Identical devices is one of the big problems with simulation and I'm amazed it never gets mentioned more. It can be instructive using different devices for each transistor sometimes, just to see the effect and how the circuit copes.
(I come from a repair background and so I have what you might call a "practical instinct" of what looks right and where (possible) problems might lie and that is partly what I based my comments on. I have seen many cases of component failure where devices are seemingly operated within their limits)
(I come from a repair background and so I have what you might call a "practical instinct" of what looks right and where (possible) problems might lie and that is partly what I based my comments on. I have seen many cases of component failure where devices are seemingly operated within their limits)
In response to OP CharlieLaub,
For linear applications that never switch off:
R1 (across VT2's 0.65VBE junction) draws very near constant current.
This should be used to bias VT1 at its most favorable operating point.
And to swamp with excess, the small variable current into VT2's base:
Thus VT1's operating point won't be disturbed by VT2.
For switching applications:
R1 helps remove carriers from VT2's base, so it can turn off faster.
The big final transistor is usually the slow one of the pair anyway.
Will sometimes find another turn-off resistor across VT1 as well.
If VT2 is switching on/off, there will not be constant VBE, and we
lose the convenience of a constant current through resistor R1.
For linear applications that never switch off:
R1 (across VT2's 0.65VBE junction) draws very near constant current.
This should be used to bias VT1 at its most favorable operating point.
And to swamp with excess, the small variable current into VT2's base:
Thus VT1's operating point won't be disturbed by VT2.
For switching applications:
R1 helps remove carriers from VT2's base, so it can turn off faster.
The big final transistor is usually the slow one of the pair anyway.
Will sometimes find another turn-off resistor across VT1 as well.
If VT2 is switching on/off, there will not be constant VBE, and we
lose the convenience of a constant current through resistor R1.
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