Matching transistors - Measuring hfe

[CBS240]

Hi Shawn,

'snip' What set up did you use to match the Vgs of your devices.

A simple circuit I got from this forum once. I used an old 741.
http://www.diyaudio.com/forums/showt...77#post1096777

What devices were you using and how many pieces did you start and end with, I'm very curious.

http://www.fairchildsemi.com/ds/KS%2FKSK595H.pdf

I have not seen on this forum anyone using this part, so I'm kinda out on a limb here. Regardless of the tiny operating current, I like it and haven't found any real drawbacks in performance or sound. In fact the sound is excellent if used properly. The drawback is in the fact that there is a wide % variance in Vgs on these, at least much more than the j201's I measured. Went through 100 or so in the first measurement run. Measured typical Vgs@100uA of around -0.160V to -0.300V for the KSK595.
First I measure each device and sort them as similar groups of 10-20% or so. When I need a MP, I re-measure from a group to chose the closest two. Once you have them sorted into groups, it isn't so hard to find many matched pairs. In this circuit, there is about 5V Vds on each input transistor and on each cascoding pair. 5 pairs total...one could grow old looking through a pile of 100 jfets just to find matches. The input diff looks like a ladder ...one of my so called 'experiments' ...that actually seems to work.


I use these cheap storage boxes to keep them sorted. When I need more MP's, I now have a narrower margin to look through.

(The top row is some 5087 BJT's sorted by Hfe)

[darkfenriz]

Originally posted by TomWaits

darkfenriz, do you care much about hfe in pre/vas stages? Or does it all depend on the topology? Sounds like a huge factor?

Well, honestly I do my best making a design as Hfe-independent as possible, but that's probably not the answer you expect.
I tend to measure output devices hfe to check if they are genuine.
As far as diff and vas stages are concerned hfe of more than 100 is OK for me, same for drivers and predrivers of output buffer.

[TomWaits]

Originally posted by CBS240
Hi Shawn,
A simple circuit I got from this forum once. I used an old 741.

Thanks for sharing that twice. Slowly sinking in...I downloaded it this time.

Originally posted by darkfenriz
Well, honestly I do my best making a design as Hfe-independent as possible, but that's probably not the answer you expect.

Good answer.

I tend to measure output devices hfe to check if they are genuine. As far as diff and vas stages are concerned hfe of more than 100 is OK for me, same for drivers and predrivers of output buffer.

Thanks,

Shawn.

[AndrewT]

Hi Cbs,
your Vgs measurement set-up shows the DMM connected to the FET gate.
Is there a chance of causing oscillation doing this?

Since there is no current through the 100r could the DMM be connected to pin 6 instead? Thus avoiding risk of oscillation.

[CBS240]

Hi Andrew T

Not sure, but you did point out a mistake in drawing. The DMM was measured across the 1K resistor on pin 6. Didn't notice that

[Mark A. Gulbrandsen]

Shawn,

I think getting yourself a good used curve tracer might be the better route if you're really serious about this. You have measured HFE... but at what operating current? The curve tracer will measure all that at what ever given operating current you want up to device destruction . They can be had cheaply on E-Bay. I paid just 30.00 for mine!

Mark

[TomWaits]

Originally posted by Mark A. Gulbrandsen
Shawn,

I think getting yourself a good used curve tracer might be the better route if you're really serious about this. You have measured HFE... but at what operating current? The curve tracer will measure all that at what ever given operating current you want up to device destruction . They can be had cheaply on E-Bay. I paid just 30.00 for mine!

Mark

Mark, I'm looking higher in the tree and I do want a better result than some $5 DMM. This for the price may beat the pants off anything else right now in my budget.

Thanks,

Shawn.

[Mark A. Gulbrandsen]

Now yer cookin! I may try one of those out myself. Ita alot lighter than the old tektronix is.

Mark

[TomWaits]

Originally posted by Mark A. Gulbrandsen
Now yer cookin! I may try one of those out myself. Ita alot lighter than the old tektronix is.

Mark

I picked up on it in Zero Cool's Curve Tracer Thread. Seems there may be downloadable software upgrades as it evolves.

Screen capture/print and share + easy for achiving. You can't beat it unless you have mucho $$$ to spend. I'm sold... I got to have one!

Shawn.

[dutch diy]

One thing which is adressed in 'curve tracing' in this topic is that hfe is not constant in relation to Ic.

With a multimeter only a small Ic is used to measure Ib and thus hfe can be calculated.

When matching small signal devices there is not a huge miss.

When matching output devices the intended Ic should be used.

I did match 100 MJE15003 and found there was a + and - 2 % spread between measurements with Ic of .7 A and an Ic of .9 A. The average Hfe for .7A was 65.2 and for an Ic of .9 it was 64.7.

The hfe from the datasheet for this device was hfe min 25 and hfe max 150.