Phase Shift Circuit Oddity

Thread Starter

dannybeckett

Joined Dec 9, 2009
179
Hi everyone,

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

http://www.tedpavlic.com/teaching/os...se_shifter.pdf

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:

http://snag.gy/yfcPO.jpg

At 80KHz the output waveform is somewhat distorted:

http://snag.gy/0wA5i.jpg

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,

Dan
 

ramancini8

Joined Jul 18, 2012
473
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.
 

bertus

Joined Apr 5, 2008
20,650
Hello,

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.

Bertus
 
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