Basic question about Differential Amps

[Ethan]

Here's something I don't get about diff amps. If the idea is that the positive and negative signal "difference" is what is being amplified, what happens when the balanced positive and negative signals are ideally identical, and there is no "diff" to amp?

A very silly and basic question I'm sure but I had to ask

 

[AnalogPackrat]

Not silly...gets to the heart of the matter. When there's no difference between the inputs, there's no output signal (ideally) and we have "common mode rejection." Tada! A basic feature of the diff amp. Cool, no?

A P

 

[rodabod]

[quote author="Ethan"]...what happens when the balanced positive and negative signals are ideally identical, and there is no "diff" to amp?[/quote]

If the positive and negative signals are identical, then this signal is "common" to both of them.

Hopefully if this ever happens then it will be a small amount of noise or interference which is the same on each line. These will then cancel-out because when the differential amp goes to amplify, it only amplifies the difference - if the noise is the same on the (+) and (-) line then they will not be amplified. The pure microphone or line signal is usually opposite on each line (hence (+) and (-) ) and is therefore amplified.

Now you can look at "common mode rejection ratio". It is the ratio which shows how good the differential amp is at rejecting this noise / interference.

 

[PRR]

what happens when the balanced positive and negative signals are ideally identical, and there is no "diff" to amp?

Balanced signals are equal but opposite. There is a large difference.

If the signals are equal and not-opposite, nothing should happen. That's not the signal we put on the line. Ignore it.

 

[Snatchman]

(SOT)...Hi everyone. Not meaning to hi-jack this thread, but I 've wondered about this "differential mic pre design" myself! Not knowing much about it, my Tascam M-2600mkII console has this design.( so they say) Is this a good mic pre design? Can it be improved or modded with good results? Thanks......

 

[rodabod]

[quote author="Snatchman"]"differential mic pre design"... my Tascam M-2600mkII console has this design.( so they say) Is this a good mic pre design? [/quote]

It depends what Tascam means... Almost all mic pre's are differential amplifiers (ie. balanced).

I have seen designs where the signal is kept balanced throughout the circuit though, and this could be what Tascam means. I am not familiar with the unit. There is debate as to whether the benefits of this sort of design are really worth it (in the DIY building sense), but I doubt there is any draw-backs since it is probably quieter.

 

[NewYorkDave]

differential amplifiers (ie. balanced).

Any amplifier is a differential amplifier, strictly speaking. A regular old triode amplifies only the difference between what it sees at its cathode and at its grid! :green:

 

[rodabod]

[quote author="NewYorkDave"]
Any amplifier is a differential amplifier, strictly speaking. A regular old triode amplifies only the difference between what it sees at its cathode and at its grid! :green:[/quote]

Ahh. I see what you mean. So it's any difference... :idea: