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Design >> Analog Design >> opamp
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Message started by raja.cedt on Sep 10^th, 2008, 12:53am

Title: opamp
Post by raja.cedt on Sep 10^th, 2008, 12:53am

hi,
i have designed a two stage op amp with 1v supply in 65nm tech.
input common mode voltage=0.4 to 0.7
output bias point =0.5(swings from .2 to .9 )
now i connected in non inverting configuration with gain 3 then it is saturating because of output swing limitation(actually op amp trying to go 1.2 because non inverting input i gave .4 as bias ) means op amp is trying to amplify both signal as well as bias. Is there any method to subtract bias voltage in the feed back so that we can get wide range of gains
thanks

Title: Re: opamp
Post by aidnaal on Sep 10^th, 2008, 5:07am

Hi,
Could you please send the schematic of the circuit (non inverting configuration)?

Title: Re: opamp
Post by thechopper on Sep 11^th, 2008, 7:26pm

It sounds to me you are connecting the feedback resistors to DC GND rather than an AC ground whose DC bias point should be the same as the input common mode.
In this way you will not amplify the bias point and will take advantage of the output dynamic range.
This is the only way a non-inverting amplifier can work without amplifying the DC operating point. Otherwise you will have to use an inverting configuration

Regards
Tosei

Title: Re: opamp
Post by raja.cedt on Sep 12^th, 2008, 3:42am

hi Tosei,
thanks for your answer.but how to give that bias.if you do some bias generation circuitry then it will directlly come with feedback network and all most all bias circuits are active networks.So the greatness in feedback networks is output is accurately defined by feedback passive elements now output is not accurately defined.
How inverting amp will help in this regard.There also we have to give bias for non inverting pin rather than dc ground.

Thank you.

Title: Re: opamp
Post by thechopper on Sep 18^th, 2008, 7:42pm

Hi

In order to make it work without amplifying the bias circuit you have to separate the bias from the signal to be amplified. Otherwise you will amplify the DC bias input voltage. See the drawing for reference

Tosei