https://open-source-silicon.dev logo
#analog-design
Title
# analog-design
e

Eslam Morsie

05/18/2021, 7:35 AM
Hello all, I am designing Miller OTA but there is a problem in the design always the transistor M5 off although in my calculation I ensure it is in sat could anyone help me what is the problem in these design steps?
j

J. Scott Elder

05/18/2021, 12:16 PM
How do you know the transistor is off? Did you run a simulation?
e

Eslam Morsie

05/18/2021, 12:16 PM
Yes after simulation
j

J. Scott Elder

05/18/2021, 12:17 PM
Did you close the loop in your simulation?
e

Eslam Morsie

05/18/2021, 12:19 PM
I run it as open loop to check first the DC operating point so I didn't proceed to closed loop
j

J. Scott Elder

05/18/2021, 12:20 PM
You can't simulate such a circuit open loop. Too much forward gain.
e

Eslam Morsie

05/18/2021, 12:31 PM
I simulated the same OTA with input pair NMOS and it works when I tried the PMOS input pair I faced this problem.
Although I used the same methodology.
j

J. Scott Elder

05/18/2021, 12:57 PM
Then you were lucky. You had a perfectly balanced circuit. If you want to study your amplifier, close the loop, then study the performance. If you want to study the AC performance, then close the loop through a huge pole in the feedback. Put a huge inductor from the output to the minus input. Put a huge capacitor from the minus input to ground. Then you can see the AC performance in the open loop and the dc performance in the closed loop. This is how engineers in industry study closed loop systems, although there are multiple other methods to break the loop for AC.
👍 4
e

Eslam Morsie

05/18/2021, 1:03 PM
ok, thanks sir.