How does the bandwidth of OPAMP depend on biasing current ?

How does the bandwidth of OPAMP depend on biasing current ?

Bandwidth of OPAMP

It is not simply, defined Bandwidth, if you OPAMP have complex structure. But for two stage amplifier, Gain Bandwidth is gm/Cc, where gm - transconductance of the input dif pair, and Cc - compensation capacitor.

Bandwidth of OPAMP

normally, more current mean wider bandwidth.

Re: Bandwidth of OPAMP

more current means the output capacitance can be charged faster and hence meaning more speed meaning more bandwidth.....

Re: Bandwidth of OPAMP

A.Anand Srinivasan said:

more current means the output capacitance can be charged faster and hence meaning more speed meaning more bandwidth.....

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BUT! Parasitic capacitance will be more large too! And you may get additional poles and zeros!! Be careful.

Re: Bandwidth of OPAMP

BUT! Parasitic capacitance will be more large too! And you may get additional poles and zeros!! Be careful.[/quote]

i meant just increasing the current by increasing the bias current and it doesnt introduce additional poles or zeros.... as far as i know the already present poles and zeros have more effect or less effect according to their position....

Re: Bandwidth of OPAMP

usually the bandwidth of any amplifier can be dependent on gm, R and C
( sometimes L if peaking method is used). gm is a transconductance of an amplifier, R is resistance at the load and C is capacitance seen at the load.

gm is increased when bias current increases, and it helps to achieve a wider bandwdith

in CMOS Opamp, R is usually composed of rds of nmos and pmos. rds is inversely proportional to the current due to the channel length modulation, therefore, increasing bias current reduces rds, which pushes a pole into higher frequency resulting in wider bandwidth.

usually C is fixed even though a bias current is increased if the size of device isnt
changed. but C can be affected if C is miller cap that is related to the gain of a amp cell. usually gain can be expressed as gm*R. generally gm is increased by sqrt of current and R is decreased by 1/current. so gain is decreased by 1/sqrt
of current which leads to decrease in C as well, and it achieves a wider bandwidth
by pushing a pole into higher frequency.

therefore generally, by increasing a bias current, you get wider bandwidth in opamp

Bandwidth of OPAMP

in opamps using large geometry devices the unity gain bandwidth is directly proportional to the sqrt of bias current

Re: Bandwidth of OPAMP

shobhit said:

How does the bandwidth of OPAMP depend on biasing current ?

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larger current larger bandwidth

Bandwidth of OPAMP

I think that as the gain of op-amp and also its tranfer function is related to parameters such as gm and it depends on current bias, your band width would be dependent on current bias.

Re: Bandwidth of OPAMP

can any ony give the relation between UGB and operating frequency of OTA?

yhanks

Re: Bandwidth of OPAMP

If you place the opamp in a closed loop, as is almost always the case, it depends on the feedback. The gain-bandwidth product (=UGB) is constant in a voltage amplifier, so the lower the gain of the feedback path (return ratio) the higher the closed loop dominant pole in general. In the limit of unity feedback (unity gain buffer) the closed loop cut-off frequency is equal to the unity gain bandwidth of the op-amp. Of course you need to ensure stability first.

Re: Bandwidth of OPAMP

shobhit said:

How does the bandwidth of OPAMP depend on biasing current ?

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Pole Location=gm/Cgs

gm=2ID/(Vgs-Vth)