I asked similar question recently, which also referred to differential transistor pair, but this one is different.
I made this circuit (in real world):
I was only experimenting with DC (no signal applied). Potentiometer was used to adjust I_e1 to equal I_e2. Beta of transistors were around 300 and both of I_e were set to be exactly 1.1 mA.
After everything was connected as shown, I started to observe the I_e of each transistor when one transistor gets heated up. So, I "touched" the Q1 with two fingers for a few seconds - but nothing happened (no changes in current), same happened with the Q2. It seemed a bit strange that nothing happens. Then I took my lighter and heated one transistor to temperature about 100 degree Celsius - the current through both of transistor didn't even blink! At the end I really really heated it up - and only at this level of heating, the currents moved up/down the scale for 4-6 uA (micro amps)!
- How is this even possible? When lighter was usually approached, the I_e changed on the scale of mA, if not tens of mA.
I even replaced transistors - same results!
I also reconnected the whole circuit again - same results!
At the end I used different multimeter (in case one wouldn't work properly) - same results!
*This thing could be possible with current mirrors at collectors and constant current source at emitters, but I only used usual passive resistors.
Comparison of multimeter values: