problem with oscillator

Discussion in 'General Electronics Chat' started by shreyas_bhat, Feb 2, 2005.

  1. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
    47
    0
    I am trying to design a Twin T oscillator. The Twin T network was connected between the inverting i/p and the output of an op-amp, and the non-inverting input was grounded. I looked up this design over the net, but this doesnt seem to oscillate. However, when a feedback resistor is connected between the output and non-inv input, it started to oscillate. Could somebody please explain the working ?

    Also, I expected that the frequency of oscillation be a function of the resistor and capacitor in the Twin T circuit. However, even a variation in the positive feedback resistor seems to vary the frequency, which is not what I want. Can somebody suggest certain steps to overcome this problem.

    Thanks
     
  2. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    Only way to get oscillations is through feedback normally, especially with an opamp.

    Its like a swing. If you have negative feedback, it likes someone pushing the swing from one side while on the other side someone grabs the swing and slows it down. The whole negative thing is that the signal is 180 degrees out of phase compared to the orignal. Now, if you happen to have positive feedback is likes both people pushing the swing in the same direction so it keeps going higher and higher and your oscillator starts. For the swing, its frequency of oscillations has to do with its length of chain, for your circuit it has to do with component selection.

    The feedback resistor shouldn't have much of any effect on oscillations, rather it should effect the rate at which your oscillations reach their peak value.

    Could you possibly post a schematic? Wouldn't mind checking it out..
     
  3. vineethbs

    Well-Known Member

    Nov 14, 2004
    56
    0
    one sec ,shreyas ! u hav a twin T n/w that is basically a notch filter , i can't really understand the ckt , so correct me if i am wrong .

    the notch filter is your freq selection ckt , so actually to get an oscillation at a particular freq u can subtract the signal from the o/p of the notch filter , this wud give u a bandpass response

    so the oscillation wud occur only if u feedback the o/p (no filtering at + terminal ) - the notch filter o/p , so the addition of a feedback resistor is important , but it shudn't really affect the freq , i think

    thanx brandon for the exp of feedback!
     
  4. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
    47
    0
     
  5. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
    47
    0
    I tried to attach the schematic but it doesnt go through. Anyways, lemme explain once again. Between the output of the op-amp and its inverting input, I have the Twin T filter circuit. And in between the o/p of the op-amp and the non-inverting i/p is a resistor (R4), and there is also a resistor between the non-inv pin and the ground. The only source of power is the DC to drive the op-amp.
    Variation in R4 causes the oscillation frequency to vary, which should not ideally happen. If I had to retain the Twin T circuit and build an oscillator out of it, are there any suggestions ?

    Thanks
    Shreyas


     
  6. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    If I read you correctly, you had a circuit which had the +ve input (non-inverting) grounded, and it didn't work.

    Now you have connected a resistor from the o/p to the +ve input. That's a receipe for disaster because you are introducing uncontrolled positive feedback and the results will be unpredictable. I suggest you go back to your original circuit and find out why it is not oscillating.

    It looks like it could have been wrong biasing on the inputs. Measure the DC voltage at the output. If it is running with correct bias it should be about half the supply voltage. If it is very much higher or lower it is likely that you have a bias problem, ie. the DC voltages difference between the two inputs is too big.
     
  7. vineethbs

    Well-Known Member

    Nov 14, 2004
    56
    0
    but pebe the ckt actually takes the difference between the notch filter output and the positive feedback ,so is it uncontrolled ?

    i think mebbe the gain of the twin T network mebbe a problem
     
  8. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Hi vineethbs,
    You are most probably right. I don't know the circuit but as originally designed it, presumably, worked.

    I was thinking that as the original circuit had its non-inverting input earthed, it seemed rather drastic to introduce positive feedback which would probably not improve matters.

    It's a pity we couldn't see the circuit. shreyas_bhat has tried to upload without success. I've tried to upload with two different file formats - without success. Can anyone say which file extensions are acceptable to this forum?
     
  9. vineethbs

    Well-Known Member

    Nov 14, 2004
    56
    0
    hello all , i simulated a ckt with a twin T n/w in the feedback path , but what am getting is a square wave , too much gain i think , reducing the gain of the amp is not really solving the problem . what to do :blink:
     
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