Slew rate simulation in opamp

[-CAM-]

Hi,
i've designed a CMOS folded cascode opamp to have a SLEW rate of 150 V/us (Iss of the differential pair is 30 uA and CL = 200 fF) in unity gain configuration. I found that ICL is about 1 mA.I saw current in the differential pair is unbalanced in one branch only for a very quick time so the capacitor can't charge like i would.
So i would know if this tjhe normal behavior.
Thanks.

 

[Junus2012]

Hello
the slew rate is a large signal behavioural of the circuit. when you connect the op-amp as a buffer and apply a large pulse signal (like 1vp-p) there is at time where all the current is passing from one side of the op-amp (depending whether it is rising edge or falling edge) before it settle and the end , this time is the slew rate.

in the folded OTA the maximum current at the load is the OTA differential biasing current (ISS) that is in your case is 30uA if you didnt use a buffer.
if you used a buffer the the load current will be defined by the buffer and the slew rate will be defined by the compensation capacitors Cc and Iss

 

[-CAM-]

Hello
the slew rate is a large signal behavioural of the circuit. when you connect the op-amp as a buffer and apply a large pulse signal (like 1vp-p) there is at time where all the current is passing from one side of the op-amp (depending whether it is rising edge or falling edge) before it settle and the end , this time is the slew rate.

in the folded OTA the maximum current at the load is the OTA differential biasing current (ISS) that is in your case is 30uA if you didnt use a buffer.
if you used a buffer the the load current will be defined by the buffer and the slew rate will be defined by the compensation capacitors Cc and Iss

Thanks for the answer. The point is that Iss is reachead only for an istant after there the balancing between the currents of the differential pair. So, i can t say CL is charged by Iss, cause this is true only for an istant. Instead, if i use open loop configuration, i can see the theorical behavior.

 

[Junus2012]

Hi again
at the steady state small signal condition and if your curcuit is balanced there will be no current pass in to capacitor. the charging and discharging is only when a large differential input signal is applied.

as I told you before that if you are not using any kind of buffer then Iss is your charging and discharging current. therefore if you go to any literature for the folded you will find this expression
S.R = Iss/CL.

I dont know how you are getting 1 mA from 30 uA biasing current ??? are using buffer ??

 

[-CAM-]

Hi again
at the steady state small signal condition and if your curcuit is balanced there will be no current pass in to capacitor. the charging and discharging is only when a large differential input signal is applied.

as I told you before that if you are not using any kind of buffer then Iss is your charging and discharging current. therefore if you go to any literature for the folded you will find this expression
S.R = Iss/CL.

I dont know how you are getting 1 mA from 30 uA biasing current ??? are using buffer ??

Sorry, i try ti explain better. If my opamp is in unity gain configuration the transient current in the capacitor reaches only for one istant the value Iss, so during the input ramp, the capacitor doesn't charge with Iss (because this value is reached only for one instant). I know theory so i' don't understand why in unity gain configuration the transient current is 1 uA average and not Iss

 

[Junus2012]

So now you are talking about 1µA but in your first post you said 1mA ??

of course if you take the average of the output current it will be less than Iss because Iss is not passing in the load when it settles to the final value. after the op-amp settle the op-amp is under small signal condition where (VID=0) and no current passing in the load
if you increase the frequency of the input pulse signal you will see that the average power is increasing because the slewing time is taking more place in the higher frequency signal

Sorry, i try ti explain better. If my opamp is in unity gain configuration the transient current in the capacitor reaches only for one istant the value Iss, so during the input ramp, the capacitor doesn't charge with Iss (because this value is reached only for one instant). I know theory so i' don't understand why in unity gain configuration the transient current is 1 uA average and not Iss

 

[-CAM-]

yes, i was wrong...the correct value is 1 uA...

 

[Junus2012]

Ok, then please refer to my previous post, there is your answer