Phase Shift Circuit Oddity

Discussion in 'General Electronics Chat' started by dannybeckett, Aug 7, 2012.

  1. dannybeckett

    Thread Starter Senior Member

    Dec 9, 2009
    Hi everyone,

    I am building a lock-in amplifier, of which a quadrature phase shift section is required. I have constructed the circuit here:

    Which does work very nicely. I have chosen 5K1 resistors for the feedback, a 100K trimpot and a 3.3nF capacitor for the RC circuit. This gives me a range of about 500Hz to 100KHz at 90 degrees phase. The problem I am having is that at higher frequencies, the output waveform is somewhat distorted.

    At 1KHz everything is OK:

    At 80KHz the output waveform is somewhat distorted:

    At each frequency I adjusted the trimpot to achieve a 90 degree phase shift. I am just wondering what could be the cause of this distortion? Could I be running into bandwidth issues with the op amp I'm using? (OPA4137)

    Thanks for any help,

  2. ramancini8

    Active Member

    Jul 18, 2012
    I am suprised that you can get any response at 100KHz because the 741 open loop gain is very low at that frequency. You need a higher frequency op amp for higher frequency operation. Try a TL081.
  3. bertus


    Apr 5, 2008

    The used OPA4137 has a GBW that is comparable with the LM741 (1 Mhz).
    The suggested TL081 ,by ramancini8 ,has a GBW of 3 Mhz.
    If you want a better opamp than that you could take a look at the NE5532, wich has a GBW of 10 Mhz.

  4. ramancini8

    Active Member

    Jul 18, 2012
    Or you can use an LMH6514 that has a 100 MHz BW.
    Or you can get higher frequency op amps.