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Design >> Analog Design >> Comparator, OTA, Opamp
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Message started by The_One on Feb 6^th, 2004, 8:13am

Title: Comparator, OTA, Opamp
Post by The_One on Feb 6^th, 2004, 8:13am

What are the differences between Comparator, OTA, and Opamp?
What are they used for?
what are their main functions?

Title: Re: Comparator, OTA, Opamp
Post by Paul on Feb 6^th, 2004, 11:38am

Hi,

all three blocks are amplifiers, but they serve different purposes.
A comparator outputs a logic signal saying whether one input is smaller or larger than the other. It typically uses positive feed-back to saturate its output to either logic zero or logic one values (in most designs VDD and VSS).

OTA stands for Operational transconductance amplifier. "Transconductance" means that the output signal is a current which is proportional to the difference between both input signals. Most integrated amplifiers (opposed to discrete amplifiers) are OTAs. They are used in switched-cap circuits, A/D-converters etc.

Opamps deliver an output VOLTAGE proportional to the difference of its input voltages. While we may consider integrated amplifiers as being opamps, most of them are in fact OTAs. Discrete amplifiers usually are opamps. Compared to OTAs, they typically use a voltage follower output stage to be able to deliver relatively large output currents to resistive loads, which typically is not useful in switched-capacitor circuits (where you hence use OTAs).

When I speak of "discrete" amplifiers, I mean an amplifier IC in a package used with discrete external components. "integrated amplifier" means an amplifier used inside an IC with on-chip components. Obviously a "discrete" amplifier in this sense is also on a chip...

Paul

Title: Re: Comparator, OTA, Opamp
Post by The_One on Feb 6^th, 2004, 7:03pm

Hi Paul, would you mind to give some schematic examples of Comparator, OTA, Opamp, so that i will have better understanding about them. thanks.

Title: Re: Comparator, OTA, Opamp
Post by Paul on Feb 9^th, 2004, 12:28am

Hi,

you can find these in most classical text books, like
- Laker/Sansen, McGRaw-Hill, OTAs and amplifiers
- Gregorian/Temes, Wiley, switched-cap amplifiers
- Johns/Martin
- Gray/Hurst/Lewis/Meyer
and many more...

Paul

Title: Re: Comparator, OTA, Opamp
Post by kid on Jun 21^st, 2004, 5:31pm

OTA + Buffer = Op Amp

OTA's are used to drive capacitive loads ( < 20 pF) and Op Amps to drive resistive loads. So you see when you have a resistive load you add a buffer to the output stage of the OTA to prevent loading it and reducing the gain.