# Wien Bridge Oscillator with PSPICE

Discussion in 'The Projects Forum' started by Hyperian, Apr 25, 2008.

1. ### Hyperian Thread Starter Member

Apr 25, 2008
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I've read the other wien bridge project in this forum but while they've been helpful, in the end i cant get it to work.

I actually got the real circuit to work, the original Wien Bridge described in wikipedia: http://en.wikipedia.org/wiki/Image:Wien_bridge_classic_osc.png
I used actual light bulb and everything.

But with PSPICE(have to use it, stupid uni) i cant put a light bulb in so i've been looking around and found this:
http://www.ecircuitcenter.com/Circuits/opwien/opwien.htm
I installed LTSPICE so i can look at the .clr file and it works great. But when i try to move that circuit to PSPICE i am left with the current pulse that i couldnt put in.

I think i'm stuck with not knowing how to put a current pulse in the beginning. I read in some other thread with the solution where you add initial condition but i have no idea how i can do that with PSPICE.

2. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
The Initial Condition command is explained on that same site.
Code ( (Unknown Language)):
1.
2. .IC - Initial Transient Conditions.
3.     .IC { {vnode} = {value} }*
4.     Examples:
5.     .IC  V(2)=3.4  V(102)=0
6.
In the example, the voltage for node 2 is set to 3.4v, and node 102 is set to 0v
Various other SPICE commands:
http://www.ecircuitcenter.com/SPICEsummary.htm

3. ### Hyperian Thread Starter Member

Apr 25, 2008
13
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thanks for the reply to that site. I added the .IC and put it into the non inverting amp side and it worked! thanks!

but now i have another question... Since in PSPICE circuit you dont have a bulb, how come the circuit still oscillates? I thought the circuit oscillates because it has a light bulb? does that mean my real circuit will oscillate too if i just put a resister in place of a light bulb?

4. ### Wendy Moderator

Mar 24, 2008
21,823
3,039
The light bulb is a form of AGC (Automatic Gain Control). It will limit the gain of the circuit so a symmetrical and pretty sine wave is generated by increasing it's resistance as AC (the sine wave) goes through it. Any amp will oscillate if it has more than unity gain in feedback.

I have seen bulbs used for RF gain control also, think of them as a current controlled resistor, the more current, the more resistance.

5. ### Caveman Senior Member

Apr 15, 2008
471
1
They also act as if they have a low-pass filter on the resistance change as well. This is due to the fact that the resistance changes due to heat. And the heat added is defined by the rms power through the bulb.
Also, it works best if only slightly on because when you get too hot, the difference between the bulb temperature and the ambient cause nonlinearities in the input power to resistance curve.

6. ### Hyperian Thread Starter Member

Apr 25, 2008
13
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So.. if i understand right.. the amp will always oscillate if it has more than unity gain in feedback, but the light bulb gives make the oscillation nice?

i get how the RC in the non-inverted part of the circuit works, but i dont know how both sides of the circuit works together

7. ### Caveman Senior Member

Apr 15, 2008
471
1
The light bulb causes the amplitude to stop growing by reducing the gain to exactly one. This creates a nice low-distortion sinewave output. Otherwise, the op amp will simply clip and output a distorted wave.

Okay, so basically the negative feedback and positive feedback cancel one another.

A Wein bridge has a magnitude feedback of Vout/3 at its center frequency. The negative feedback is the light bulb divider. When there is a low amount of rms voltage going through the light bulb, it has a low resistance and therefore has very little negative feedback going back to the input. So the positive feedback will dominate and the output amplitude will grow.

However, as the amplitude grows, the negative feedback path will become stronger and stronger until the sum of the positive and negative paths combine to a gain of 1.

8. ### Hyperian Thread Starter Member

Apr 25, 2008
13
0
Ok.. but i still dont get how I can simulate the same thing in PSPICE with a resistor instead of a lightbulb.