What's the MOS function added between 1st and 2nd stage ?

• May 30, 2005

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huangjw

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about the mos'S effect

in two stage op-amp or comparator ,between the first stage and second stage ,we add a diode connect MOS.please ,the MOS's function

 

• May 30, 2005

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airace

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Re: about the mos'S effect

compensation??

 

• May 30, 2005

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huangjw

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about the mos'S effect

maybe,but i am not sure.
i don't know how it does work

 

• May 30, 2005

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Btrend

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Re: about the mos'S effect

1. level shift between 1st & 2nd stage
2. clas AB biasing

 

• May 31, 2005

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tuza2000

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about the mos'S effect

is it for compensition series with Cc in order to cancel the right plane zero introdecued by Cc?

 

• May 31, 2005

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huangjw

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about the mos'S effect

what is cc

Added after 3 minutes:

but
this application is in comparator

 

• May 31, 2005

• #7

opamp741

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Re: about the mos'S effect

if comparator is not using any offset cancellation then there is no need of using compensation.
the only possible reason is level shifting..

 

• May 31, 2005

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noiseless

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Re: about the mos'S effect

Diode connected mos is used as a resistor with Cc to move the zero to hte lefe plane and then try to cancel one pole in the two stage topology.

 

• Jun 1, 2005

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raghav

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Re: about the mos'S effect

The answer is that it is used mainly for level-shifting.But the same diode acts as a compensator as well.it facilitates the action of next opamp stage

 

• Jun 1, 2005

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huangjw

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Re: about the mos'S effect

it's a comparator.
I simulate the circuit in two ways, one has the MOS,the other doesn't have MOS,but the result is the same.
why do we use level shift.the advantage?

 

• Jun 1, 2005

• #11

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about the mos'S effect

it acts as a nulling resistor

 

• Jun 1, 2005

• #12

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huangjw

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Re: about the mos'S effect

xusoso said:

it acts as a nulling resistor

Click to expand...

?

 

• Jun 1, 2005

• #13

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Re: about the mos'S effect

huangjw said:

it's a comparator.
I simulate the circuit in two ways, one has the MOS,the other doesn't have MOS,but the result is the same.
why do we use level shift.the advantage?

Click to expand...

in order to fit the input range of ur 2nd stage amplifier,level shifting is required for the output of ur first stage

 

• Jun 1, 2005

• #14

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ezt

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Re: about the mos'S effect

Hi.
yes, I think it is for level shifting. for compensation issues usually, not diode connected MOS trs are used. generally, MOS trs with some fixed bias voltage on their gates are implemented. However, I can't understand why should somebody use diode connected MOS in comparators?! if you could send a schematic of your circuit it would be more better.

Regards,
EZT