# Differential pair: is the differential input always supposed to be an AC voltage?

by eliotsbowe
Tags: differential, input, pair, supposed, voltage
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 P: 35 Hello, I'm studying the MOS/BJT differential pair on Microelectronics by Sedra-Smith. The book refers to voltage signals with the following notations: $V_{AB}$ = DC voltage $v_{ab}$ = AC voltage $v_{AB}$ = generic time-varying voltage When it comes to differential pairs, it uses a quite confusing notation for differential inputs: they are always called $v_{id}$, like they could be only AC signals, but I'm not positive on this restriction. I happened to find some exercises on the MOS differential pair based on Sedra-Smith's figures (link) and they seem to confirm that $v_{id}$ may be both a DC and an AC signal. Is this definitely correct? If it is, I have a second question: what would be the point of applying a DC differential input to a differential pair? I mean, is there any application in the real life in which a DC voltage needs to be amplified? Thanks in advance for your help.
 P: 589 There is no universal nomenclature. Depends on which datasheet or book. Many (if not most) high speed differential technologies need their receivers to be biased to a specific DC voltage in order to function correctly. This can be accomplished by using a driver that conforms (such as with ECL), or by AC coupling and incorporating biasing into receiver (self biasing).
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P: 5,473
 Quote by eliotsbowe If it is, I have a second question: what would be the point of applying a DC differential input to a differential pair? I mean, is there any application in the real life in which a DC voltage needs to be amplified?
You might position multiple thermocouples (each with its own buffer) around a heatsink, and the temperature difference between locations can be gauged by measuring the voltage difference between any pair of thermocouples. So use a differential amp for this.

The term DC is often used to simply mean "of much lower frequency than the principal time-varying signals", and means the differential amplifier inputs are directly coupled and can operate right down to DC (so the diff amp design can't use capacitor coupling).

P: 5,462
Differential pair: is the differential input always supposed to be an AC voltage?

 If it is, I have a second question: what would be the point of applying a DC differential input to a differential pair? I mean, is there any application in the real life in which a DC voltage needs to be amplified?
The differential input pair or longtail pair is one of the most used configurations at all frequencies, including zero or DC.

This is partly because it is easy to build in compensations for disturbing influences like temperature and because you can stably and controllably achieve substantial voltage gain.

The configuration can not only offer differential input put also single ended input, and differential oputput so can be used in four modes.

Single ended input single ended output
Differential input single ended output
Single ended input differential output
Differential input differential output

Use of the differential input increases common mode rejection (noise etc).

Use of the differential output allows driving of such devices as the opposing plates of electrostatic deflection systems eg in cathode ray tubes.

The differential output also allows phase splitting operations such as driving push-pull amplifiers.

All forms of amplifying devices can be used in this mode, valves (tubes), transistors, FETs, even op amps.

go well
 P: 35 I think the answers made my ideas clear, many thanks to everybody

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