A Transistor is an electronic component created by joining two types of semiconductors. One type of semiconductor is "P-type" and the other is "N-type". The two types of bipolar-junction transistors (BJTs) that result are explained below.
It is equivalent to two nose-to-nose diodes, but as before, just connecting two diodes will not work. Generally, a PNP diode is identical to an NPN diode, but with all currents reversed.
Bipolar transistors have 3 terminals : Base, Collector, Emitter
Transistors are commonly used in amplification , switching, and buffering signals or applied voltages.
A bipolar transistor can be regarded as a current amplifier. The (small) base current is amplified by a factor (typically in the range of 10 to 500) to yield a proportionally larger collector current. But in order for a transistor to perform its task properly, there have to be voltages applied to make the transistor conduct current. These voltages are called bias voltages.
Depending on how the transistor is used, it can behave as a (relatively) linear amplifier or a switch.
A bipolar transistor has three terminals, base, emitter and collector, and two internal PN-junctions; the collector to base junction (CBJ) and the emitter to base junction (EBJ). A junction is conducting a significant amount of current in the forward direction (P to N) if the voltage across it is larger than a diode forward voltage drop (Vd), which is around 0.6 V in silicon. The operation of the transistor is largley affected by how the terminals are biased. The table below shows how to bias the base-emitter voltage (Vbe) and the collector-emitter voltage (Vce) to bring the transistor into the different modes. The table shows the voltages for a silicon NPN transistor:
|Reverse-Active||Vbc = Vd||negative|
When a transistor operates in the active mode it will act as an amplifier meaning that a small change in base current will cause a proportional change in collector current. In the active mode, the EBJ is conducting and the collector to emitter voltage is bigger than the collector to emitter saturation voltage (Vcesat). Typically, Vcesat is around 0.2V.
When a transistor operates in the cut-off and saturation modes, it will act as a switch. In the cut-off mode, there is no base current and therefore no collector current. The collector voltage is not important as long as it is larger than the emitter voltage. In the saturation mode, the transistor is fully on, meaning that an increas in base current does not result in any significant increase in collector current.
The reverse active mode is normally not used. This mode occurs if the emitter and collector terminals are swapped for a transistor in the active mode so that emitter and collector change roles. Based on the descriptions of a transistor given so far, one might expect that a transistor is symmetrical and that it would work identically with collector and emitter swapped. However, the physical design of real transistors is such that the current amplification is much smaller in the reverse active mode compared to the forward active mode, although it is still typically larger than one. This can be useful knowledge when troubleshooting a prototype circuit. If it seems to act as if the gain of the transistor is too low, it might be due to swapped emitter and collector terminals.