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What is the function of capacitor in this current source?

 
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John Xu



Joined: 22 Jul 2005
Posts: 59
Post14 Nov 2005 9:27   What is the function of capacitor in this current source?
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

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sunking



Joined: 25 May 2004
Posts: 914
Helped: 46
Post15 Nov 2005 2:30   What is the function of capacitor in this current source?
for setup.
initial the cap will make the lower is zero.
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alvays



Joined: 07 Jul 2004
Posts: 64
Helped: 3
Post15 Nov 2005 8:18   What is the function of capacitor in this current source?
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
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Davood Amerion



Joined: 01 Mar 2005
Posts: 588
Helped: 90
Location: Persia
Post15 Nov 2005 9:26   What is the function of capacitor in this current source?
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
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mady79



Joined: 30 Mar 2004
Posts: 88
Helped: 6
Post15 Nov 2005 10:11   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 ..
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