the voltage and current on the diodelaser

[chunjonas]

Hi everyone,
i have designed a circuit to amplify a signal(max.Voltage 2V, pulswidth 2 ns,frequency 1MHz) so that it can drive a laserdiode. the request should be a peak current >1A which flows over the laserdiode. Right now i am using only a HF transistor(NTE 48), the laserdiode is as last between Vcc and collector. the emitter is shorted to GND.

right now i can measure the voltage on the laserdiode with peak 3V, pulswidth ≈2,5 ns. But when i meauserd the current( i used a coil which have 36 windings and parallel to a resistor 36Ω that means 1V is 1A),the peak current was only 100 mA and the pulswidth got wider...it should be more narrow...because acrroding the datasheet 3V should have 3A current...(see pic.)

does someone know how it was going? if anyone of u did so just share me ur expirence i appreciated kindly....

by the way i dlike to use a ulztrafast mosfet to amlify, does anyone know which typ pass this? the request is the rise and fall time of the FET should be in 1 ns range.

Thank you
chun

 

[ronsimpson]

NTE48 is not fast! There is no way it responds in 2nS.

The data sheet is not good! Some where between 100mhz and 1,000mhz the gain=1 and thats with 5 volts CE. You are using the part with maybe 1V from CE so the transistor will be much slower!

You want 3A form a 1A transistor?

I see more problems. What diode are you using?

 

[alec_t]

when i meauserd the current( i used a coil which have 36 windings and parallel to a resistor 36Ω that means 1V is 1A)

I don't understand that. A 1 ohm resistor will drop 1V when passing a 1A current. Why 36 ohms? What is the coil for? Are you trying to make a 1 ohm resistor?
Unless the coil's resistance is 1 ohm (actually slightly above, since it apparently is connected in parallel with 36 ohms) your measurement won't be what you expect.

 

[ronsimpson]

Alec_t,
He might be building a current transformer. Probably not.

 

[chunjonas]

the pic is the IU- Cureve of diode laser (SPL PL90_0). the continouse current of NTE48 is 1 A. what i want to have is a peak current with 3A in a pulswidth 2 ns. I also changed the tansistor by BLY 88A. but the result was not better than NTE48.
or du u have some suggestions on the transistor?

thank u!

 

[chunjonas]

right i built a tansformer with 1:36. so with a 36 ohm resistor it have such relationship.

 

[ronsimpson]

Much faster transistor. Built for RF amplifier. Has good gain at 3A. Has good gain at 150mhz. (good gain=20 to 40)
This is not a "saturated switch transistor" but should be fast. You will have to drive the base with more current than you planed. You might look up "baker clamp" to increase the speed. 1nS edges I don't think so but worth a try. Will be faster if you can keep it class A. I dod not find data on laser.

 

[chunjonas]

Much faster transistor. Built for RF amplifier. Has good gain at 3A. Has good gain at 150mhz. (good gain=20 to 40)
This is not a "saturated switch transistor" but should be fast. You will have to drive the base with more current than you planed. You might look up "baker clamp" to increase the speed. 1nS edges I don't think so but worth a try. Will be faster if you can keep it class A. I dod not find data on laser.

ok. the data for laser u can get from: https://www.electro-tech-online.com/custompdfs/2011/08/OSR_SPLPL90.pdf
thanks and i will try and share the result...

 

[ericgibbs]

hi chun,
Have you considered driving the laser using a transistor in the 'avalanche' mode, this will give a very fast rise time pulse.???

EDIT:
examples

 

[chunjonas]

hi chun,
Have you considered driving the laser using a transistor in the 'avalanche' mode, this will give a very fast rise time pulse.???

EDIT:
examples

Hi
thank u for answer. i have considered such solution. i have read some relatives about that. but i think the supplz for zetex is too large and the generated current is also too large. what i want is a peak current in range 1A to 3A. to decrease the supply i have used some RF transistors (BFR540).make them in alavanche properties. so supply is Vcbo(20V). but unfortionetly it didnt work,hehe.... it seemed it cannot be used in alavanche...

 

[chunjonas]

Much faster transistor. Built for RF amplifier. Has good gain at 3A. Has good gain at 150mhz. (good gain=20 to 40)
This is not a "saturated switch transistor" but should be fast. You will have to drive the base with more current than you planed. You might look up "baker clamp" to increase the speed. 1nS edges I don't think so but worth a try. Will be faster if you can keep it class A. I dod not find data on laser.

do u know some mehtods how to supply the bias more current, if the input signal not changed? i have measured with BLY 88A with a laserpointer as last. the peak voltage on the laserpointer could be 3.8 V. this value stayed almost unchangend if the supply voltage or the resistor between supply and bias changed.
another problem is by using BLY 88A the input signal shap was changed.it didnt happen by using NTE48...