What is the function of capacitor in this current source?

[John Xu]

Hi,
Pls. see the attached schematic. It is a supply -independent current sourrce, there is a capacitor C1(which is always implemented by a PMOS cap). Can anyone pls. tell me the functions of this capacitor?

Is it a decoupling cap to increase the PSRR??? And how we can select the value of it?

Thanks in advance

 

[sunking]

for setup.
initial the cap will make the lower is zero.

 

[alvays]

sunking:
do you really think it that? yet i consider it as a decoupling capacitor to suppress the interference and noise.
If i'm wrong, would you please annotate it?
INDEX: reference decouple capacitor

 

[Davood Amerion]

John Xu, I agree with you. I think it is a decoupling cap to increase the PSRR.
it's value depend on current flow on the middle leg of figure!, noise attenuation and frequency which we want to eliminate (eg. 50Hz).

Regards,
Davood Amerion

 

[mady79]

Re: What is the function of capacitor in this current source

Normal practice in any designs to have a small decoupling cap at bias node for psrr and ac coupling reasons.

cap at bias node will remove any noise injected from other circuits in reality or noises coupled from supply .
hope this will answer ..