I need some help designing an op amp phase shift ckt. Its the same ckt as here - http://www.maxim-ic.com/appnotes.cfm/appnote_number/559
The problem is it works fine in the audio range (upto 500khz). But when I increase the frequency to 1MHz and above, the amplitude of the output starts to drop as I shift the phase and increases to its original value as it reaches max phase shift.
I simulated the ckt in multisim, the ckt works fine at 6Mhz, but acts the same way when the frequency is increased to 20Mhz.
The op amp I used was LM7171.
Any help would be appreciated.