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how to compensate for pulse broadening in Darlington Pair

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Newbie level 6
Apr 20, 2011
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Hi all,

a Darlington pair with two 2N2222 transistors were used to drive a laser diode working at 1.2A, however, there is about 100ns delay in the optical output as well as two or three times the pulse broadening wrt to the electrical signal. delay can be tolerated but not the optical pulse broadening.

is there a way it can be corrected somehow? is there a better circuit with other components i can achieve the same current and speed. ?

my thought is that 1.2A is out of spec for 2n2222 even if the current is pulsed. Whats more your probably over-saturating the the transistor, hence the pulse
broadening (ie slow turnoff). An additional resistor between the 2nd transistor base and emitter circuit can help some.....

a better circuit and easier to achieve your requirements is using a FET/MOSFET.
thank you very much i will check that.
is there a FET or Mosfet included fast switching circuit structure you would recommend?

There are purpose-made laser diode driver ICs out there.
Which will do things like give you control of threshold
current. Not to mention being way, way faster and
likely more current-drive-capable.

If you want the Darlington to turn off quicker (like the
same timescale as turnon) you have to pay attention
to the second base, its discharge time (which is handled,
how?) needs to be on the order of turnon time that is
fed by emitter follower front end at very low net
impedance. You probably don't want that impedance
presented as a full time load at that node. You might
look at a squaring circuit, but I think you just want to
walk away from '70s discrete technology and embrace
the '90s.

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