BC550C hFE matching

[erik777]

Out of the bunch of BC550C NPN trans.

I have two pairs that are the best out of the bunch.

The hFE for one pair match 563,563
The other pair are 607,595.

Would it be better to get the perfect match of 563 hFE
Or get the higher gain of 607, 595, but be within 2% of the hFE value?

Thanks
El

 

[erik777]

The build directions call for two matched BC550C’s for each mono block in positions T1, T2
I have matched pair at hFE 560, and another at 550 hFE, but not for both monos symmetrically
Option 1: Buy more transistors until I get 4 matched
Option 2: Mono block (1): T1 hFE 560, T2 hFE 560 | Mono Block (2): T1 hFE 550. T2 hFE 550
Option 3: Mono block (1): T1 hFE 560, T2 hFE 550 | Mono Block (2): T1 hFE 560. T2 hFE 550
As a rule of thumb, which is the better layout?

 

[AndrewT]

Out of the bunch of BC550C NPN trans.

I have two pairs that are the best out of the bunch.

The hFE for one pair match 563,563
The other pair are 607,595.

Would it be better to get the perfect match of 563 hFE
Or get the higher gain of 607, 595, but be within 2% of the hFE value?

Thanks
El

couple them together thermally.
then insert them into an LTP test jig.
Couple the emitters and take that to -ve.
couple the bases and take that to the adjustable Vbe voltage.
Attach accurately matched 1k000 resistors to each collector. Take them to +ve.

Now ajust the Vbe so that the LTP passes some current start with 0.1mA
Measure the Vdiff at the collectors. A second DMM that check Vr1k0 checks the collector current, but must be removed else it affects the accuracy of the 1k000
remeasure for 0.2mA, 0.4mA, 0.6mA, 0.8mA, 1mA, etc up to you maximum test current.

Vdiff shows how well your two transistors are matched at any Ie.