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FET Based Phase Shift Oscillator

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Mehrzud

Mehrzud

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Hi
I need help with design of a FET based wien bridge oscillator and simulate it using pspice. Any suggestion?
 

Re: FET based Wien-Bridge Oscillator

Mehrzud said:
Hi
I need help with design of a FET based wien bridge oscillator and simulate it using pspice. Any suggestion?
Click to expand...

Yes. My suggestion is that you specify your question a little bit deeper.

Just for a start, you can always search for it on the web.

Get some ideas what you're dealing with (what is it? how it's workin?). That's my suggestion :D
 

Re: FET based Wien-Bridge Oscillator

Can anyone help with parameters and how to get the desired results?

f=1/(2*pi*R*C*√6)
|A|=gm*RL
RL=(RD*rd)/(RD+rd)
55_1212502958.jpg
 

Re: FET based Wien-Bridge Oscillator

Your schematic does not show a WIEN bridge oscillator.
It is a phase shift oscillator based on high pass elements - not the best choice.
The gain of the FET stage has to be at least G=-29.
Moreover, the load must contain also the input impedance of the cR-stages.
It is better to use an opamp.
 

    Mehrzud

    Mehrzud

    Points: 2
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Re: FET based Wien-Bridge Oscillator

1) You are right. It's a phase shift oscillator.
2) I have no problem with the op-amp one, the problem here is how to see the oscillation results in Pspice by using FET?
 

Re: FET based Wien-Bridge Oscillator

Mehrzud said:
1) You are right. It's a phase shift oscillator.
2) I have no problem with the op-amp one, the problem here is how to see the oscillation results in Pspice by using FET?
Click to expand...

What do you mean with "...to see the .......results in PSpice by using FET " ??
If the circuit is designed well you´ll see the results. Or did I misunderstand the question ?
 

FET based Wien-Bridge Oscillator

The first RC phase-shift stage is supposed to be driven from a very low impedance, not the high resistance of the 10k drain resistor.

The very high value 10k drain resistor must be loaded by a much higher impedance, not the low impedance of the phase-shift stages.
 

Re: FET based Wien-Bridge Oscillator

What do you mean with "...to see the .......results in PSpice by using FET " ??
If the circuit is designed well you´ll see the results. Or did I misunderstand the question ?
Click to expand...

I mean what the value of elements should be to see the oscillation in time domain analysis of PSpice?(maybe a sine wave)
 

Re: FET based Wien-Bridge Oscillator

I don't think a single FET will work in that circuit because its voltage gain is too low and its output impedance is too high.
 

Re: FET based Wien-Bridge Oscillator

Would you please suggest some changes while the main idea is unchanged? I mean a phase shift oscillator with just a single FET even with imaginary parameters(high gain, low output impedance).
 

Re: FET based Wien-Bridge Oscillator

Practically, having a RC chain charateristic impedance >> 10k would allow circuit operation. This seems possible with a FETs input impedance, not considering the disadvantages of a high impedance network.

However, I suggest to take LvW's doubts seriously. I fear, the circuits performance would be rather bad anyway.
 

    Mehrzud

    Mehrzud

    Points: 2
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Re: FET based Wien-Bridge Oscillator

Mehrzud said:
I mean a phase shift oscillator with just a single FET even with imaginary parameters(high gain, low output impedance).
Click to expand...

So you intent to create a circuit with "imaginary" behaviour ? High gain and low output impedance with a single FET seems to violate the physics.
You have to decide what you really want.

I think, in principle it could be possible to work with the phase shift principle and a single FET - however the design is very critical because it contains several parameters which have large tolerances (Rout of the FET, Gm of the FET). And in addition, all CR stages should be designed with a high input impedance (C small and R high).
And dont forget: the gain of the FET has to be larger than G=-29 !!
 

Re: FET based Wien-Bridge Oscillator

Hi LvW
What values do you offer?
 

Re: FET based Wien-Bridge Oscillator

Mehrzud said:
Hi LvW
What values do you offer?
Click to expand...

It isn´t so easy ! The circuit has to be designed carefully. Analog circuit design means to select parts and values out of an unlimited number of alternatives.

In your case: Which Fet ? Which transconductance ? Which output resistance ? Which bias scheme ? ........?

I am really not in a position to "offer" you something. Try different alternatives and find out via simulation if you find one set of parameters which satisfiy you.
Is it really an absolute requirement to use only one single FET ?
 

Re: FET based Wien-Bridge Oscillator

Yes, Just a single FET, no matter the amplitude of oscillation or it's frequency just a single FET.
It's something like a competition between friends!:|
 

I simulated a Jfet amplifying stage. It has a high impedance load and has an output of 1.35V peak with an input of 0.1V peak so its voltage gain is only 13.5.
The phase-shift oscillator will not oscillate if the transistor's gain is less than 29.
 

Operational example, using high impedance RC-chain as suggested.

 

    Mehrzud

    Mehrzud

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Thank you, Thank you all indeed. Just a last question. Why j110? I tried the same bias configuration using 2N5555/6/7/8 and nothing happened!
 

To achieve a gain of >29, the FET must have a sufficient transconductance. It must be >3 mmho (mS) respectively > around 5 considering the output load by RC network and FET output conductance. This is most easy with a low ohmic switch FET type as J110, already difficult with suggested 2N3819 and impossible with many popular signal FET.
 

    Mehrzud

    Mehrzud

    Points: 2
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Very good. Is there any website to find the desired electronic elements having something like advanced search or the choose is made by experience?
By the way I used Orcad 16.0.1 and the result was nothing even when I changed the gm parameter of J110 PSpice model. but the EWB version worked fine just with a change in transistor!
 

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