Hi Nick,
Actually, the darlington does add a diode drop
to the Vce of the output trasistor, since the
Vce of the output transistor is in parallel with
the sum of its own Vbe drop and the input transistor's
Vce drop. Even if the Vce for the input transistor is
very small, the Vce of the output transistor will be
held an additional Vbe drop above this value.
Guess you gotta pay the price of the extra current
gain somehow, eh?
73 de Mike, W4EF........
----------
From: Nick Kennedy[SMTP:nkennedy@cswnet.com]
Sent: Tuesday, January 26, 1999 8:51 PM
To: TRLOG@contesting.com
Subject: Re: [TRLog] Need help with Keying/PTT/Paddle interface
Date: Monday, January 25, 1999 11:07 AM
Subject: RE: [TRLog] Need help with Keying/PTT/Paddle interface
>
>> Good point - the darlington adds one diode drop to
>> the Vce saturation voltage! Exactly how low do the
>> modern transceivers need to go anyway? One would think
>> that pulling them down to 1 volt would do the trick.
I don't think a darlington adds a drop to Vce, it would add one diode drop
to Vbe. In a darlington, two base-emitter junctions are in series but there
is nothing in series with the "main" collector-emitter junction.
If you're pulling voltage down low enough for a TTL logic zero, or if you're
turning off a transistor, you need to go a lot lower than 1 volt. Maybe a
couple of tenths ...
73,
Nick, WA5BDU
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