Do the transistors in the VAS stage need to be matched PNP/NPN pair?

[jazzclassics]

hi,

In the VAS stage, there are usually two transistors, one being the VAS and the other perform the current source like shown in the below diagram. But Since the current source only supplies a fixed current and does nothing to change the amplification, why it always appear in NPN/PNP pair in most designs?

Jazz

 

[rsavas]

Do not think so, since one is acting as a constant current load. If it was a push-pull VAS then I think it would matter a lot more.

 

[godfrey]

IMHO, they don't need to be matched. However their power dissipation is about the same, so it makes sense or them to be roughly the same size.

 

[jazzclassics]

IMHO, they don't need to be matched. However their power dissipation is about the same, so it makes sense or them to be roughly the same size.

Thanks godfrey.

Do they need to have the same Vce/Ft etc? If they do not need to be a complementary pair, then we should be fine to use any transistor that supplies the current and sustains the 2x rail voltage. For cost reasons, it can be recommended to just use the most readily available and cheap parts to perform the CCS, as long as it satisfies the forestated current and voltage requirements.

Jazz

 

[mcd99]

For single ended VAS I would just use the best transistor for each position/job. For example, BC5x0c if there is low Vce (if you decide to use a 2 transistor current source) and 2SA992/2SA1845 or 2SA1381/2SC3503 where low Cob is important within the active part of the VAS (Q5).