Bipolar transistor Hfe values

[k1jroth]

Hello

I`m buildin amplifier with bipolar onsemi transistors.
http://sound.westhost.com/project68.htm

I`v measured Hfe values with multimeter and all NPN transistors are 120+-1 and PNP 160+-1.

Now i don`t have a clue that should all transistors be same Hfe value or does it matter if PNP and NPN have different gain valua?

regards jyri

[Mr Evil]

Ideally, the NPN and PNP parts would be precisely matched, howerver in practice it is futile to try, since a lot more than Hfe differs between them.

It would definitely be worth matching Q2 and Q3, but none of the others matter much.

[richie00boy]

I assume from those gain values that you are talking about small signal transistors. For P68 the NPN/PNP gains do not have to be that close as the design is not symmetrical. It's good to have close values for each type though, and it looks like you have nicely matched devices

Only the output stage (which is symmetrical) should have matched gains, in an ideal world. But there are other factors that are more important here such as transfer characteristic.

[k1jroth]

I`m building dual versio of the amplifier.
Transistors which i measured are Q9,Q11,Q9,Q11 Hfe 120+-1 (NPN)
and Q10,Q12,Q10,Q12 Hfe 160+-1 (PNP).

I have lot of these transistors.

What kind of gain should i search for Q7 and Q8?

[k1jroth]

transfer characteristic.

richie could you explain what you mean by that.
Measured values are from output stage transistors.

[Sch3mat1c]

Originally posted by k1jroth
http://sound.westhost.com/project68.htm

Yipe.

I`v measured Hfe values with multimeter and all NPN transistors are 120+-1 and PNP 160+-1.

Either they've already been matched or your meter is dead. In any case, you've certainly got the temperature even on those...

Tim

[jwb]

How do you use a multimeter to measure Hfe?

[wrenchone]

A lot of multimeters have a set of sockets where you can plug in small signal transistors and measure their HFE. The rub is knowing what collector current is used for this measurement, as HFE can vary quite a bit over several decades of collector current. For output transistors, I would think that VBE at the intended quiescent current would be a more important measurement, as it will allow one to more accurately predict the needed values for a bias network. As for other transistors, VBE matching is important for an input differential pair, as this will ultimately determine the output offset one must deal with to reduce DC through the speakers. Output transistor HFE is important to know at the maximum intended peak output current for the design, as it will help one to size the driver transistors.

[Sch3mat1c]

Or just use standard Good Practice and use large Re's. Matched silicon is a misnomer.

Tim

[richie00boy]

Transfer characteristic is the correlation between base current or voltage, and the collector current. If you look at the datasheet for your devices there will be a graph that explains it visually and it should be quite easy to understand.