Transistor beta repeatability

[MacGyverS2000]

The recent post on collector/emitter reversability has prompted me to ask a long-time question I've had...

In general terms, is there a family of inexpensive, small-signal transistors that has decent repeatability of gain from batch to batch?

For example, I commonly use 2N2222-style NPNs in a lot of my circuits (SOT-23 package). The gain is decent for my typical current and frequency range (<50mA, <1kHz) in the area of 120-150, and they're just so darn inexpensive (<<$0.25/pc in quantity, if memory serves).

Difference in part-to-part Beta is usually counted on one hand when comparing devices on a reel (and I would therefore assume from the same batch). However, comparing reel to reel the Beta can vary 20-30 or more, and that can cause issues with circuit repeatability.

Would there be a more appropriate line of transistors to use that are equally inexpensive, but have a tighter tolerance from batch to batch (never bothered comparing a large batch of 3906s, etc.)? Maybe a more recent process? This is more of a curiosity to me than a pressing design issue...


Dan - Owner

http://www.Hi-TecDesigns.com

 

[MacGyverS2000]

For the smaller units, compact size and low cost is of the essence... adding extra components (especially something the size/cost of an op-amp) is difficult.

Circuit 3 would be downright scary in a production circuit as the forward voltage of an LED can vary significantly from lot to lot (depending upon binning, even from unit to unit). For a one-off, it would be great.

Circuit 4 would probably be the extent to which I could modify, so long as I stick with SOT-23 transistors, but even then I don't know where I would get the extra space from. There will be a lot of experimenting going on, that's for sure...


Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

Dan,

What level of accuracy or matching are you trying to achieve?

TTFN

 

[benta]

macgyver, I understand your concern over the voltage drop over two series connected LEDs.

In that case, you could drive the transistor base from a voltage divider, giving perhaps 2.5 V base voltage? Even at 9 V supply, you should have 6 V available for the 2 LEDs.

Again, if you transistors have enough Beta, base current is so small that it should not be a problem to make the voltage divider with resonable resistance.

D.mn I just added a resistor per LED ;-)

Benta.