triple darlington?

Its called, now get this, "a Triple"

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

is there any reason you can't make a triple darlington transistor, with
Hfe=Hfe^3 by adding a third transistor to a darlington pair in the same
configuration?

 

ok, thanks.

 

Subject: triple darlington?

Hi, Andy. I would guess you mean something like this (view in fixed font or M$
Notepad):

"Triple"
C
o
|
.---o---o
| | |
|/ | |
Bo-| Q1 | |
|> | |
| |/ |
o-| Q2 |
|> |
| |/
o-| Q3
|>
|
o
E

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

First you need to understand that all transistors have some leakage current
when they're "off". With the combined beta of the "triple", the low microamp
or high nanoamp leakage of Q1 will be amplified by Q2, and then be enough to
turn on Q3. So, even if you have no current whatsoever going into the base,
significant current will be flowing from the collector to the emitter. Not
only that, but the leakage current is temperature dependent, so your triple
transistor characteristics would change dramatically once it was powered up.

Darlington transistors are great for switching things on and off, and they do
have their uses, but there's reasons you don't see them everywhere. Keep
learning about transistors, you'll see why.

Hfe of a regular darlington is usually well over 1,000, anyway. That should be
enough. But it is a good question. I remember asking this myself back when I
was learning this stuff (midway through the Pleistocene Era, as I remember)

Good luck, and keep learning
Chris

 

just tried this out under linux using 'wine' - neat program.

thanks - that was the sort of thing i wanted to know - i.e. how long can
you keep adding transistors b4 it gets ridiculous.

 

Note that practical Darlington's and triples usually have base emmiter
resisters to reduce leakage and speed up turn off.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.