Differential amplifiers - why amplify the difference?

[BetaTripp]

Hi all,

A bit of a rookie question maybe but what is a differential amplifier used for and what is the point in amplifying the difference between the two inputs? If you are feeding back the output of the amp to the negative input wouldnt you just end up with the input minus the output which would leave nothing.. I'm guessing this is what is supposed to happen but i'm not sure what it would be used for. I am trying to get into electronic design and see differential amplifiers in schematics but can't work out what they are used for or even how I would use them if designing myself.

Thanks!!

 

[LvW]

The most important property of a diff. amplifier is that the signal output at one collector node is (a) in phase with the input signal at the other transistor and (b) inverted with respect to the "own" input signal (as in a common-emitter stage). Now - such an diff. amplifier is ALWAYS the input stage of an operational amplifier. Because of the mentioned phase relations we can say that - with respect to the opamp output - one signal input is in-phase with the output (positve gain) and the other signal input is reversed in phase (neg. gain). These properties allow us to use the opamp for a variety of applications because we can realize many feedback functions (negative or positive or both at the same time).

 

[duke37]

Suppose you have a microphone which produces a voltage at its output. Connect it to an amplifier with a cable which picks up interference on both wires. Connect the wires to a differential amplifier and what is amplified would be the signal from the microphone and the common mode interference would be the same at both inputs and so would not be amplified.

I am sure this could be explained better but I am not awake yet.

 

[AnalogKid]

Are you asking about a differential transistor stage, sometimes called a long-tailed pair, or the differential inputs to an opamp IC? The overall discussion is the same, but the explanation is different depending on the starting point.

ak

 

[Ratch]

Hi all,

A bit of a rookie question maybe but what is a differential amplifier used for and what is the point in amplifying the difference between the two inputs? If you are feeding back the output of the amp to the negative input wouldnt you just end up with the input minus the output which would leave nothing..

You don't have to feed all the output back to the input. Just a fractional amount.

I'm guessing this is what is supposed to happen but i'm not sure what it would be used for. I am trying to get into electronic design and see differential amplifiers in schematics but can't work out what they are used for or even how I would use them if designing myself.

Thanks!!

An operational amplifier is one big differential amplifier. Look at how useful they are. There is plenty of elementary material on op-amps that you should find interesting.

Ratch

 

[BetaTripp]

Suppose you have a microphone which produces a voltage at its output. Connect it to an amplifier with a cable which picks up interference on both wires. Connect the wires to a differential amplifier and what is amplified would be the signal from the microphone and the common mode interference would be the same at both inputs and so would not be amplified.

I am sure this could be explained better but I am not awake yet.

So how is the signal amplified and not the noise? Is it something to do with the phase of the signal? Say if the phase was 180 degrees out, I thought all of it would cancel out and if the phases were the same they would add together. Maybe it is nothing to do with phase..

 

[Ratch]

So how is the signal amplified and not the noise? Is it something to do with the phase of the signal? Say if the phase was 180 degrees out, I thought all of it would cancel out and if the phases were the same they would add together. Maybe it is nothing to do with phase..

You seem to have a problem with an amplifier's functionally, and how it is implemented. Lets discuss a single-input amplifier first. Within the amplifier's frequency range, both noise and signal are amplified. There is no way to distinguish between noise and signal if both are in the same frequency range. The trick to noise reduction is to not let noise into the signal path in the first place. That means doing whatever needs to be done to reduce noise pickup at the signal source, and designing the amplifier so that it does not pick up stray signals and amplify them. The second thing that can be done is to filter the noise. If it is in a frequency band that can be disregarded, like high frequency, use a low-pass filter. A differential amplifier will not amplify a noise or signal if it occurs simultaneously of both inputs because they cancel out. How well it does that is designated the common-mode rejection ratio (CMRR).

Moving on from the functionality aspect of the amplifier, you can get involved the implementation. Here you decide on bandwidth, low noise level, amplification factor, I/O impedances, and whatever. However, implementation is a different subject from functionality.

Ratch