Vasile Surducan wrote:

> > The 350MHz op amp and -ve supply rail can
> > probably be replaced with a cheap 200MHz
> > transistor giving the same overall circuit
> > performance, and what's with C1 and diodes? Are
> > you compensating the slow turn-on of the diodes?
>
> No, I compensate the slow turn off of the transistor !
> The capacitor is charging at about 1V...1.4V and
> discharging at off command with reverse potential on Vbe
> allowing fast turn off.
> The problem with a switching transistor is not the turning on
> time but the turning off, and the self storage time.

I understand. :o)

> >
> > I really think it's worth making the thing as
> > dc-coupled as you can, you are driving it with
> > a sharp edged square wave, so just think of it
> > as a very fast output driver.
>
> you have right, but if I don't want a square wave
> but a controlled shape and  amplitude
> by the input signal ? See my schematic from this
> point of view, and as a simple alternative to
> change from LEDs to laser.

Ouch! I thought it was for simple on/off modulation.
My circuit is of no use to you. :o)


>   I'm agree with you here... except that all serious
> laser driver hasn't a simple resistor for constant
> current but a specialised chip soldered very close to
> the laser. That's because of high current treshold
> ( greater than 7Amps ) needed to lasser effect begun.
> I'm talking now about high power lasser diode which are
> not working in CW, but also about 3mW CW laser diode
> from CD-rom drive, both have a specialised circuit.

I don't understand this, i've driven 5mW red lasers
fine with a 100mA constant current supply. You are saying
they need 7A to start? Is that to get them turned on
quickly enough, ie effects of the internal R and C
of the laser?
-Roman

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