Transistor issues-saturation mode

Hi all,

I am using a 2N2219A bipolar NPN transistor for my project.
I want the transistor to work in saturation region so that the drop across Vce=Vce(sat)=.3V.

but i need Ic not to increase more than 70mA.
i tried 2things :

1. made Ib=7mA, so that the transistor is in saturation which worked but Ic got more than 100mA.

2. made Ib=.7mA as hfe=100 but the transistor was not in saturation mode so the drop across Vce was more than 0.3V.

Can anyone please suggest me how to design the transistor biasing so that I have Ic=70mA and also Vce=Vce(sat).

I have also attached the datasheet of 2N2219A.
View attachment 2N2219A_datasheet-7.pdf


Thanks
Mukund

I wouldn’t try to find exact Ib current that produces Ic=70mA @ Vce<0.3V as if you use different transistor the values will be different ..
Try to use decent Ib current that will for sure produce saturation and then limit the Ic current to whatever level you need (70mA or so) with a resistor connected between [C] and [Vcc] ..

IanP
:wink:

I don't see how you can achieve what you want. If you want the transistor to saturate then the collector current will be whatever value is dictated by the load resistance and power supply voltage. You cannot limit the current and still have saturation. You can have one or the other.

Keith

I think, the problem should be rephrased like this: The original poster wants an output stage with low voltage drop, that limits the current at exactly 70 mA.

Obviously, it's impossible with a simple transistor output stage. Or strictly spoken it's completely impossible unless you're specifying two current levels: A minimum current that still achieves Vce,sat < 0.3 V and maximum short circuit current. In any case, both values must be different. Most devices implementing a short circuit protection are achieving ratios between both specifications of e.g. 1:1.5 or 1:2. This can be done with a simple current mirror. For an exact current limiting slightly above the specified output current, you need real current measurement, e.g. by a shunt resistor and a sense amplifier.

The original post says he wants 70mA and VceSat=0.3V. If you fix the power supply then the load resistor must be a specific value to meet those conditions or vice versa. The requirements must be changed to something that makes sense.

Keith

please give the following data:

Vcc
min/typ/max hfe of the transistor.
(Ic=70mA as given)

If you want tight control then you need a feedback scheme.
If you want simple current limiting then maybe just a plain
current mirror would do. We don't know why you want what
you want, or under what conditions, so can't say whether
you're asking the right questions.

IanP said:

I wouldn’t try to find exact Ib current that produces Ic=70mA @ Vce<0.3V as if you use different transistor the values will be different ..
Try to use decent Ib current that will for sure produce saturation and then limit the Ic current to whatever level you need (70mA or so) with a resistor connected between [C] and [Vcc] ..

IanP
:wink:

Click to expand...

I used a higher value resistor between vcc and collector and limited the current to 70mA but the problem was that the voltage drop across the resistor is so high that am not able to get a decent voltage drop to the load am connecting after the resistor which in this case is my laser diode which needs an operating voltage of 2.5v and 70mA current..

---------- Post added at 10:20 ---------- Previous post was at 10:18 ----------

srizbf said:

please give the following data:

Vcc
min/typ/max hfe of the transistor.
(Ic=70mA as given)

Click to expand...

Vcc=5V and am connecting a laser diode after the resistor(Rc).
hence i need to limit the current as well as provide a potential drop of 2.5V too.
if i increase the resistor value , the drop across is increasing proportionately...
can u suggest me the most suitable mode of operation for my application. ?

try:

Rc=33ohms--->laser diode(anode )-->laser-cathode -to- collector of 2n2219a.
Rb=2.2k-->base of 2n2219a
emitter gnd.

You really should read the replies in your other thread:https://www.edaboard.com/posts/810145/#post810145

A CONSTANT CURRENT/VOLTAGE IS NOT THE WAY TO DRIVE THOSE LASERS.

Keith.

have not seen the thread pointed by keith1200rs earlier.

so my suggestion may not be appropriate .

keith1200rs said:

You really should read the replies in your other thread:https://www.edaboard.com/posts/810145/#post810145

A CONSTANT CURRENT/VOLTAGE IS NOT THE WAY TO DRIVE THOSE LASERS.

Keith.

Click to expand...

Keith,

Thanks for all the help.
I read all the replies from everyone about my issue.
I even mailed thorlabs about this and they suggested me to buy a laser diode driver.
I talked to my prof under whom am doing this bio-mems project and he still asks me just to control the current for the laser diode to work properly.

So am stuck with it.

This forum is really helpful and I would like to thank everyone for their sincere and prompt suggestions.

Thanks all.

Mukund

Well, I don't know what to say. I have been designing laser drive circuits for over 20 years and have used two types - ones with internal monitor photodiodes which need a closed loop optical power control circuit and high power pulsed ones which can be driven with a high current pulse. Yours is the first type and needs an optical feedback loop using the internal photodiode. If you don't, you will blow them up. You need an optical power meter to set them, if I haven't already mentioned that.

That is it.

Keith

keith1200rs said:

Well, I don't know what to say. I have been designing laser drive circuits for over 20 years and have used two types - ones with internal monitor photodiodes which need a closed loop optical power control circuit and high power pulsed ones which can be driven with a high current pulse. Yours is the first type and needs an optical feedback loop using the internal photodiode. If you don't, you will blow them up. You need an optical power meter to set them, if I haven't already mentioned that.

That is it.

Keith

Click to expand...

Keith,

I understand your point. I am very new to laser diodes and this is my first experience with designing 8 laser diodes that switch at 100us (that s the need for the project)...

Do you have any designs with the closed loop optical power control circuit which suits my needs ? Can you please post them or send it to my mail id ? that would be really helpful and I can atleast talk to my prof about this ..

Thanks
Mukund

While the circuits are not well drawn, there is lot of useful information here: Sam's Laser FAQ - Diode Laser Power Supplies

Be careful - he shows bad circuits as well as good ones! I saw a lot of bad circuits when visible laser diodes came out in the 1980s. Reliability was poor because a lot of people simply didn't take any notice of the data sheet.

I wouldn't recommend opamps as the basis for a control circuit. While they can be used you can often get an unpredictable startup surge. It doesn't take many microseconds to blow up a laser.

All those circuits are fairly slow. You can modulate them by adding a resistor to source/sink extra current into the laser. That relies on knowing the slope efficiency (mW/mA) and setting the laser power at half the rated power. Then you can get maybe 80% modulation very simply, and up to tens of MHz or more with no problem.

For genuine on/off control at speed you really need to look at proper laser drive ICs. I haven't used them, but people like Analog Devices, Maxim and iC-Haus are worth looking at.

Keith.