stability and compensation

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pillar_chen

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voltage follower stability pole zero

I designed a opamp with rail to rail input and push pull output. there are 77 degree phase margin and 12dB gain margin from the ac simulation. but the voltage follower built by the above opamp was oscillating when the follower is stimulated by a pwl pulse with step 100mV.
what's the reason? how to design the opamp to get a stable voltage follower?
 

What your mean "the voltage follower built by the above opamp"?
Oscillating always comes from the un-stable opamp, attach the phase margin, open loop gain and unit gain bandwidth.

Bg,


 

Ok, using the opamp form a voltage follower. phase margin is 77 and unity gain is 4.6MHz, and the DC gain is 89dB.
thanks
 

what is your time duration of the 100mV pulse and Tr/Tf...as ugf is ~4Mhz, so it is important
 

the rise and fall time are all 1ns, and the period is 5us in transient simulation.
 

does the load of .ac and .tr is the same?
 

Change ur pulse source parameter,for example tr=tf=100ns,pulse width=100ns,etc.then run transient analysis, see whether the opamp is oscillating or not!
 

1. How many poles and zeroes do you have before the unity gain point?
2. Are you sure your circuit for simulation works in the correct region?
Well, you have rail-to-rail input, what is your output range? also rail-to-rail or not?
 

check if you got any pole zero doublets ,if phase margin looks good in open loop gain .
but again check the common modes for input and output ,if your output common mode voltage margins so that both nmos and pmos of push pull are in saturation i.e.,s vdd-vov and vov (vov is over drive voltage vds-vgs-vt) .you input has to be with in the allowable ouput common mode voltages if you want to operate this opamp as a voltage follower .rail -rail input does not make much difference until you find voltage range common to input and output common modes.

Hope this helps !
 

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