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A question about bandwidth of opamp

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naalald

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Hi,
In an opamp, I want to increase the bandwidth without increasing the current. I think I should increase the gm of the output transistors whose drain constitutes the high impedance node of the circuit and the dominant pole. I do so but the bandwidth does not increase. What is wrong with the procedure? Any suggestion?
Thanks.
 

I assume that we talk about 2 stage amplifier and you mean GBWP, not BW.
If you increase gm of the output transistors you will increase only Phase Margin. You need decrease compencate capacitor.
 

Hi,
Thanks for your reply.
I do mean GBWP, which is the unity gain frequency of the amplifier. But the GBWP can be defined so:
ωu = Av × ω3dB
in which Av is the amplifier gain and ω3dB is the -3dB frequency of the amplifier. If we rewrite the equation we have:
ωu = (gm×Rout) × 1/(Rout×Cout)
in which case, a dominant pole at the output node is assumed. So, If we simplify the equation we have:
ωu = gm /Cout
Therefore, by increasing the gm, the ωu should increase.
Isn't that right?
 

Hi naalald,

you don´t speak about an opamp (as indicated in your first post) but about a Transconductance amp (OTA), right ?

The bandwidth of the device alone (without any load) is determined internally and will NOT change by adjusting gm of the output transistors.

If you connect a capacitive load you create another pole and the result is something like a lowpass filter/integrator. But this pole will be dominant only if it´s frequency is lower than the internal pole. And only in this case the transconductance gm will determine the bandwidth of the whole device (consisting of the active unit and the external components).
 

Yes, in case of an OTA wu = gm/2pi*Cload. Where gm is the transconductance of an _input_pair_. And pay you attention in which mode works your pair. If you try to increase gm by scaling transistor but transistors are working in small inversion, gm will NOT increase! Because gm reaches its maximum value in weak inversion.Thus you have the only one way if you don't want to go up current - to decrease Cload
 

great replies..........this forum is awesome
 

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