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Push-pull output stage configuration, common emitter or common collector

Jiri Panacek
Prodigy 10 points
Other Parts Discussed in Thread: LM324

Hello Everyone, 

I would like to ask you, if one of you can describe in detail reasons, why standard push-pull configuration of bipolar transistors is mostly used in common-emitter output configuration rather than common collector. Is there any reason for that ? I understand this configuration in pure analog design (class-B amplifier), but I don't understand this in motor control applications. Especially driving top NPN transistor, keeping Vbe on reasonable level with respect to various load seems to be tricky..

Many thanks for every hint.

Jiri

Sample image from the internet for the configuration with common emitters.:

  • 0
    TI__Guru*** 142796 points

    Jiri,

    Each transistor is a common collector amplifier. The input is on the base and the output is on the emitter, therefore the collector is "common" to both.

    This common collector (emitter output) setup gives a low output impedance and the base currents are used by the load (not wasted). No phase splitter is needed.
    As drawn, crossover distortion is a potential problem. The output is also not rail to rail. This circuit is used for old designs like LM324.

    However, almost all modern design have common emitter (collector outputs) that will give rail to rail output performance.
    Many times the bi-polar transistors are replaced with MOSFETs.