# How is an OTA driving a Capacitor faster than an OPamp with similar load ?

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#### engrMunna

OTA has a high output resistance and its outpole is equal to 1/ (Rout * Cout) since Rout is quite high than this frequency will be low and making the ota not good to be used at high frequencies.......

OpAMP has low output imepdance and hence its output pole will be at a higher frequency for the same Cout ?

Where am i going wrong in this analysis becuase it is a kind of universally accepted truth that OTA are better for high frequency operation?? please help i am confused

#### dick_freebird

A general purpose op amp often embeds general purpose
assumptions, such as a need for unity-gain (or low fixed
loop gain) stability. Then you get a built in frequency
compensation tat rolls off bandwidth. OTAs can "go bare"
have to stabilize some arbitrary worst case, only its
present reality, you can do better in all but that case.

#### engrMunna

but its output resistance too large...and if it drive a large load capacitance then wont this place a pole at very low frequency and hence affecting its bandwidth?

#### LvW

but its output resistance too large...and if it drive a large load capacitance then wont this place a pole at very low frequency and hence affecting its bandwidth?

Yes, of course. But that is the task of a current output that drives a capacitor. Thats an integrator.
You shouldn't think in amplifier terms (like for voltage opamps). It's not the primary task of OTA's to act as a simple voltage amplifier because in this case an additional output buffer would be needed (that - on the other hand - should have a very low input capacitance).

#### engrMunna

if OTA's can't process high frequencies then why are they used in active filters? and other high frequency applications?

---------- Post added at 03:54 ---------- Previous post was at 03:48 ----------

from dick_freebird and LVM's reply....what I gather is that compared to op-amps they dont need any compensation as they dont have any feedback...so this makes them faster than op-amps for a capacitive load? am I right?

#### LvW

if OTA's can't process high frequencies then why are they used in active filters? and other high frequency applications?

Who has claimed that OTA's can't process high frequencies?
The only point is you shouldn't use OTA's as voltage amplifiers with capacitive loads.
As I have mentioned: Dont think in voltage amplifier terms only!
OTA's work fine in filter circuits in which their frequency dependence - if capacitivly loaded - is exploited.
More than that, OTA's are electronically tunable and can be configured as voltage tuned resistors.

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