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PICList Thread
'DCC vs Darlington Transistor'
1998\07\26@094616 by Thomas McGahee

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Dwayne,
Have you considered using a Direct Coupled Compound (DCC) arrangement? This
has the gain advantage of a Darlington, but has only one forward
diode drop (Vbe drop). A DCC has gain = Beta1*Beta2. The 'device' has the
characteristics of the input transistor. A disgusting ascii drawing follows.

*****************************************************
Direct Coupled Compound (DCC). "NPN" Equivalent Shown.
One Vbe forward diode drop. Gain=Gain_npn*Gain_pnp

            Effective Collector
                     |
                     |
                PNP  V (E)
                   |/
             ------|
       NPN |/      |\
Base--------|         | (C)
           |\        |
             V       |
             |       |
             --------*
                     |
                     |
              Effective Emitter

*******************************************************
Add this to your 'collection'.

Hope this helps.
Fr. Tom McGahee

>
> > Hi!
> >
> > I'm looking for recomendations for TO-220 NPN and PNP transistors that have
> > moderate current gains at about 1 Amp.  Should be good for at least 40V,
hfe
{Quote hidden}

1998\07\26@100721 by paulb

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Thomas McGahee wrote:

> Have you considered using a Direct Coupled Compound (DCC) arrangement?
> This has the gain advantage of a Darlington, but has only one forward
> diode drop (Vbe drop).

 I'm not sure it was the *input* bias (effective Vbe) that was
concerning the original poster.  More likely the effective Vcesat, which
is identical to a darlington and consists of the Vcesat of the input
transistor *plus* the Vbe of the second.

> A DCC has gain = Beta1*Beta2.

 Virtually identical to a darlington of course, where it is
Beta1*(Beta2+1).

--
 Cheers,
       Paul B.

1998\07\26@124910 by Morgan Olsson

picon face
This is very useful.
I once used five transistors in a serial connection like this.
I could almost read my thoughts.
Had to use implement b-e resistors to keep it off.
Has also slow turnoff without them.

The LP324 OpAmp i think i remember has a smart connection of three
transistors that first work like this, then when PNP saturated, if
sufficiently low collector current, there was anither NPN that could
achieve a effective saturation under 0,1V or something.
Anybody know how that trick is done?
/Morgan

At 09:41 1998-07-26 -0400, you wrote:
{Quote hidden}

have
>> > moderate current gains at about 1 Amp.  Should be good for at least 40V,
>hfe
>> > greater than 100, reasonably robust.  My old standbys TIP31, TIP32,
TIP41,
>> > TIP42 don't have enough gain and the TIP120 - TIP127 are darlingtons and
>> > have too much voltage drop.  I don't need complimentary pairs so
unmatched
{Quote hidden}

/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\  mrtspamKILLspaminame.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /

1998\07\28@055531 by Tom Handley

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  Tom, thanks for posting this. While I take this for granted in ICs I
have'nt considered this circuit in a long time. I was also thinking
Darlington. I had earlier made a comment about how many questions asked
in this group force us to visit subjects we have'nt looked at in a long
time and this is a good example.

  - Tom

At 09:41 AM 7/26/98 -0400, Tom McGahee wrote:
{Quote hidden}

Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs...

1998\07\29@110931 by Peter L. Peres
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On Tue, 28 Jul 1998, Tom Handley wrote:

>    Tom, thanks for posting this. While I take this for granted in ICs I
> have'nt considered this circuit in a long time. I was also thinking
> Darlington. I had earlier made a comment about how many questions asked
> in this group force us to visit subjects we have'nt looked at in a long
> time and this is a good example.

Well, since we're at it, have you considered a MOSFET with a Resistor (and
maybe a diode or two) in parallel with Gate and Source ? Because this will
do precisely what you are after, and outperform both the Darlington and
the other configuration in the process, and may be cheaper on account of
the lower part number used.

regards,

       Peter

1998\07\30@055429 by Tom Handley

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  Peter, I was just commenting on Tom's circuit which is something I had
not considered in a long time. In my own work, I normally use MOSFETs.
Actually, I have'nt used discrete bipolar transistors in a long time other
than IGBTs. His comment triggered a `rusty old brain path' that said;
"Oh yeah, I forgot about that" ;-)

  - Tom

At 06:53 PM 7/28/98 +0000, Peter L. Peres wrote:
{Quote hidden}

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