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Old 26th January 2008, 07:16 AM   #1
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Default differential amp question

I've been reading up on differential amplifiers and have a question

when running the diff amp in single ended mode i see the gain is calculated by RC / 2re'.

I thought having the gain mostly dependant on the dynamic emitter resistance (re') would lead to gross distortion on the output.
However with supply rails of +/-9v and an input signal of 1mv the output is quite clean.

From experience running even 1mv into a common emitter stage with bypassed RE leads to gross non linearities in the output waveform.

Why doesnt this hold true in the differential amp?

thanks for the help guys
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Old 26th January 2008, 12:06 PM   #2
AndrewT is offline AndrewT  Scotland
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are you talking of a discrete amplifier?

Post a schematic so we can see which components you are referring to.
regards Andrew T.
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Old 27th January 2008, 11:05 AM   #3
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Click the image to open in full size.

like this
however one input is grounded
and one is connected to a signal
eg. single ended mode...
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Old 27th January 2008, 11:25 AM   #4
jcx is online now jcx  United States
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try reading Doug Self's diff pair distortion explanation:

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Old 28th January 2008, 11:02 PM   #5
CBS240 is offline CBS240  United States
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THIS paper is a good reference for this subject.

All the trouble I've ever been in started out as fun......
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Old 29th January 2008, 01:26 AM   #6
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Maybe the mathematically inclined can explain this further:

the collector current of the common emitter circuit goes exponentially with Vbe, while the collector current of the differential amplifier goes with the tanh of the differential voltage. This leads to a greater linear input range of the differential amplifier. (I guess the tanh is more linear around zero than the e-function)

(source: Tietze/Schenk: Electronic Circuits, very good book)

Donīt ask me for a derivation ;-)

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