- BJT Definition: A Bipolar Junction Transistor (BJT) is defined as a three-terminal semiconductor device used for amplification and switching.
- History of BJT: BJTs replaced vacuum tubes, were invented by Bardeen and Brattain in 1947, and further developed by Shockley, leading to the Nobel Prize in Physics.
- Applications of Bipolar Junction Transistor: BJTs are used in switching applications, where they act as open or closed switches, and in amplification applications, providing voltage and current gain.
- Transistor as a Switch: BJTs operate in saturation or cutoff regions for switching, acting as closed or open switches based on the input voltage.
- Amplifier Operation: In amplification, BJTs use coupling capacitors to block DC and pass AC signals, with single-stage or multistage setups for higher gain
History of Bipolar Junction Transistors
The transistor (BJT) was not the first three-terminal device. Before transistors, vacuum tubes were used in electronics for almost 50 years. In electronics, vacuum tube triodes were used almost for half a century before the BJT’s. Thomas Edison’s light bulb, invented in the early 1880s, was an early use of vacuum tubes. Vacuum tube triodes were used in computers until the early 1950s. However, as circuits became more complex, more triodes were needed, making computers large and power-hungry. Vacuum tubes also consumed a lot of power and were prone to leakage, making them unreliable.
Scientists and engineers wanted to create a new kind of three-terminal device. Instead of controlling electrons in a vacuum, they explored controlling them in solid materials. In 1947, physicists John Bardeen and Walter Brattain at Bell Labs discovered that placing two point contacts close together could create a three-terminal device. This led to the invention of the first point-contact transistor using germanium, a paper clip, and razor blades.
William Shockley then developed the junction transistor (BJT) by combining thin slices of different semiconductor materials. Transistors replaced vacuum tubes, revolutionizing electronics. Bardeen, Brattain, and Shockley received the Nobel Prize in Physics in 1956 for the transistor effect. Transistors were made as individual components until the late 1950s when integrated circuits (ICs) emerged, placing all components on one chip. This is a key part of the history of BJTs.
Applications of Bipolar Junction Transistor
There are two types of applications of bipolar junction transistor, switching and amplification.
Transistor as a Switch
In switching applications, a transistor operates in either the saturation or cutoff region. In the cutoff region, the transistor acts as an open switch, while in saturation, it acts as a closed switch.
Open Switch
In the cutoff region (both junctions are reversed biased) the voltage across the CE junction is very high. The input voltage is zero so both base and collector currents are zero, hence the resistance offered by the BJT us very high (ideally infinite).
Closed Switch
In saturation (both junctions are forward biased), a high input voltage is applied to the base, causing a large base current to flow. This results in a small voltage drop across the collector-emitter junction (0.05 to 0.2 V) and a large collector current. The small voltage drop makes the BJT act like a closed switch.
BJT as Amplifier
Single Stage RC Coupled CE Amplifier
The figure shows a single stage CE amplifier. C1 and C3 are coupling capacitors, they are used for blocking the DC component and passing only ac part they also ensure that the DC basing conditions of the BJT remains unchanged even after input is applied. C2 is the bypass capacitor which increases the voltage gain and bypasses the R4 resistor for AC signals.
The BJT is biased in the active region using the necessary biasing components. The Q point is made stable in the active region of the transistor. When input is applied as shown below the base current starts to vary up and down, hence collector current also varies as IC = β × IB. Therefore voltage across R3 varies as the collector current is passing through it. Voltage across R3 is the amplified one and is 180o apart from the input signal. Thus voltage across R3 is coupled to the load and amplification has taken place. If the Q point is maintained to be at the center of the load very less or no waveform distortion will take place. The voltage as well as current gain of the CE amplifier is high (gain is the factor by which the voltage of current increases from input to output). It is commonly used in radios and as low frequency voltage amplifier.
To further increase the gain multistage amplifiers are used. They are connected via capacitor, electrical transformer, R-L or directly coupled depending on the application. The overall gain is the product of gains of individual stages. Figure below shows a two stage CE amplifier.
