why use OTA in current-mode?

why the voltage mode compensation use external capacitor which connects between error amplifier output and inverted input , and the error amplifier is opamp with buffer stage but not gm amplifiler (OTA).
As we know the current mode is gm amplifier ,whether the voltage mode can use OTA or not , why ?

why the voltage mode compensation use external capacitor which connects between error amplifier output and inverted input

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Voltage mode have High impedance node, that adds one extra dominant pole and hence the stability gets affected. This needs compensation capacitor, for pole splitting. (say way?)

whether the voltage mode can use OTA or not , why ?

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yes sure, infact first stage of 2-stage error amplifier is OTA itself. Due to very low voltage gain of gm stage you require another stage for voltage gain,to use it as voltage amplifier.
Hope you understood the point. By the way do you understand clearly, the difference between current mode/voltage mode?

Hi Varunkant2k:

Thanks for your help.

Voltage mode have High impedance nod

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and the High impedance you mentioned is the Rout of the EA?
And now I am confused about the function of EA, usually, its function is described as "amplify the error signal ", however, after adding the compensation net, the gain is determined by the ratio of the resistors in the net. And since the EA'S function is to amplify why use a OTA in the current mode?

the difference between current mode/voltage mode?

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as you know, i am not clearly understand why in the current mode the inductor can be seen as a current source?

and the High impedance you mentioned is the Rout of the EA?

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Yes

however, after adding the compensation net, the gain is determined by the ratio of the resistors in the net.And since the EA'S function is to amplify why use a OTA in the current mode?

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You need to know the compensation effect on the high impedance node.(check if it is reduced or increased).
No problem if you use high impedance at first stage itself, (folded cascode device). As you said, due to low impedance node, OTA does not give voltage gain, but current gain. Stability is also not a problem because poles are very far. This is the reason we use OTA.

am not clearly understand why in the current mode the inductor can be seen as a current source?

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This you need to read inductor behavior.

Thx a lot!