functioning of transistor in simple words

Discussion in 'General Electronics Chat' started by PG1995, Apr 20, 2011.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    Hi ;-)

    1: I was reading that a transistor is an active component. How is it one? It doesn't produce any power. We feed 'power' to it. It is just like a resistor which is a passive component. Could you please explain this to me in simple words? At the bottom you will find the quoted passage from a Wikipedia page on this which I was not able to understand. So far I have been told any component which adds power to the circuit is an active component. In this context only battery is an active component. We feed the current to the base of a transistor which in turn lets more current to pass from the collector to the emitter. We can say adding current to the base makes the bore of the 'transistor pipe' a little wider. In other words adding current to the base decreases the resistance the transistor.

    2: The word transistor is portmanteau of words "transfer resistor". How does it transfer resistance? I don't get it. Perhaps you could help.

    I can understand the following voltage vs. current graph of a diode:


    I was unsuccessfully trying to understand this load-line (I think this is what it's called!) of a transistor. I don't even get what that 'dark' line AB is. Please help me in simple words.

    The link I was reading this load-line graph on was:
    Analysis Of DC Load Line

    Electronic component:
    A component may be classified as passive or active. The strict physics definition treats passive components as ones that cannot supply energy themselves, whereas a battery would be seen as an active component since it truly acts as a source of energy.

    However electronic engineers performing circuit analysis use a more restrictive definition of passivity. When we are only concerned with the energy due to signals it is convenient to ignore the so-called DC circuit and pretend that the power supplying components such as transistors or integrated circuits is absent (as if each such component had its own battery built in) although it may in reality be supplied by the DC circuit which we are ignoring. Then the analysis only concerns the so-called AC circuit, an abstraction which ignores the DC voltages and currents (and the power associated with them) present in the real-life circuit. This fiction, for instance, allows us to view an oscillator as "producing energy" even though in reality the oscillator consumes even more energy from a power supply, obtained through the DC circuit which we have chosen to ignore. Under that restriction we define the terms as used in circuit analysis as follows:

    Passive components are ones which cannot introduce net energy into the circuit they are connected to. They also cannot rely on a source of power except for what is available from the (AC) circuit they are connected to. As a consequence they are unable to amplify (increase the power of a signal), although they may well increase a voltage or current such as is done by a transformer or resonant circuit. Among passive components are familiar two-terminal components such as resistors, capacitors, inductors, and most sorts of diodes.

    Active components rely on a source of energy (usually from the DC circuit, which we have chosen to ignore) and are usually able to inject power into a circuit although this is not part of the definition[1]. This includes amplifying components such as transistors, triode vacuum tubes (valves), and tunnel diodes.

    Passive components can be further divided into lossless and lossy components:
    Lossless components do not have a net power flow into or out of the component. This would include ideal capacitors, inductors, transformers, and the (theoretical) gyrator.

    Lossy or dissipative components do not have that property and generally absorb power from the external circuit over time. The prototypical example is the resistor. In practice all non-ideal passive components are at least a little lossy, but these are typically modeled in circuit analysis as consisting of an ideal lossless component with an attached resistor to account for the loss.
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    BJT is a active device thanks to its property, the ability of a amplifying the power.
    Amplifier is a device that allow as control the flow of "high power" by helps of a low power. To the Amplifier effect occurred, two things are necessary: source of energy and a device for controlling the flow of this energy - > the amplifier.

    BJT act more like control by base current current source
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    1. From a strict physics point of view, transistors could be considered passive, but this is not the view in electronics. The transistor (together with a suitable power supply) can be used to create active circuits which can amplify or oscillate.

    2. The transistor does not transfer resistance - it transfers current. This rather archaic term comes from early analysis of common-base circuits, where current is seen to be transferred between the low-impedance emitter-base circuit into the high impedance base-collector circuit, so that there is a net amplification.

    3. The load line illustrates the interaction of the power supply voltage and the load impedance. The example given shows a simple resistance R, where the slope is equivalent to the resistance value, and the intersection of the line with the X axis at B is the supply voltage VCC. The intersection A with the Y axis is given by VCC/R.

    Intersections of the load line with the family of output characteristic curves for different IB show the VCE and IC working points given by different base currents.
  4. acelectr


    Aug 28, 2010
    2- Actually the -> transfer resistor sounds right. Because for instance a resistor in the base or at the emitter would not be treated the same as at the other terminals of the bjt. Thus for instance a resistance at the emitter, say a resistance R ohm, will have to be transferred to the base as R(Beta+1) ohm, where beta is as it would be already known Ic/Ib. So the word transfering the resistance is logical I guess. Same to transfer a resistance at the base to the emitter you need to divide the resisntace at the base by again (Beta+1). This is all related with the currents actually or maybe the current relation are coming from this resistance relations. That is a question, infact. Is this transferring issue comes from the bjt current relation or are the current relations are coming from this resisntace relations of (Beta+1)?
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    The transistor can operate as a kind of impedance converter, but that is not how the name was derived. In early transistors, the phenomenon of current transfer between the emitter-base and base-collector circuits was described, and so the name was coined. The device was viewed as transferring current between a low resistance and a high resistance circuit. This was particularly relevant to transistors used in the common-base configuration, as was common at the time.

    The contraction to "transistor" is also said to be because the device can be considered to have transresistance, the dual of the property of transconductance posessed by the electron tubes which were in use at the time. This ignores the fact that the output characteristic of most transistors is closer to a current than a voltage source, but it's a bit too late to worry about that now...