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BJT transistor matching

What to measure / match: hFE

IB = base current
IC = collector current
hFE = DC current gain = IC / IB

The specs of a transistor (including hFE) varies with current draw, temperature, applied voltage, ... so if you want to be precise you should measure under the circumstances the transistor will see in his working environment.

complementary pair
a pair of a coresponding NPN and PNP transistor, e.g. BC550/BC560 or 2n5088/2n5087

how close to match the hFE values
5 is almost perfect. 10 to 20 is just fine. Probably you'll have to accept larger deviations (about 30), even with large quantities from one shop, from one manufacturer and one manufacturing batch matches are not guaranteed

time to settle
When testing there will be some initial figure on the display in the first moment, soon after that the figure changes going up (or down). hFE is affected by temperature, while holding the transistor between your finger tips and applying voltage the transistor heats up and thus it's hFE changes (particulary with small TO-92 parts). Just make sure all transistors have the same time to settle, a few seconds are enough.

Don't become obsessed about matching, I've build my PPAs without having any transistor matched and they are working fine. Probably you can measure the difference but it is unlikely that you can hear the difference.

Furthermore you probably won't find matching pairs among the output transistors anyway. Just pick all transistors from a single order, they should come from a single manufacturing batch. Only when you are going to pick transistors from several orders (and several shops) matching should make a significant difference

When you have complementary pairs of different value, choose the high hFE pair over the middle and low pairs.

the easy way: using a DMM

This is the easiest, fastest and most simple way: use a digital multi meter (DMM) that sports a hFE mode. Just put the transistor to the supplied socket, select hFE mode and read the value on the display. The DMM calculates the hFE for IB being a fraction of a mA but that's precise enough for most applications.

deviations between DMMs
most probable reasons:

  • different loaded batteries
  • different current measure points

simple grouping by collector voltage

just measure the collector voltage and group the transistors by their voltage without calculating the hfe (easy + fast)


calculating hFE

extensive and time-consuming, you need to measure two voltages and calculate the hFE values via a spreadsheet


VRB = voltage across base resistor
VRC = voltage across collector resistor

base current IB = VRB / RB

collector current IC = VRC / RC

hFE = IC / IB

hFE = ( VRC / RC ) * ( RB / VRB )

please note that you have to reverse the polarity when switching from npn to pnp transistors and that there is the risk of cooking the transistor when you connect it in a wrong way. Double check the pinout of the transistor (refer to datasheet!) and your wiring before applying voltage.