differential amp - pnp or npn

[gearheadgene]

Is there any basis for using pnp vs. npn for a differential amp? I've seen both flavors but not sure why one would be favored over another.

[edl]

Hi,

Better transistors (low output capacitance, high transition frequency) for voltage amplifier stage (VAS) are mostly NPN. Then the differential amplifier stage needs PNP devices.
That's why PNP-diffamp are more common.

Regards,

[tom31415926]

Sometimes the reason for selecting npn or pnp transistors is based on bias conditions for the desired load and the following stage. For example, if the input voltages of the differential amplifier are near ground, and the outputs of that stage need to be close to the positive power supply, then an npn differential pair would be the choice. The same is true for pnp transistors and the negative supply rail.

In power amplifiers with high supply voltages, one sometimes sees two differential pairs - one of each type - in order to achieve symmetry. That way, the differential pairs create drive signals appropriate for gain stages operating from each supply voltage. the outputs of these gain stages drive output buffers through level shifters.

As was said in another post, npn transistors are capable of a higher ft (transition frequency). This is not always an overriding concern.

PNP transistors can have a lower equivalent input noise voltage due to the base material being n-type and having a lower resistance. Changes in device geometry can be applied to both types, however, that reduce the base resistance.

I work with analog CMOS design, but not for audio (unfortunately!) We use differential pairs of n-channel MOS and P-channel mos all the time. There choice is very often dictated by bias concerns.

Tom