When making an amplifier, I know it is preferable to use complementary transistors, but why is that? What would be the consequence if they were not complementary? I guess I am confused about what it means to be complementary.

 

Generally it means that they are designed to have similar performance parameters so that when they are used on designs using both, such as push-pull stages, the resulting amplifier behaves very similar regardless of which transistor is driving resulting in better linearity and, hence, less harmonic distortion. If you don't use complementary transistors, then you probably are not getting as clean an output as you might if you did, but you might not be able to tell much of a difference, either.

 

The desire is to achieve symmetry of the sine wave. That's easier and more likely if you uses complementary transistors. If you don't care about great precision, it probably doesn't matter much.

 

1) The complimentary transistors are going to be in the output stage.
2) The transistor/resistor stack that drives their bases and keeps their bases a certain voltage different from each other is one series string using one amount of current.
3) If the transistors need different amounts of current to drive their bases, which one do you design for?

 

Complimentary refers to using two transistors of opposite sex (PNP and NPN) in a symmetrical circuit configuration.
This circuit configuration provides symmetrical electrical performance for both positive and negative output swings.

The matching of transistors properties becomes important to make the performance symmetrical as well, this is why you will hear the term "matched complimentary pair" thrown around in audio amp design circles.

If the two transistors have different gain for example, this can compromise the symmetry of the output.

 

The more common term I have seen is a "matched pair" which amounts to taking transistors of the same part number and running them on a curve tracer to find pairs who have the same conductance characteristics. In many circuits they then share a common heat sink. Complimentary more implies as mentioned, opposit sex or one each NPN and PNP, an example of which would be 2N3904 NPN and a 2N3906 PNP. Examples of transistor complementary pairs. Matched pairs can also be found marketed in a single case.

Ron