Problem with current in gm-stage

[Winny_Puuh]

Hi guys,
I want to convert signals in the range of 10 µV to a reasonable current (1 nF).
I’m using a gm-stage like shown in the picture but somehow the output current is 160 fA (actually it should be iout = gm*uin = 27 µS *10µA). I think there has to be a major mistake as the results are off by a factor of 1000 and more. If someone of you sees the problem, because it’s obvious, please tell me. I need help quickly!

 

[nitishn5]

Your input node i.e. the output of V1 has no DC offset to set the input common mode.

You need a DC source same as V10 in series with the AC source.

 

[Winny_Puuh]

Thank you for your answer but even though it's not shown in the schematic I have a DC offset for V1 of 900mV. So that should not be the problem... Just don't get what's wrong there... <__<

 

[nitishn5]

Check the DC Operating point and make sure all the transistors are in saturation.

I don;t know how you have designed the circuit. Your sizes look a bit strange. Your NMOS input pairs are L=0.3u and your PMOS are at L=1u. how did you arrive at this?
You can also use a simple biasing circuit for the tail current instead of driving a voltage at the gate. Use a current source and mirror it into the tail current.

 

[monsoon]

Your dc operating point is not proper because the way the load resistance has been connected. Because one terminial of R0 is connected to ground, drain voltage of M3 will be 0 under DC conditions.
Also 10G is too large a resistance. It requires 27 µS *10µA*10G=2.7V swing at the output. Connect the R0 to 0.9V (instead of 0) and use a smaller resistance (say 100M)

 

[mretsh91]

I know it is a small note but ...why are u connecting the body of the input transistors to the source, normally we connect the body to the lowest voltage in the circuit

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also the lower transistor, shall be biased by a current source to reject the cm signal

 

[yuvan]

Hi Winny,

How much is the current in M8? I guess you wanted it to work as a current source!