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

 

I believe the reason you are seeing such a linear relationship between the input signal and the output is that in the differential configuration the transistors are typically matched and therefore any variation in the re' term of one transistor is matched or balanced by the same variation in the re' term in the companion transistor in the differential pair.

Here is a short article on the differential amplifier configuration. Review it to see if it helps you better understand the behavior.

hgmjr

 

I believe the reason you are seeing such a linear relationship between the input signal and the output is that in the differential configuration the transistors are typically matched and therefore any variation in the re' term of one transistor is matched or balanced by the same variation in the re' term in the companion transistor in the differential pair.

Here is a short article on the differential amplifier configuration. Review it to see if it helps you better understand the behavior.

hgmjr

Due to the symmetry you are talking about, almost all the distortion is odd harmonics. In a single-ended common emitter amp, the even harmonics are much more prominent. Small amounts of even harmonic distortion is more obvious (to the eye, and maybe the ear) on a sine wave than is odd harmonic distortion.

 

ok. seems to make more sense now. cheers