Collector current less than base current in saturation

Thread Starter

Zotto

Joined Mar 3, 2016
45
Hey I was wondering for the circuit I have here if I am doing the transistor analysis correctly. I am finding the base current to be 4.3/330 ~= 13mA, and the collector current, because the bjt is in saturation, the current through the collector is 5-Vce(sat ~=.3V) - Vf(~2V)/330 ~= 8.2mA? Can this happen in a circuit, or is my analysis incorrect?

The picture is pretty poor, but essentially the base of the PNP transistor is connected to a 330ohm resistor and ground, the collector is connected to a 7seg display, which is connected to a 330ohm resistor aswell, and the emitter is just connected to the Vcc rail which is 5V. Thank you for any help.
 

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wayneh

Joined Sep 9, 2010
16,400
Yup. As you are seeing with your own eyes, it's happening. Your emitter current should be the sum of the two, if you want to check it.
 

Thread Starter

Zotto

Joined Mar 3, 2016
45
Yup. As you are seeing with your own eyes, it's happening. Your emitter current should be the sum of the two, if you want to check it.
Don't have the circuit in front of me just yet, just doing some theory calculations right now.
 

#12

Joined Nov 30, 2010
18,210
Confirm. It is entirely possible, sometimes even practical, to overdrive the base and starve the collector.
It's just a saturated switch being very good at acting like a switch.
 

wayneh

Joined Sep 9, 2010
16,400
I'm pretty sure you can even roast the transistor with excessive base current without even connecting the collecter to anything. Been there...
 

hp1729

Joined Nov 23, 2015
2,304
Confirm. It is entirely possible, sometimes even practical, to overdrive the base and starve the collector.
It's just a saturated switch being very good at acting like a switch.
"Practical" to overdrive the base? Why?
 

#12

Joined Nov 30, 2010
18,210
"Practical" to overdrive the base? Why?
For instance, several loads might be switched on from the high side with this one switch in series with the ground for them all.
When only a few of the loads are switched on, the grounding switch is over-driven, but entirely capable of grounding the remaining loads when they switch on.

The first thing that comes to mind is a 7 segment display where segments are driven from the high side and the NPN on the low side is a master shut-off for blanking.
 
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#12

Joined Nov 30, 2010
18,210
I'm pretty sure you can even roast the transistor with excessive base current without even connecting the collecter to anything. Been there...
A quick calculation shows about 500 ma through the base-emitter junction to smoke a 375 milliwatt transistor.
That's assuming I actually remember a 375 milliwatt transistor.:rolleyes: What's a TO-92 good for in watts?

625 mw for a 2n2222
Gonna take most of an amp to smoke that.
 
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MrAl

Joined Jun 17, 2014
7,849
Hi,

I was going to say the same thing that has already been said here, but let me restate it.

For example, you connect a 10 amp current source to the base emitter, the base current is 10 amps, and you do not connect anything to the collerctor, the collector current is a big fat ZERO. Is the base current larger than the collector current? Sure is :)
Not satisfied with that extreme example? Then connect a 1ma current source to the collector emitter. What now? The base current is 10 amps and the collector current is 1ma, so still the base current is much greater than the collector current.
Still doesnt seem practical? Then connect a 9 amp current source collector emitter. Do the same analysis.

Now wasnt that fun? :)

Next you can look at how the base collector might become forward biased in saturation.
 

hp1729

Joined Nov 23, 2015
2,304
For instance, several loads might be switched on from the high side with this one switch in series with the ground for them all.
When only a few of the loads are switched on, the grounding switch is over-driven, but entirely capable of grounding the remaining loads when they switch on.

The first thing that comes to mind is a 7 segment display where segments are driven from the high side and the NPN on the low side is a master shut-off for blanking.
Hmmm, thanks.
 
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