this is my first post here. I’ll just dive in and hopefully learn something. I’m relatively new to electronics, and getting to grips with the terminology etc. Please correct me if I am describing things incorrectly, I want to learn. Thanks all in advance for reading/responding.

Here is a circuit I am looking at: NPN Amplifier circuit I’ve constructed it on a breadboard and indeed it works…. the description in the booklet I have here is:

Current gain

"The circuit shows the basic functioning of the NPN transistor. There are two circuits. A small base current flows in the control circuit; a larger collector current flows in the load circuit. Both currents together flow through the emitter. Since the emitter is at the common reference point of the circuit, this circuit is also called the emitter circuit. Once the base circuit is opened, no more load current flows. The base current is much smaller than the collector current. The small base current is thus amplified to a larger collector current. The base resistor is 470 times larger than the series resistor in the load circuit. The small base current can be identified by the low brightness of the green LED. The transistor BC548B amplifies the base current about 300-fold so that the red LED is substantially brighter than the green LED.  Connect a second resistor of 470 kΩ parallel to the existing base resistor. The base current thus increases, and the collector current also becomes larger. The transistor is now fully interconnected, i.e., an even larger base current can no longer increase the collector current. If you connect a 22 kΩ resistor in parallel, the red LED does not become any brighter. The transistor now works like a switch. Between collector and emitter there is only a very small voltage drop of about 0.1 V. The collector current is already limited by the consumer and cannot increase anymore. Between base and emitter there is a voltage of about 0.6 V which changes only slightly with a change in current. The LEDs are for indicating the currents. The red LED shines brightly, the green one barely. Only in a completely darkened room can the base current be seen as the weak shining of the green LED. The difference is an indication of the large current gain.”

ok. In general I understand it. But I have some questions.

I understand that the resistor in the load circuit is smaller, and as a consequence not slowing the current down much, so a higher amount of current could ‘potentially’ flow through the circuit (if sufficient current is applied to the base to enable electrons to move from emitter to collector ->1. am I correct in saying that electrons move from emitter to collector, when the base current is applied sufficiently?) I understand that up to a certain point, the amount of current applied to the base collector incrementally increases the amount of electrons that are allowed to pass from emitter to collector, and therefore the LED in this circuit becomes progressively brighter. It is ‘amplified’ according to the above description - although I find the word ‘amplified’ confusing, it seems more that a gate or door is made slightly open, permitting a greater degree of current to pass through - for me, using the term amplify seems misleading somehow, ->2. although it may just be my limited understanding at this point that causes this confusion?

Perhaps I just need to change the way I think about it….. a rough analogy I suppose is holding my hand over the end of a vertical pipe full of water, I’m holding my hand over the pipe, stopping the flow coming out of the end, then I gradually uncover part of the pipe by sliding my hand slightly to the side, and the water starts coming out of the pipe, but not at full speed - so with respect to the NPN circuit above, it could be said that by moving my hand gradually to the side, I am gradually ‘amplifying’ the water flow, by gradually moving my hand away from the pipe, (and therefore increasing the water flow out of the pipe). ->3. Is it just a simple case of perceiving it this way, or do I have it all wrong?

Another thing I find confusing, is trying to visualise the flow of current/electrons within an NPN circuit such as this, by means of understanding electron flow (from - to +), not conventional flow. It is my elementary understanding at this point, ->4. however what is occurring, from an electron flow point of view, when I close the control circuit and apply base current? ->5. By closing this control circuit, electrons actually begin to move from emitter to base first? Sorry if this seems a ridiculous question, I seem to be stuck on something here and need to move past it before comprehending more on a deeper level.

It is important to me to perceive circuits from an electron flow point of view. It may not be important to some, and perhaps in the long term I don’t need to understand it from this perspective, however I at least need to reach a deep enough level of understanding of circuit behaviour from this perspective first, before letting it go and moving on to other perhaps more important things.

->6. Finally I don’t understand this aspect of the description: “Between collector and emitter there is only a very small voltage drop of about 0.1 V. The collector current is already limited by the consumer and cannot increase anymore. Between base and emitter there is a voltage of about 0.6 V which changes only slightly with a change in current.”

Could someone please elaborate on this for me?

I have numbered and highlighted my questions to hopefully make it a bit easier to respond to my confusion.

Any assistance would be greatly appreciated. many thanks.


  • 2
    \$\begingroup\$ Boy, there's a lot of text and questions in here. The way our site works, it would probably be better to break it down into separate questions. That way you'd get better answers. So why don't you extract the first question and the context and start with that? Oh, and before I forget: Welcome to our site!! \$\endgroup\$
    – Ricardo
    Apr 25, 2014 at 21:18

2 Answers 2


Bipolar junction transistors (BJT) have 3 primary modes of operation:

  1. Off: no significant current flows from the collector to the emitter
  2. Saturated: the transistor behaves like a diode (which is why there's a fixed voltage drop across the collector/emitter pair under saturation)
  3. Linear: this is where the transistor behaves like a current amplifier. The current flowing from the collector to the emitter is proportional to the current flowing from the base to the emitter. Note that in this region the voltage drop across the collector/emitter pair is NOT roughly constant, but linear.

In cases 2 and 3 there is current flowing from the collector to the emitter.

You can saturate the BJT by attaching a smaller load (i.e. higher resistance) than the linear limit allows. Think of a pipe with a valve. If there's something clogging the pipe upstream, at a certain point no matter how much you open the valve you're not going to get any more fluid flowing through the valve.

Yes, the electrons do literally move to generate the current, though this isn't directly used for most electrical engineering analysis.

For most EE analysis we don't consider the motions of charged particles directly, instead relying completely on the abstract concept of current. This is an approximation of reality, but it turns out to be remarkably good in most cases.

Ironically, in my day-to-day work with plasma physics we do consider the motion of charged particles instead of relying on abstract currents because the analysis does differ depending on what species are moving and where/how fast they are moving.

  • \$\begingroup\$ Helloworld, much appreciated! \$\endgroup\$
    – HokuBoi
    Apr 27, 2014 at 11:56

An amplifier never creates power, it always limits it. The amount it is limited is the "gate door opening and closing". We call it an amplifier because the amount of electrons allowed through the door is much greater than the number of electrons controlling the door, so it is the control signal that is amplified, or appears to be amplified.

For instance the small signal of millivolts and milliamps entering the signal input of an audio amplifier controls an output of tens of volts and several amps at the speaker.


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