Let me try this post to see if I can properly ask the question.

I have the common emitter configuration which works for me, the math (Ie = Ic + Ib) works. The load is in series to the collector. And all it takes is a very very little current through the base to turn on the path from the collector to the emitter:

enter image description here

Now I also have been playing around with the following. After further reading, it appears it is similar to the common base configuration except, that I'm using a NPN transistor not a PNP which means the whole thing is wrong. I actually get current from the base through the emitter, the collector has no part in it. I disconnected the power source to the collector but the LED still lit with no variation. Based on what I have read, the configuration really just acts like a diode from the base to the emitter. The load is in series with the emitter. Am I getting this 2nd diagram's explanation right? enter image description here

Follow up info #1 regarding figure #2. If I remove the resistor on the collector leg, the following is what is being measured: enter image description here

the Re in the table means after the resistor on the emitter leg
1- now I do not see emitter voltage = input voltage (even if I account for the 0.7v for the register-LED).
2- when I disconnect the collector leg, the LED actually is brighter.
3-when I reduced the collector voltage from 4.98v to 2.2v, I thought the emitter voltage would be reduced too.
So obviously I'm missing something regarding the common collector/emitter follower.


2 Answers 2


The first configuration is useful if you want to easily saturate (fully turn on) the transistor so it basically "gets out of the way" (becomes almost a short) so that you end up with a lit LED, current-limited by a 1K resistor.

In the second configuration, you have an emitter-follower. Approximately speaking, the voltage at the top of the 200 ohm resistor above the LED is about 0.7 volts below the voltage at the base of the transistor. The current through the LED then follows from this in a straightforward way.

In this configuration, it behooves you not to have the collector resistor. So hereafter let us assume that it has been removed from the circuit.

The transistor just acts as a current source: that is to say, roughly speaking, the voltage at the base "programs" the voltage on the resistor-LED stack, and the transistor supplies the current via its collector.

The circuit basically acts as a buffer, making it look like the resistor-LED stack has a much higher impedance than it really does. The circuit driving the base "thinks" it is driving the LED with only a fraction of the current that it actually requires.

The circuit is useful if you want to be able to vary the intensity of the LED by varying the input voltage. It's also useful if you want a high turn-on voltage. (Remember, to get X volts on the resistor-LED stack, you need to input X + 0.7V).

For instance, we could use a circuit based on an emitter-follower if we had an DAC (digital to analog converter) which we wanted to use to drive the LED, but discovering that it doesn't have enough current-driving ability to do the job directly. (Of course, a better way to control LED intensity from the digital realm is to use pulse width modulation (PWM): and that basically calls for the first circuit, rapidly turned on and off with varying duty cycles.)

  • \$\begingroup\$ using arduino is what got me here in the 1st place. I was trying to drive a motor with 9v on the collector and 5v (or less) on the base. Transistor got real hot very quickly. So I figured I needed to step out of the cookbook and figure out how to really work with a transistor. Software developer turned hobby hardware??? Anyway further info regarding figure #2 posted above. I am using adruino as the power supply, two PWM pins for the collector and base. \$\endgroup\$ Dec 6, 2013 at 22:49

Your second configuration is much more useful when you leave out the collector resistor. In that configuration (called common collector, or emitter-follower) you essentially get no voltage amplification (output voltage is the same as input voltage), but a lot of current gain (the input current is very very low).


responding to the comment:

Better or worse requires a criterium.

If your aim is to switch a LED that is fed from a voltage that is higher than your input voltage, go with your first (common emitter) configuration, that is what it is for.

If your aim is to supply a voltage to your load that is the same as your input voltage, without drawing much current from your input voltage, then you go with the emitter follower configuration.

There is of course one more configuration (common base), but that is seldom used because is has a very low imput impedance (but a very high voltage amplification). (It was (is?) common in HF circuits.)

  • 1
    \$\begingroup\$ Doesn't it cause the transistor base to only draw as much as is necessary to light the LED, given a fixed VCC? It seems like common collector like this would make the transistor operate right at the saturation point based on Ib and hFE. Correct me if I'm wrong :) \$\endgroup\$ Dec 6, 2013 at 19:42
  • \$\begingroup\$ @Wouter,actually, I found the common emitter config did better..due to an amplification? The experiment with the common collector (no resistor in series to the base or collector): I measured: collector = 5v(0.5mA), base=255mV (0.0mA meter showed), emitter shows 0.55v (0.5mA). When I completely disconnected power source to the collector, got even more interesting effect. At least with the common emitter config, results can be predicted? comments? \$\endgroup\$ Dec 6, 2013 at 19:58

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