Why Isn't My Gm-C Integrator Design Working as Expected?

[samiurrehman]

Gm-C Integrator design...

Dear all,

I have been asked to design Gm-C continuous time integrator with no rigid specifications. I have used a two stage OTA with 46dB gain and applied a capacitor at the end as integrating element. But the circuit is merely acting as an amplifier and not integrator. It does not convert square wave into triangular... What modifications do i need to bring? Circuit schematic attached...

http://images.elektroda.net/32_1284460009.jpg
the circuit quite well acts as a low pass filter after installation of capacitor at the end...I believe just a capacitor at the end can't transform an amplifier into integrator...

 

[Bob S]

I believe you need a large series resistor R between the amplifier and capacitor C, such that RC is long compared to the integration time. For 1% linearity, RC>=100 x square wave integration time.

Bob S

[added]
Suppose after t=0 for square wave input, V_out(t) = V₀[1-e^-t/RC]

Then dV_out(t)/dt = [V₀/RC]e^-t/RC

To achieve integration linearity of 1% (meaning dV_out(t)/dt = constant) , e^-t/RC >= 0.99 for all t