positive feedback in an op-amp

Discussion in 'General Electronics Chat' started by shreyas_bhat, Aug 9, 2005.

  1. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
    Hello All,

    I have a Twin-T oscillator, where the Twin-T (RC network) is connected between the negative input and output and the positive of the amp is grounded. The frequency of this oscillator is f = 1 / 2pi RC. (where R and C are the resistors and capacitors forming T-network in the feedback path).

    However, with the same connections between the negative and output, if I short the positive and the output terminal (so now, I have a -ve feedback and a positive feedback), and connect a resistor between the positive and ground, the frequency of oscillation is tremendously reduced and does not follow the f = 1/2pi RC.

    Can somebody explain to me the role of the positive feedback here and how it can cause a variation in the frequency. Or is there a way (any formula) by which I can exactly predict the frequency of oscillation with the positive feedback.

    Any suggestions are welcome.
  2. freetek

    New Member

    Aug 10, 2005

    Had to look myself since it has been some decades since I did any design work with them.
    One of the best explanations I have seen (that didn't drone on for days) is right here on this site:


    Since positive feedback in its most primitive form can make a latch of an op amp, the hysterisis will definitely drop the frequency from the point you might expect based on the RC time constants.
    It kinda wants to stay lit - not relax.