rail to rail opamp...urgent need help

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maori

Joined: 02 Feb 2007
Posts: 11

12 Apr 2007 12:16 rail to rail opamp...urgent need help

im a newbie..an still didn't quite understand about this rail to rail operation.

here i attach my circiut....can anyone explain it..if in detail..i really appereciate.im using HSPICE as my tool.

1)i hope i can know about this circuit..how it flow..starting from the input
differential pair to the output of common source stage.

2)why we need current source in this cirvuit.

3)function of Cc in this circuit.

4)from the circuit in the middle is differential pair...and it is the input stage..in the
right it is the output stage...but in the left side...i really doono..what is that n the
function of it...please help me...explain it.

i really need help from anyone here...coz im quite stuck right now...PLEASE..

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acbalbason

Joined: 08 Apr 2007
Posts: 118
Helped: 10
Location: Chung Li, Taiwan

12 Apr 2007 18:38 Re: rail to rail opamp...urgent need help

hi!

i'll do my best! first your differential pair turns on if vin is correct. which will simultaneously turn on M9 and M10. given the right vds for m1 and m2, m3 and m4 will turn on, so and so forth until all transistors are on. from the 200 uA current source, the current will split into 100 uA each per leg (ideally). you are forcing 100 uA to pass through m1 for example. it is basically two pairs of pmos cascoded (m3 m4 and m9 m10) with nmos pairs (m1 m2 and m11 m12). the common source stage is an inverting amplifier. it is usually needed for very high gain. you need a very high gain for your amp to swing (voltage gain versus input voltage) from 0 to say up to 5 V (vdd). Cc is a compensting capacitor. since very high gain op amps have relatively small bandwdith, you need a compensating capacitor to increase its bandwidth effectively.

- al

xusoso

Joined: 18 Aug 2004
Posts: 91
Helped: 3

13 Apr 2007 8:45 rail to rail opamp...urgent need help

constant gm

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Vabzter

Joined: 18 Jan 2007
Posts: 31
Helped: 1

13 Apr 2007 9:19 Re: rail to rail opamp...urgent need help

maori wrote:

Hi,
You have a pair of differential input stage- the PMOS in the left and then followed by NMOS. Rail to Rail means that full swing from 0 to Vdd. Hence u need to have a parallel of NMOS and PMOS stages at input.
You need current source to have proper constant current in the input which will then be converted to voltage. When you are building this circuit you have to replace the ideal current source by a combination of MOS and current mirrors.
The Cc is used as a compensation capacitor so that your opamp will be stable. Are you sure that the other end of the Cc is grounded? and not connected to gate of M16?
Cheers
Vabzter