OTA transconductance test bench for simulation

Hi,

I am designing a simple two stage OTA as show in the figure. can anyone suggest the test bench for simulating the transconductance of the OTA.

Pls do explain with figures if possible. I tried looking into lot of test benches already uploaded but it did not make much of sense to me.

ex: https://www.edaboard.com/threads/248195/

but mine is a single ended output OTA these figure in the link mentioned was confusing for me, and also the Cmref output. I could not understand what it was.

Hi,

it depends a bit of what are your specifications and what do you want to see!
For me, when I design OpAmp, I want to know (for example) what is the DC gain, the gain bandwidth (GBW), the phase margin...
The testbench is quite simple, I'm using Transient and AC analysis. With DC sources, sets the input voltages where your OTA should work and place 2 AC sources. (1V AC, 1mV Amplitude, 50Hz)
Here is a snapshot of my test bench. It's for a fully differential OpAmp but it's quite similar. I'm also simulating the input common mode in order to see the effect at the output.

Hope it helps...

vijith133 said:

Hi,

I am designing a simple two stage OTA as show in the figure. can anyone suggest the test bench for simulating the transconductance of the OTA.

Pls do explain with figures if possible. I tried looking into lot of test benches already uploaded but it did not make much of sense to me.

ex: https://www.edaboard.com/threads/248195/

but mine is a single ended output OTA these figure in the link mentioned was confusing for me, and also the Cmref output. I could not understand what it was.

Click to expand...

Ok when simulating the ota tranconductance we want to see the output current divided by input voltage. Lets assume you have a single ended ota that is designed properly. So you have two inputs vinpos and vinneg and one output vout. Connect a dc source to both vinpos and vinneg. Set its value to be your input common mode ( set it as half of your supply voltage) . Then connect another dc source at the output with the same voltage ( half of supply). The reason for this is that you need to have an ac group at your output to be able to measure the output current. To find Gm ( transconductance) run a dc sweep on the input ( sweep the input dc voltage from 0 to your supply voltage) and then measure the output current at the top pin of the output dc source ( your measuring the output current that passes through this source ) . After this you'll get a curve that is the current vs voltage ( it should be linear for a range of the voltages) . Then simply take the derivative of this graph . The result would be your gm vs vin graph. the vpoltage range for whoch the gm is almost constant is the max signal that eould result in the same gm value . This has been also been explained in detail in chapter 2 of razavi' s book

vijith133 said:

Hi,

I am designing a simple two stage OTA as show in the figure. can anyone suggest the test bench for simulating the transconductance of the OTA.

Pls do explain with figures if possible. I tried looking into lot of test benches already uploaded but it did not make much of sense to me.

ex: https://www.edaboard.com/threads/248195/

but mine is a single ended output OTA these figure in the link mentioned was confusing for me, and also the Cmref output. I could not understand what it was.

Click to expand...

I am a bit surprised. You have designed this circuit that certainly is slightly more advanced than just a basic amplifier stage - and you are asking how to test it´s main property?
Are you really asking how to simulate the output current as a function of input differential voltage?

LvW said:

I am a bit surprised. You have designed this circuit that certainly is slightly more advanced than just a basic amplifier stage - and you are asking how to test it´s main property?
Are you really asking how to simulate the output current as a function of input differential voltage?

Click to expand...

Sorry I guess i have confused you with my question, i was inquiring about the Gm V/s Frequency sweep.