Why usually the size of input device are bigger than others?

[shaq]

Dear all,

I have a little question about designning a opamp.

If I have a pmos input device, we usually make them bigger than other mosfets.

But, why?

Any theoretical foundations on it ?

 

[safwatonline]

i think large size to get low o verdrive voltage and hence high gain

 

[Areky_qin]

input device is large than others can lead big gm, that's means big gain, at the same time some device supply rs. Second, input device is large enough can reduce the offset voltages, increase the input common range etal. Other devices is small size, can lead small paratic caps, it is benifit fo frequency compensation , speed and bandwidth.

 

[shaq]

Re: Why usually the size of input device are bigger than oth

input device is large than others can lead big gm, that's means big gain, at the same time some device supply rs. Second, input device is large enough can reduce the offset voltages, increase the input common range etal. Other devices is small size, can lead small paratic caps, it is benifit fo frequency compensation , speed and bandwidth.

Why if "input device is large enough can reduce the offset voltages" ?

 

[zyyang]

Re: Why usually the size of input device are bigger than oth

may be bigger dimension will reduce the 1/f noise?

 

[Sudhir.K.A]

Hi all,
This is in reference to shaq's question.

Bigger devices are less prone to offset due to process variation and mismatch.More over input reffered offset is will be more for input stages than after the gain stage.

regards,

 

[yschuang]

Re: Why usually the size of input device are bigger than oth

Hi, all

I was curious about how big the input devices can be. Usually, the input pair are big, then they are in the subthreshold region. Is it ok to do that?

 

[sengyee88]

Re: Why usually the size of input device are bigger than oth

In general, differential pairs should always biased in saturation region. Increasing size of the differential pair transistors, reduces input offset, increases gm (hence gain), it also reduces bandwidth and slew-rate of the amplifier if bias current remain unchanged.

Therefore, transistor sizing should not only consider offset or gain, but speed, current consumption and noise are important too.

 

[raduga_in]

One more reason what i think is that you can increase the input common mode range by using
large devices. As the size is large hence you need
less overdrive to get the required current through
the devices.

 

[fanrong]

Re: Why usually the size of input device are bigger than oth

To decreas the noise if the device is mos

 

[pkalavakuru]

Re: Why usually the size of input device are bigger than oth

1) U can get theoritical xplanation r urself can calculate them .

Take two current equation and do partial differntial checkt matching parameter(dv/dt). w.r.t width , length.

To get 1 percent matching u 'll find that u must define the PMOS transistor bigger in size.

ciao

 

[femtopiconano]

Re: Why usually the size of input device are bigger than oth

In general, differential pairs should always biased in saturation region. Increasing size of the differential pair transistors, reduces input offset, increases gm (hence gain), it also reduces bandwidth and slew-rate of the amplifier if bias current remain unchanged.

Therefore, transistor sizing should not only consider offset or gain, but speed, current consumption and noise are important too.

Why increasing the input pair size reduce bandwidth ???
GainBandwidth is proportional to gm and so the sqrt of W/L, isn't it ?
SlewRate depend on current and Cload, not on the input pair ... isn't it ?