V forward voltage.
 

I agree that the diodes are not parallel with the 22.1k feedback resistor. If they were then the output level of the oscillator would be much less and the distortion would be horrible.
 

A diode is "off" for 0 volts - and, of course, when it is reversely biased.
In case of forward biasing, the diode starts to conduct for voltages > 0 V.

However, the voltage-current relation is exponential and the current for - let`s say - 0.3 volts is remarkably smaller than for 0.6 volts.
It is an oversimplified view to think that the diodes are "off" for voltages < 0.6 Volts. That`s simply wrong!
In particular, for smooth amplitude limiting in a WIEN oscillator the voltage-current relation for voltages < 0.6 volts is important.
Otherwise (on(off at 0.6V) , we would have a hart limiting effect and we wouldn`t need the diodes at all.
However, the real behaviour of the diodes allows a smooth increase of the feedback factor for rising output amplitudes.
As a result, the total harmonic distortion of the output signal will be rather low (depending also on the ration of both feedback resistors (100k/22k)

(Please note that the oscillator was named after Max WIEN - not "WEIN".
 

Yes. I didn't see the typo, and as far as it is called sometimes Wien-Robinson Bridge oscillator.
 

In a Texas Instruments report SLOA060 about sinewave oscillators, they avoided using distorting diodes for AGC and instead used a Jfet for distortion less than 0.2%.
 

In a Texas Instruments report SLOA060 about sinewave oscillators, they avoided using distorting diodes for AGC and instead used a Jfet for distortion less than 0.2%.

In fact, there are some other methods for smooth amplidude limiting: OTA as voltage-controlled resistor, thermistors (pos. oer neg. temp co), phase-sensitive mechanisms, slew rate control,...
 

Like Hewlett and Packard, my first Wien Bridge oscillator used a light bulb for its AGC. The signal level bounced a lot when the frequency was changed. Now it uses a low distortion opto-Jfet.
 

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