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two stg OTA (Read 3889 times)
ontheverge
Junior Member
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Posts: 31
two stg OTA
Nov 01
st
, 2010, 1:47am
Hi,
I noticed that in miller-compensated two stage OTA, if the first stage is PMOS driven, the output stg is NMOS and vice versa, is there a special reason for that?
thanks,
Steve
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raja.cedt
Senior Fellow
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Posts: 1516
Germany
Re: two stg OTA
Reply #1 -
Nov 1
st
, 2010, 2:09am
hi,
1.common mode voltage of the 1st stage o/p can easily drive 2nd stage with out any level shifting.
2.Supply rejection will be good.
Thanks.
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ontheverge
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Posts: 31
Re: two stg OTA
Reply #2 -
Nov 1
st
, 2010, 5:39pm
Hi Raja,
thanks for the reply, can u elaborate a little bit on supply rejection?
thanks,
Steve
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raja.cedt
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Germany
Re: two stg OTA
Reply #3 -
Nov 1
st
, 2010, 7:58pm
hi,
Assume you have nmos input diff pair in the first stage and pmos input 2nd stage, so due to pmos diode connected load at the first stage entire supply noise will come to output, so if 2nd stage is pmos then pmos gate will change by same amount of supply noise and at the same time source will be changed by same amount due to VDD connection. hence you would not see any change in 2nd stage output.
Thanks.
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aamar
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Posts: 57
Germany
Re: two stg OTA
Reply #4 -
Nov 2
nd
, 2010, 9:40am
There are other two advantages which are
1. for the highest gain of the second stage, the vin (common mode as raja mentioned) for the second stage input transistor should be biased near the vth or vdd-vth in case of nmos and pmos respectively, which is then best implemented using the opposite type transistor from that used in the input differential pair.
2. The load of the second stage is a mirror from the tail current of the first stage and therefore no need for an additional bias voltage.
Best regards,
aamar
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«
Last Edit: Nov 3
rd
, 2010, 12:56am by aamar
»
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HdrChopper
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Posts: 493
Re: two stg OTA
Reply #5 -
Nov 15
th
, 2010, 6:28pm
one advantage more...
the second stage being the same type of the first stage active load allows for perfect balance of voltages if properly sized, thus systematic input referred offset can be minimized
Best
Tosei
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Keep it simple
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raja.cedt
Senior Fellow
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Posts: 1516
Germany
Re: two stg OTA
Reply #6 -
Nov 15
th
, 2010, 8:31pm
hi chopper,
if i am correct you are talking about sharing bias between both stages rite....
Thanks.
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