# biasing differential amplifier

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#### mathi

##### Junior Member level 2
biasing analog IC

Hi all,

where can i learn how to bias analog MOSFET ICs? I know all the conditions but i have trouble calculating by hand the of values of the bias voltages and sizes. Is there a good book or website or anything that i can use to learn to calculate these values? I have looked at the Razavi book but it doesn't show how to calculate the biasing points and values.

Thanks.

#### gujm1

##### Newbie level 1
biasing analog IC

Normally, the bias voltages are selected through simulation. Currently, no one will calculate them by hand unless you want to verify the accuracy of the simulation software.

### mathi

Points: 2

#### evilguy

##### Full Member level 4
Re: biasing analog IC

i also facing the same problem when i design my 1st analog circuit. As gujm1 said, we bias our transistor in simulation. however you can try calculating by hand using simple equation. such equation is like MOS level 1 equation. the most popular equation is the square law equation. using the formula, choose what value of current you want to drive in you transistor, choose the overdrive voltage. all of this value you must assume. the goal for hand calculation is to find W/L ratio. try it and good luck.

### mathi

Points: 2

#### DZC

##### Full Member level 3
biasing analog IC

Allen's book provide more detailed calculation.

### mathi

Points: 2

#### evilguy

##### Full Member level 4
Re: biasing analog IC

DZC said:
Allen's book provide more detailed calculation.

which chapter?

### mathi

Points: 2

##### Newbie level 1
Re: biasing analog IC

yes, we seldom try to calculate the accurate bias point for one MOSFET, that's impossible!! we often estimate the bias voltage by first-model of MOSFET. what u should know is how to get the right bias point through the initial bias point and simulation.

### mathi

Points: 2

#### jimway

##### Member level 1
biasing analog IC

U should set the oppoint by the specs before hand calculation. And the hand calculation can help you to find the bias range.

### mathi

Points: 2

#### benchen

##### Member level 2
Re: biasing analog IC

I think you can find the answer in Gray's book

### mathi

Points: 2

#### evilguy

##### Full Member level 4
Re: biasing analog IC

benchen said:
I think you can find the answer in Gray's book

i think it will be to broad for him if he must read the whole book. a lot of book dont state clearly the value of voltage or current that we have to use to bias a circuit. most of books discuss about the theory (DC analysis and AC analysis). however in designing circuit one have to be creative and decide themself how many current/voltage they want to use. after decide it, the transistor is bias to specific region with care taken into account that the decided bias current/voltage must be achieved while at the same time the ratio of transistor is varied.

### mathi

Points: 2

#### mathi

##### Junior Member level 2
Re: biasing analog IC

Thank you all for responding, do you think being able to hand calculate biasing points is critical. To get a good value you would have to take into consideration all the transistors in the branch. For a simple common source amplifier with a current source (mos in saturation) load you would have to consider both transistor simultaneously and that is difficult. When you have more it practically becomes impossible to solve.

##### Member level 4
biasing analog IC

For BSIM3 model, it's not easy to do hand calc.

You can do dc sweep of pmos/nmos for a few w/l cases and get lists of their parameters such as vdsat, vth, beta for future reference.

### mathi

Points: 2

#### evilguy

##### Full Member level 4
Re: biasing analog IC

mathi said:
Thank you all for responding, do you think being able to hand calculate biasing points is critical. To get a good value you would have to take into consideration all the transistors in the branch. For a simple common source amplifier with a current source (mos in saturation) load you would have to consider both transistor simultaneously and that is difficult. When you have more it practically becomes impossible to solve.

i have some tips to you. yes you need to know the basic of hand calculation. however when you deal with smaller process, you cant rely on hand calculation anymore. you dont need to consider both transistor at the same time. you can analys it independently (in DC analysis) always bare in mine, any transistors that are in series tend to have the same drain current. as you said if design common source amp with current source, you need to bias both transistor in saturation. if one transistor not in saturation region, let say linear region, the transistor in saturation will lower down its current and enter linear region. this happen due to the physic characteristic of MOS device.

dont look large circuit as it. try to see what circuit is that. then you need to break the circuit into smaller circuit and analys it independently. most large circuit has differential pair, current mirror/sink/source, common source/gate and source follower. all of this circuit can be analys independently. when you have bias it correctly then you can combine all of that together to to form large circuit. however you need to adjust a few things such as transistor geometry and assure all of your transistor is bias in correct region.

this is what i'm discover myself. maybe others has better technique than me.

good luck on your quest in learning analog ic design

good luck

### mathi

Points: 2

#### mathi

##### Junior Member level 2
Re: biasing analog IC

Thanks for your tips guys. You can only consider the transistors individually only if you neglect channel length modulation. In sub-micron devices you have to consider the channel length modulation, which means you have to consider both transistors at the same time in the case of common source stage with current source load.

#### evilguy

##### Full Member level 4
Re: biasing analog IC

yes.. you are right. however in doing manual analysis/hand calculation, we'll neglect all the 2nd order effect, short channel effect and CLM. yes... when you do simulation you will get result that far from your prediction when doing manual analysis. then you must be some sort of creative. tweak here and there to suite your desire. when your design a alot you will get the step.

### mathi

Points: 2

#### mathi

##### Junior Member level 2
Re: biasing analog IC

Thanks for your explaination, really helped.

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