[sdiy] Interesting PNP's

[Ian Fritz]

Hi all --

Nosing through my Digikey catalog I noticed that Zetex has some interesting 
high-gain, medium-power PNP devices.  To check these out, I ordered 10 ea 
ZTX795A.  Here are some of my characterization results.  I will compare 
these with the MAT03 pairs from AD.

The devices are reasonably priced at $6.75 for 10.  (MAT03F,  $14 per 
pair.)  Measured Beta's were in the range 340-380. (MAT03F,  typical, 
120).  Four separate pairs had Beta matched to 2% or better (MAT03F, 
typical, 0.5%).  Vbe matching for the one matched-Beta pair I looked at was 
smaller that I could measure, i.e., less that 0.1mV.  (MAT03F, typical, 
0.01mV).  To measure the bulk resistance Rbe I had to drive up to about 40 
mA through the b-e junction, and I obtained a value of a mere 0.12 
Ohms.  (MAT03F, typical, 0.3 Ohms).  I didn't try to measure leakage or 
noise.  Maybe someone else would like to take this on.

My initial impression is that these would make pretty decent exponential 
converters -- four, well-matched, high-gain pairs for $7 seems pretty 
reasonable to me.  These also look interesting as multiple mirrors for 
multi-stage filters, etc., since the parameter spread is really quite 
small.  For critical current mirror applications it might still be worth 
paying for the MAT03, although I characterized the one pair differentially 
in a mirror configuration and couldn't see any change in the measurements 
when I swapped the two units.

Does anyone have a simple way to measure log conformance, without fancy lab 
equipment?

   Ian