spendorspain
New Head-Fier
- Joined
- Apr 10, 2005
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Hi
I’ve searched this forum regarding BJT hFE matching and I’m a little confused. There are head-fiers that favour Vbe measurement (which I don’t know how to make) over simple hFE testing. Others say the hFE exact value is less important than the relative values between pnp, npn and npn-pnp complementary pairs (so the quality and absolute accuracy of DMM is less important, as it is used only to measure the difference).
I’m not sure if I can trust the measurements made with my digital multimeter (a cheap “YFE YF-602”) using its “b-c-e” hFE test socket, but
I have measured these hFE values for the transistors to be used in a WNA class A headphone amplifier:
For the four BD139 (used as a pair with BD140 in the output stage):
161, 161, 162 and 162
For the four BD140:
172, 174, 177 and 177
For the two MPSA42 (used as a pair with MPSA92 ahead of the two pairs of BD139/140 output devices):
152 and 153
For the two MPSA92:
165 and 171.
As you can see the hFE difference in output devices (both ST Micro-made) is around 1% between the pnp and npn respectively and below 10% between each npn-pnp pair (e.g.: the worst case is a pair with BD139 hFE=162 and its complementary BD140 hFE=177, about 9% difference) .
However, I’ve read that it is more important to match the input transistors rather than the output ones (as the overall negative feedback can compensate a small difference in the latter case), and unfortunately the hFE values of MPSA42/92 have more difference (around 4% between the pnp and npn respectively and around 10% between the npn-pnp pair).
My questions are:
1) A transistor mismatch in an amplifier can lead to sonic degradation or more dc offset at output, so is the matching above precise enough, both for the BD139/140 and MPSA42/92?
2) If the BD139/140 are used in a rail splitter is its hFE matching so important?
Thanks in advance for your comments
Best regards
Jose
I’ve searched this forum regarding BJT hFE matching and I’m a little confused. There are head-fiers that favour Vbe measurement (which I don’t know how to make) over simple hFE testing. Others say the hFE exact value is less important than the relative values between pnp, npn and npn-pnp complementary pairs (so the quality and absolute accuracy of DMM is less important, as it is used only to measure the difference).
I’m not sure if I can trust the measurements made with my digital multimeter (a cheap “YFE YF-602”) using its “b-c-e” hFE test socket, but
I have measured these hFE values for the transistors to be used in a WNA class A headphone amplifier:
For the four BD139 (used as a pair with BD140 in the output stage):
161, 161, 162 and 162
For the four BD140:
172, 174, 177 and 177
For the two MPSA42 (used as a pair with MPSA92 ahead of the two pairs of BD139/140 output devices):
152 and 153
For the two MPSA92:
165 and 171.
As you can see the hFE difference in output devices (both ST Micro-made) is around 1% between the pnp and npn respectively and below 10% between each npn-pnp pair (e.g.: the worst case is a pair with BD139 hFE=162 and its complementary BD140 hFE=177, about 9% difference) .
However, I’ve read that it is more important to match the input transistors rather than the output ones (as the overall negative feedback can compensate a small difference in the latter case), and unfortunately the hFE values of MPSA42/92 have more difference (around 4% between the pnp and npn respectively and around 10% between the npn-pnp pair).
My questions are:
1) A transistor mismatch in an amplifier can lead to sonic degradation or more dc offset at output, so is the matching above precise enough, both for the BD139/140 and MPSA42/92?
2) If the BD139/140 are used in a rail splitter is its hFE matching so important?
Thanks in advance for your comments
Best regards
Jose