ota vs op amp
Actually there will be different kinds of amplifiers like for those whose output voltage is a function of the input voltage, out put current - as a function of the input current...
they can be named in short hand notation as:
1.)VCVS--voltage controlled voltage source
2.)CCVS
3.)CCVS
4.)CCCS
OTA is an op-amp-like device (operational transconductance amplifier, or OTA) exemplified by the LM13600, a dual amplifier in a 16-pin DIP. It has inverting and noninverting inputs, and an output, like a a normal op-amp, but it is a very different device, and cannot be used like a voltage op-amp. Its applications are specialized, even more so than those of the Norton op-amp, which at least could be persuaded to do the things a normal op-amp does. The OTA is frequently used as an open-loop amplifier. Its gain is not high, and it is not useful in most of the familiar feedback circuits.
The idea of the OTA is to provide a transconductance amplifier, whose input is the differential voltage between its inputs, which are the bases of the transistors of a standard differential amplifier, and whose output is a current proportional to the differential voltage. The transconductance is the ratio of the output current to the input voltage, denoted by gm. The transconductance of an OTA is proportional to the bias current IABC established by an external resistor. At a bias current of 500 μA, the gm of the LM13600 is typically 9.6 mS, but can vary from 6.7 mS to 13.0 mS.
And a comparator is which compares the input with a standard value and give the output accordingly. It is the main device as a state of transfer from analog to digital devices.
An opamp operating in it's saturation regions can also be visulaised as a comparator, but it's functionality is not a perfect one. As you proceed learning op amps you will come across some better circuits for comparators especially the schmitt inverter being used as a comparator.