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When the load is a low value resistor or other alike, the OTA unable to drive it because of its' high output impendence. At this situation, you must add an output stage whch features a low impedence based on the OTA and provide enough current to load , generally, this an OP-amp.
ota is equivalent to opamp but its output is current... ota can be converted to opamp by using a common source amplifier at its output which converts the output to voltage... there is not much a difference other than this....
the output impedance impedance of an opamp is 0, but, that of an ota is a finite quantity. so the opamps that we actually design are ota's. when it is followed by a source follower it's output impedance is reduced thereby approximating an opamp.
OTA relies mainly in its gain on high output resistance. Gain of any amplifier can be represented by Gm*Ro where Gm is the amplifier's transconductance and Ro is the output resistance. On using high values for Ro, higher gain can be achieved. The major drawback is that the OTAs can't drive loads of resistive nature because that would reduce Ro thus reducing gain. At this case (resistive loads), output buffers are required to keep the required gain (this buffer should have high input impedance) and the OTA becomes OPAMP. Unbuffered OTAs are used extensively when loads are of capacitive nature as in switched-capacitor circuits.
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