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fotios 26th July 2009 10:41 AM

Lower closed loop gain? Higher slew rate.
From experiments on workbench:
In an amplification arangement (regardless if it is a Leach or Stochino or Slone or Self architecture) as much the closed loop gain of amplifier is reduced, in the same time its slew rate is increased - and vice versa.
Why this?


infinia 26th July 2009 11:49 AM

BW increases with dominant pole compensated amps for lower CL gains. Perhaps if you adjusted that pole (Vas stage) to make the final BW equivalent, SR would remain constant. You may have to adjust the phase margin too as a function of CL gain, to keep stabilty and transient loop performance equal. I think if we make the CL gain very low then BW can get too high making things unstable, as other stages poles come to play. That's really the whole rational for dominate pole compensation topologies (ie insuring Vas stage determines the BW and stability above the other stages).

jcx 26th July 2009 04:45 PM

usually we are concerned with/talk about slew rate limit - not the linear circuit low pass filter square wave response initial slope/rise time

for conventiaonal topologies slew rate limit is a hard saturation of the input stage with all of the input diff pair tail current charging the Cdom - this limit doesn't change unless you change the input stage current limit or the Cdom

fotios 26th July 2009 04:48 PM

Thanks Infinia for your explanation. You are correct, and i can perceive what you refer.
I talk about a small - discrete implemented - operational amplifier with double LTP in input and seperate VGS per each side, operating in non-inverting mode. The Miller capacitance is compensated in each VAS. I am experimenting with the gain obtained by changing the ratio of the main feedback loop resistors. Accross the legs of feedback resistor it is connected a compensation capacitor. If i give a ratio of: Rf / Ri = 1, then the Av = 2....( Av = 1 + Rf / Ri ) but the rise time it is big. The value of the cap accross the Rf resistor it is OK to not presented overshoot. By reducing the value of this cap to get a lower rise time, i have overshoot. Increasing the value of Cdom caps, the overshoot dissapears but the rise time it is worsen again. Then, i turned in the changing of Av. If it is reduced at 1,1 by placing a ratio of Rf / Ri = 0,1 then the things are impoved. I have a vey low rise time ( arround 800nsec) without overshoot by increasing the value of cap accross the Rf resistor. The Cdom capacitors are not changed. It is very interesting that, this rise time of 800nsec remains the same from 1 to 100000 Hz. And that there is not any overshoot in this whole bandwidth, and the amplitude is stable. I have not checked above 100KHz.
To make the thing short, i was playing with the ratio of Rf / Ri and the value of compensation capacitor accross the Rf. All the above are obtained on the workbench with the trial and error method and with the aid of square waves produced from a 10MHz Hameg function generator and a 150MHz Hameg DSO. I don't know if my conclusions are wrong, but the instruments shows good things.


infinia 27th July 2009 01:41 AM

I would't expect SR to change too much unless you drop the Vstep level finding if you are large signal SR limited.

be interesting to determine the f-3dB BW for Av = 1 a follower... you know sweep the sine wave until it's a triangle? thinking it must be more than 500KHz.

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