is this normal BJT behavior?

put two transistors in parallel hoping they'd share I_c evenly

I_bb is connected to arduino PWM and i varied it

i put an ammeter at each collector site. and here's the current distribution i get.

(so on the arduino i only wrote down the PWM analogWrite value i forgot to check the actual corresponding voltages)

PWM I_C1(mA) I_C2(mA)

190 125 148.7
200 124 151
210 115 154
220 111 155
230 90 159
240 92 154

why the inverse current sharing? how can i fix?

Are they the exact same transistor, from the same wafer, with the same Hfe, and thermally coupled?

You can sort of "balance" them by using small emiter resistors.
Or a (~100ohm) trimpot, outside pins to the two emiters and wiper to the resistor.
Or buy a matched "double-transistor".
Why this circuit?
How did you measure those currents.
Leo..

So really, you are looking for a thing called a “current mirror”, are you not?

I'm intrigued: why do that?

Just didn't have the "right" bjt handy, same as using whatever resistors you have handy to make the required resistance?

hey guys i actually i figured out why. the resistances on my two ammeters haha. but thanks all

Paul__B:
So really, you are looking for a thing called a "current mirror", are you not?

hey question about transistors in general though, your diagram implies collector base junction does not need to be reverse biased right?

Hi,
caution..
You are measuring a pulsing DC current due to the OP using PWM to switch transistors.
If the transistors were fully on, which they cannot be.
Max collector current will be.

Assuming Vcesat = 0, just humour me.

5/2570= 0.0019A

i put an ammeter at each collector site. and here's the current distribution i get.

How are you measuring over 100mA in each collector?
What are the transistors? That is their part number.

Tom..... :slight_smile:

bmarconi:
hey question about transistors in general though, your diagram implies collector base junction does not need to be reverse biased right?

If you mean that the collector voltage can drop to the same or less than the base, then yes it can. In saturation, the collector voltage can drop to 200 mV or less, while the base will still be on the order of 700 mV or more.

bmarconi:
hey guys i actually i figured out why. the resistances on my two ammeters haha. but thanks all

More likely the different Vbe and Hfe characteristics of the transistors.

Russell

You should always use a balancing resistor in each emitter, when parallelling bipolar transistors.
Because bipolar transistors have a positive temperature coefficient, tending to increase the Collector emitter current with temperature.
This means that one transistor will always take the lead and get hotter than the other, thus overtaking the current flow.
The individual emitter resistor is a simple means to counteract that.

You dont need to do it, if you are using FET transistors, as they have a negative temperature coefficient, meaning they tend to drift against lesser drain-source current with temperature. That makes for a natural balance.