VCO-1 expo converter transistors

stewpye · Posted: Fri May 01, 2009 2:03 pm Post subject: VCO-1 expo converter transistors

Hi,

I've been looking for some matched PNP transistors for the VCO-1. I found these at Mouser for $0.48...

BCM857BS,115
http://au.mouser.com/Search/Refine.aspx?Keyword=BCM857BS%2c115&FS=True

I know they are SMD, but I'm experienced with soldering chips with 0.65mm pin spacing. My question is will they be good enough for the expo converter? Datasheet can be gotten from the link above.

HFE typ 290
HFE matching:
HFE1/HFE2 min 0.9 typical 1
So, worst case if one was 300 the other could be 270. Doesn't sound too good. I'm guessing they would usually be closer to 1.

VBE matching MAX 2mV. This seems pretty good.

Would I be wasting my time laying out a PCB for these? I'm planning on making a little expo converter PCB with a right angle header so I can change them if it doesn't work so well.

Cheers,
Stew.

State Machine · Posted: Sat May 02, 2009 5:57 pm Post subject:

Stew,

This seems like good choice and would say that it would work just fine. The 2mV matching is actually worst case so it's most likely better than that and probably is more like 1mV to 1.5mV. When matching transistors on your own from discreet packages, the typical number is 1mV to 2mV Vbe matching.

Let us know your results please ........

Bill