mburakbaran
Member level 2
Hello,
I'm designing a high Input common mode range opamp in order to used it in buffer configuration in ams 0.35. I'm using some kind of a folded cascode topology. In order to realize the high swing at the input terminal, i am using complementary mosfets. I mean there are 2 parts of the opamp one is built with nmos transistors other is with pmos transistors. when one side is closed, other comes into play the supply the swing need. of course there is a region where both of the parts are working at the same time.
I'm applying 450m-4.6V ideal pulse into the input terminal.
The pulse response is quite strange.
The output signal does its normal slewing, before catching up with the input (about 20mV below the input level) it does some kind of an overshoot (but it is not an overshoot, as the output signal level didnt go over the input signal level yet), it falls about 10mV or so and then it starts rising again and finally catches the input with a 4mV error.
When i apply a small pulse like 2V-3V, it behaves strangely, too. It does a pure overshoot, but it does strange ups and downs till settling.
Phase margin is 64 degrees, i dont think it is about stability. I dont think it is about the damping coefficient either.
Any ideas?
By the way, i am using a flipped voltage follower at both complementary inputs.
Note:This strange behaviour is most significant when the temp is about -196 celcius.
I'm designing a high Input common mode range opamp in order to used it in buffer configuration in ams 0.35. I'm using some kind of a folded cascode topology. In order to realize the high swing at the input terminal, i am using complementary mosfets. I mean there are 2 parts of the opamp one is built with nmos transistors other is with pmos transistors. when one side is closed, other comes into play the supply the swing need. of course there is a region where both of the parts are working at the same time.
I'm applying 450m-4.6V ideal pulse into the input terminal.
The pulse response is quite strange.
The output signal does its normal slewing, before catching up with the input (about 20mV below the input level) it does some kind of an overshoot (but it is not an overshoot, as the output signal level didnt go over the input signal level yet), it falls about 10mV or so and then it starts rising again and finally catches the input with a 4mV error.
When i apply a small pulse like 2V-3V, it behaves strangely, too. It does a pure overshoot, but it does strange ups and downs till settling.
Phase margin is 64 degrees, i dont think it is about stability. I dont think it is about the damping coefficient either.
Any ideas?
By the way, i am using a flipped voltage follower at both complementary inputs.
Note:This strange behaviour is most significant when the temp is about -196 celcius.