In Common emitter Circuit - higher amplification

[ulix]

Hi,
I was just wondering, which transitor npn or pnp has a higher amplification in the common emitter circuit?
I would say, it's the npn type, but how can I prove this?
cheers

[langwadt]

Hi,
I was just wondering, which transitor npn or pnp has a higher amplification in the common emitter circuit?
I would say, it's the npn type, but how can I prove this?
cheers

why would the amplification be different just because you flip it upside down?

[ulix]

npn to pnp doesn't mean flipping!? it's a diffrent transitor.

[Marco]

If it's degenerated sufficiently the amplification depends on the ratio of the emitter and collector resistors.

If it's not degenerated and at sufficiently low frequencies the AC amplification is purely a function of biasing, it is independent of NPN/PNP or even device material. This is because gm is purely a function of Ic, see page 24 of this class handout. I did not know this part before googling.

PS. the only time I see non degenerated common emitter amplifiers is in RF circuits.

[vk6zgo]

npn to pnp doesn't mean flipping!? it's a diffrent transitor.

If it was 1964,the standard answer would be "an NPN",but,over the years,PNPs have improved,due to better manufacturing processes,till there is no real difference.

There are still more NPNs around,though.

[dom0]

The fundamental difference between hole and electron mobility is still there, and a few in-silicon processes are different, too. So you still don't get identical transistors just by flipping N and P.

That being said the common emitter circuit will behave similar for transistors with similar key specs (fT, miller capacitance, beta), independent of NPN or PNP. Of course, finding RF PNPs tends to be not as easy as coming up with a NPN.

This is because gm is purely a function of Ic, see page 24 of this class handout

Correct. Although, as you mention, too, it is only true for low frequencies, where the Miller effect may be neglected. However, the equation Rbe = Ut / Ic is still a very important one in many fields, e.g. linear supply design, AF amplifiers etc. and often disregarded (with corresponding, i.e. bad, results).

[ulix]

Both should have the same dimensions...

[T3sl4co1l]

Complementary PNP are about 10% worse (mainly as hfe with respect to frequency, and Vce(sat)), but this is much less than the >40% difference in a single part (regardless NPN or PNP) due to manufacture and temperature variations.  If your circuit will fail due to this difference, it's a textbook Bad Circuit.

Tim