ltp transistor matching

[sakis]

can some body point to me any schematic or circuit that will help me match a pair of 2N5401 or BC556 for an ltp stage .....???

unfortunatelly i own a fluke that features a hfe meter but its damaged beyond repair ....

any other options ????

[AndrewT]

http://www.diyaudio.com/forums/showt...46#post1245546

http://www.diyaudio.com/forums/attac...amp=1183734588

I find the near matching by using one of the LTP as a fixed reference.
Then compare a batch to the reference, noting the Ic of the DUT (V+ to Tp1b) and noting the base current (common to Tp2b).

label each DUT with these two readings and pierce them into an A4sheet.

Now find sets that are very close for both these readings. Place a selected pair in the jig and note the difference in collector voltage (Tp1a to Tp1b). and note difference in base voltage (Tp2a to Tp2b). If both these differences are zero then you have an exact match for both hFE and Vbe at that current. Now see how far you can change the current both up and down and for the differences to remain near zero. A 2k0 pot across the 120r header pins allows you to swing the current and see the deltaIc and deltaIb change or stay almost fixed.
If both DUTs face each other and are clipped together and pass the same Ic then they must both be at the same temperature. Temperature is the biggest problem for us amateurs to overcome!

To check a matched pair more precisely, a 4pin header or dpst switch to short out the two 10k base resistors allows Ic to be compared when Vbe is exactly equal.

Good organisation and a bit of patience and you will find many nearly matched pairs and a few super matched pairs. If you're very lucky you might find an ultra matched pair. Keep it for something really special.

[sakis]

and actually very sophisticated andrew !!!!!

thank you very much ( i will also check the links )

thanks

[Nico Ras]

Originally posted by AndrewT
...... and a bit of patience and you will find many nearly matched pairs and a few super matched pairs. If you're very lucky you might find an ultra matched pair. Keep it for something really special. [/B]

Very good Andrew, and a great explanation.

Nico

[AndrewT]

Comparison is a very powerful tool.
We do not have the resources to do absolute measuring.
Think about what we do with a measuring bridge. It's comparison again.

[sakis]

Originally posted by AndrewT
Comparison is a very powerful tool.
We do not have the resources to do absolute measuring.
Think about what we do with a measuring bridge. It's comparison again.

since and as you said this is a painfull job to do i thought about starting with something very simple like a chinese multimeter that features hfe function ....

i was actually very surprized with what i found in my BC 557 storage for example .....

normally i buy these from a good company like sgs or philips and the all thing is like a big mess

as about BC 559 hfe in the same make and family vary from 120 to 400 ( ????? ) amazing ....

the average though between my BC 559 is arround 300 ....do you think that figures of 400 are realistic ????

[AndrewT]

the BC range go from 110 to 800 hFE @ Vce=5V and Ic=1mA and Ta=25degC.

If you use a multimeter you have no idea what the test conditions are and worse they will be different for every transistor because most use near constant Ib and monitor Ic. this means each transistor runs at different Ic if hFE varies and thus run at a different temperature and when the current varies one would probably find that the Vce has changed as well. Forget using the DMM transistor function for anything other than an indication, mostly it will be wrong!!!

[Nico Ras]

Dear Sakis,

Andrew is very right in what he is saying. You can try this with measuring Hfe on the meter, if you hold the transistor between your fingers, just the heat will change Hfe. You will find the same transistor measures different depending on the temperature.

Now the bridge method that Andrew describes, I think is a good way because the two transistors are compared under identical ambient conditions.

I tried playing a little with the idea yesterday and believe me it is a very tedious and boring exercise.

I am skeptical that there is any audible difference doing this, it would mean measuring every component that you use.

Nico

[mjf]

hello.
if i want to do it quickly,i use the hfe-test on the dmm.is better than nothing..............and it always worked well.
regards..............

[nigelwright7557]

Originally posted by sakis
can some body point to me any schematic or circuit that will help me match a pair of 2N5401 or BC556 for an ltp stage .....???

unfortunatelly i own a fluke that features a hfe meter but its damaged beyond repair ....

any other options ????

Personally I dont think matching the LTP transistors makes any discernible difference.

You are more likely to find problems from delays around the loop than in the LTP stage.