Russell,

If I recall correctly, I needed something like 1A current ability and
24V rating - not serious power but also not typical SOT-23 size,
either. I actually used a pre-packaged Darlington, not the scheme I
was advocating with the separate collectors, because in my case I
didn't care about Vsat.

I'm guessing that the true Darlington has higher switching speed due
to it keeping the higher-current BJT out of saturation?

Interestingly, heterojunction bipolar transistor microwave amplifiers
often use a Darlington configuration, like this one:

http://www.minicircuits.com/pdfs/ERA-5+.pdf

Of course, they are not switching and are not in saturation. I think
it is used strictly for increased high-frequency gain.

Is a "trilington" a buffered Darlington (three transistors cascaded)?

I had a humorous situation once where I decided to see what would
happen if I cascaded as many BJTs as possible and use it to indicate
very tiny current flow, like a 9V battery from one hand to another.
Not surprisingly, eventually the leakage current would bias it
partially on - I think at about 5 transistors in my case. I had an LED
in the collector and noticed that it was varying in brightness in a
somewhat random pattern. I replaced the LED with a speaker and heard
the "Pink Panther" theme song :) I was receiving a nearby AM radio
station because of rectification in the partially-on transistors.

Sean


On Fri, Jun 5, 2015 at 11:06 PM, RussellMc <apptechnz@gmail.com> wrote:
> On 6 June 2015 at 14:18, Sean Breheny <shb7@cornell.edu> wrote:
>
>
>> I had a project recently where I designed-in darlingtons because there
>> are very few power mosfets which are truly specified to work below 2V
>> gate drive, across temperature and manufacturing variation.
>>
>
> Sean -
>
> Read "as I know you know" in front of any (correct :-) ) assertions that
> follow.
>
> What constitutes "power" in that context - voltage, current, device
> dissipation , ... ?
> Increasing Vds ratings usually lead to increasing Vth, all else being
> equal, so your experience may depend on required voltage rating.
>
> Aside:
> Where you say you 'designed in Darlingtons'  I assume that you mean the
> arrangement that you described above (deleted) where the driver collector
> is returned to supply rather than to load.
> This is not strictly a Darlinton pair, but is commonly so called and is o=
f
> course a very useful arrangement.
> This greatly improves on (saturation) voltage at the expense of turn off
> times. (I'm always sad that the olde MC34063 uses a true
> Darlington. This was to allow then to get the """high""" switching speed
> of 100 kH using the technology of the day, but results in very poor outpu=
t
> stage Vsat.
>
> [I have some high power "trilingtons' here ex an old high power UPS.
> Saturation voltages are horrendous but not too important due to voltage
> ratings]
>
>
>
>        Russell
>
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