# Wien Bridge Oscillator Help!

#### ARealHero

Joined Dec 6, 2014
8
Hello all!

I have to design a wien bridge oscillator using a two stage BJT amplifier. I have done all kinds of research and found that I need to have a gain of roughly 3v/v, and my frequency of oscillation is equal to 1/(2pi(RC). The problem I am running into is properly biasing and then setting the gain of my circuit. I have attached a screenshot of my schematic and simulation output.

#### bertus

Joined Apr 5, 2008
22,305
Hello,

The R2 of the timing circuit is now part of the biasing of Q1.
The bias of Q1 is made with R2 and R5 now.
Is this intended?

Bertus

#### ARealHero

Joined Dec 6, 2014
8
It definitely is not intended! I will make a few modifications to my design and will post my new findings.

#### ARealHero

Joined Dec 6, 2014
8
Before I completely scrap this design (I just did some math trying to bias this circuit properly), do you have any suggestions about what could be going wrong here? I got this design from: http://www.daenotes.com/electronics/digital-electronics/rc-feedback-oscillators
I went ahead and posted the simulation as well. I can tell its doing something, but I have no clue what that something is...

#### bertus

Joined Apr 5, 2008
22,305
Hello,

Now R4 of the timing circuit and R3 are parallel and influencing the bias of Q1.
Put a capacitor between point B and the base of Q1 and see what happens.

Bertus

#### ARealHero

Joined Dec 6, 2014
8
I added the capacitor and it did not change much other than smooth out the initial spike

#### bertus

Joined Apr 5, 2008
22,305
Hello,ncy again

Try raising the timing resistors to about 50 K and lower the capacitors according to match the frequency again.

Bertus

#### ARealHero

Joined Dec 6, 2014
8
Hello!

I return with good news and bad news. The good news is it oscillates! The bad is it's oscillating at a frequency that is 10 times faster than I calculated...
As always I will post screenshots, but I was wondering if the reason it is oscillating so fast is because I am neglecting the input impedance of the BJTs or could it be that I messed up a resistor value and the gain of my circuit isn't what I think it is?

PS thank you bertus for all your help tonight!

#### bertus

Joined Apr 5, 2008
22,305
Hello,

It shows that the gain is to high as the signal is clipping now.

Bertus

#### Alec_t

Joined Sep 17, 2013
14,408

#### ARealHero

Joined Dec 6, 2014
8
Here is the schematic file!

#### Attachments

• 2.7 KB Views: 38

#### ARealHero

Joined Dec 6, 2014
8
After drawing my hybrid pi model of the two BJTs I am wondering if RB11 and RB12 effect the input impedance. I know with a typical voltage source they don't because of super position but does that change in this case due to the nature of the feedback loop? Is it necessary to include the feedback resistor in my gain analysis?

Also is it safe to assume that Vout of Q1 is equal to Vin of Q2?

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