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Design >> Analog Design >> Problem with my biasing scheme
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Message started by kidman on Jun 18^th, 2009, 5:19am

Title: Problem with my biasing scheme
Post by kidman on Jun 18^th, 2009, 5:19am

I am trying to bias a telescopic amp with PMOS input pairs. I don't use ratioing (only for the tail current source). To achieve a high bias swing, I use the quarter size transistor method. What I get is that most of the supply is across the PMOS cascode transistor. I have attached the biasing scheme. Can you please take a look at it? And also I would be grateful if you could tell me how would you bias such an amp. Thanks

Title: Re: Problem with my biasing scheme
Post by subgold on Jun 18^th, 2009, 8:01am

where is your pmos input transistor of the amp?
why do you cascode two current sources (#16 and #17) together? then neither of these two current sources is working properly.

Title: Re: Problem with my biasing scheme
Post by kidman on Jun 18^th, 2009, 9:12am

The arm on the right is the telescopic amp but only with one side shown
Transistor 17 is the tail transistor of the amp. I included it just to show how it is biased.

Title: Re: Problem with my biasing scheme
Post by wave on Jun 18^th, 2009, 10:57am

This one made me think a bit. I'm used to other structures.

Why is your gates of M8, M15, M14 on the source of the bias?
Take it up a Vgs. (down for Pmos).

Title: Re: Problem with my biasing scheme
Post by kidman on Jun 18^th, 2009, 11:11am

This is the quarter size transistor method. You can read about it in grey&meyers book

Title: Re: Problem with my biasing scheme
Post by wave on Jun 18^th, 2009, 1:45pm

kidman wrote on Jun 18^th, 2009, 11:11am:

This is the quarter size transistor method. You can read about it in grey&meyers book

Gotcha. Yeah, it's in there. Usually I use the one a few pages later, "MOS high-swing current mirror with two input branchs" which has better input headroom.
Two transistors collapse into one, atleast 1/4 or smaller after Body Effects.

cheers,
Wave

Title: Re: Problem with my biasing scheme
Post by jiesteve on Jun 18^th, 2009, 2:30pm

Watch out for input common mode level -- if it's not set in the right place the output common mode will rail. I suggest you drive both inputs and sweep the CM level until you get the amp in the high gain region (if you are free to change the CM level)

Title: Re: Problem with my biasing scheme
Post by barry on Jun 21^st, 2009, 10:53am

The current through M16 is less than the current through M10 because the voltage at the source of M10 is VDD and the voltage at the source of M16 is less.

The current through nmos source M13,M14 is equal to the current through M10. Thus the voltage at the drain of M14 goes low and most of the power supply voltage is across M15.

M16 is one of the opamp inputs, right? As jiesteve mentions, your common-mode input on the gate of M16 is too high.