- Transistor Definition: A transistor is defined as a semiconductor device with three terminals (Emitter, Base, and Collector) and two junctions (Base-Emitter and Base-Collector).
- Active Region Operation: In the active region, a transistor acts as an amplifier by increasing the strength of the input signal.
- Current Amplification: By varying the base current (IB), a large change in collector current (IC) is achieved, illustrating current amplification.
- Voltage Amplification: The transistor can amplify voltage by applying an input signal and collecting the output across a load resistor.
- Amplifier Configurations: Transistors can be configured in various ways, such as Common Base, Common Emitter, and Common Collector, to achieve different amplification characteristics.
A Transistor is a three-terminal semiconductor device: emitter (E), base (B) and collector (C). It has a BE junction and a BC junction. Cutoff is off. Saturation is fully on. The active region is used for amplification.
In the active region the device raises the input without changing its shape much. That gain comes from the motion of charge carriers. Take an npn bipolar junction transistor with BE forward and BC reverse.
In an npn transistor, the emitter is heavily doped, the base is lightly doped, and the collector is moderately doped. The base is narrow, while the emitter is broader, and the collector is the broadest.
Forward BE bias sends a small base current IB into the base, often in microamperes, because VBE is about 0.6 V in silicon.
This process can be seen as electrons moving out of the base region or holes being injected into it. The injected holes attract electrons from the emitter, leading to the recombination of holes and electrons.
The base is doped more lightly than the emitter, so electrons outnumber holes there. After some recombination, most injected electrons still cross the thin base and are swept into the collector by the reverse BC field.
That flow is collector current IC . A small change in IB therefore produces a large change in IC. An active-region npn transistor is a current amplifier. The current gain is
Now consider the npn transistor with the input signal applied between its base and emitter terminals, while the output being collected across the load resistor RC, connected across the collector and the base terminals, as shown by Figure 2.
Figure 2 applies the input between base and emitter and takes the output across load resistor RC from collector to base.
Bias voltage supplies VEE and VBC hold the device in the active region. A small change in Vin changes IE a lot because the input resistance is low under forward bias.
This in turn changes the collector current almost in the same range due to the fact that the magnitude of the base current is quite less for the case under consideration. This large change in IC causes a large voltage drop across the load resistor RC which is nothing but the output voltage.
Hence one gets the amplified version of the input voltage across the output terminals of the device which leads to the conclusion that the circuit acts like a voltage amplifier. Mathematical expression for the voltage gain associated with this phenomenon is given by
Although the explanation provided is for the npn BJT, similar analogy holds good for even pnp BJTs. Following on the same grounds, one can explain the amplifying action of other kind of transistor viz., Field Effect Transistor (FET). Further it is to be noted that there exist many variations to the amplifier circuit of transistors like
- First Set: Common Base/Gate Configuration, Common Emitter/Source Configuration, Common Collector/Drain Configuration
- Second Set: Class A amplifiers, Class B amplifiers, Class C Amplifiers, Class AB amplifiers
- Third Set: Single Stage Amplifiers, Muti-Stage Amplifiers, and so on. However the basic working principle remains the same.
