Comparison of Comparators and OTAs

[Krishnasai GA]

Can someone please explain the differences between [1]comparators and [2]Operational Transconductance Amplifiers. Can i replace [1] with [2] in applications? Is it possible? If yes, what might be the implications?

Thanks in advance.

 

[Dominik Przyborowski]

OTA works always with negative feedback so it operation is linear.
Comp. works without negative feedback or with positive feedback to obtain nonlinear operation (two state output) with hysteresis for positive feedback > .

From implementation point of view comparator is an single stage OTA with inverter stage(s) for sharpening output signal slope.

 

[vivek.roy2991]

I will try and answer the second part of your question first. I think what you mean is that how does a voltage comparator like LM311 differ from using an op-amp, uA741, as a comparator. At first glance, both may seem similar since an op-amp, in open loop, behaves as a large gain differential amplifier. If speed is not an issue, then you can certainly go ahead and use an operational amplifier in open loop configuration as a comparator For example, uA741 has a slew rate of 0.5V/us. So for a 5V peak to peak transition, the rise time would be (5/0.5)us=10us which is too long a delay for most purposes. Moreover, op-amps have saturation voltages which are not equal to the supply rail voltages. For example for a 5V supply, the output maximum voltage will be in the range of 3.9 to 4.3V. Of course you can use a rail to rail operational amplifier. But one of the first comparators, LM311, had open collector output. Imagine the flexibility!
So, in conclusion, using an operational amplifier as a comparator is not a great idea. But in case you have to use an op-amp, you can try the LM301 which does not have internal compensation and so has a response time uch smaller than uA741.

Working of comparators:

I am going to describe the operation of a comparator using the example of LM311(why you ask? Because we did this as a part of our coursework ).
LM311 has an input stage consisting of PNP emitter followers which drive a differential pair. The output of the first differential pair is amplified by another differential pair. The output of the second differential pair is single ended current which drives the base of the output transistor. If the voltage at the non-inverting terminal exceeds the voltage at the inverting terminal, then the last stage differential amplifiers ensure that almost no current reaches the base of the output transistor so that it is now in cut-off. And for the other way round, there is heavy base drive which forces the transistors to operate in saturation.

The biggest advantage is that due to open collector outputs, the output logic levels are under the control of the user.

You can refer to "Design with operational amplifiers and analog integrated circuits" by Sergio Franco for more details.