A transistor is an electronic component. It is defined as an "active" electronic component because it is able to perform more than one function at more than a single level.

The most basic of transistors is the bipolar transistor. It has three connections; those being the base, collector, and emitter. A circuit-diagrammatical representation of the bipolar transistor is shown in Fig.1 below. The collector is the connection at the top, the base is the one in the middle, and the emitter is the one at the bottom with the arrow on it. In addition to the transistor’s amplification factor, the base and emitter act as a diode. (See Fig.2 (i) and (ii).)

The direction of the arrow indicates whether the type of bipolar transistor is NPN or PNP. (Which stands for Negative Positive Negative or Positive Negative Positive.) The difference amounts to the way that the transistor is connected in a circuit with regard to the DC polarity. This polarity is caused by the transistor’s substrate layers being doped with a P-type and an N-type substrate. (See table of links.)

Fig.3 shows a PNP transistor connected into a basic circuit. Fig 4 shows an NPN transistor connected into an equivalent circuit. The resistors in the circuit limit the current flowing through the device and set the device’s voltage potential point with respect to the supply rails. The capacitor drawn in with dotted lines is a decoupling capacitor which, along with R3, decouples the collector (PNP) or the emitter (NPN) to ground; limiting distortion in the output and/or compensating for any residual ripple present in the supply rails – depending upon its value combined with that of R3 giving a certain AC reactance. (A subject beyond the scope of this article.)

Resistor, R1, is connected as a DC current-limiting resistor in both cases, to the base of the transistor; limiting the base current which in normal operation should not rise above approximately 1/10th of the current flowing between collector and emitter. (As low as 1/100th is the preferred quiescent value for maximum amplification in most high-gain devices.) The differential between the two sets the transistor’s working amplification factor or beta. This is limited by the actual electrical characteristics of the chosen device itself.

This article cannot hope to go into the full details and various functions of the bipolar transistor under all conditions, and even the AC amplification operations of said device are far too in-depth to discuss in the space allocated.

For further information on this device please visit links in the table of links below.

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