- 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 was not the first three-terminal electronic device. Vacuum-tube triodes had already provided amplification and switching for decades, but their heaters consumed power and their glass envelopes occupied substantial space. Before the BJT’s development, large electronic systems needed many tubes, power supplies and cooling components. The solid-state transistor later enabled smaller and more reliable circuits with no heater warm-up.
At Bell Telephone Laboratories in 1947, John Bardeen and Walter Brattain demonstrated transistor action with two closely spaced point contacts on germanium. Their three-terminal point-contact transistor was a semiconductor amplifier distinct from the layered bipolar junction transistor used in later mass production.
William Shockley subsequently developed the junction-transistor concept using regions of differently doped semiconductor material. The solid-state device reduced circuit size and heater power compared with many vacuum-tube applications. It also improved reliability. Bardeen, Brattain and Shockley shared the 1956 Nobel Prize in Physics for their semiconductor research and discovery of the transistor effect. Discrete Transistors were followed by integrated circuits, which formed multiple devices and connections on one semiconductor substrate.
Applications of Bipolar Junction Transistor
Two broad applications of bipolar junction transistor are switching and analogue amplification. BJTs also appear inside current sources, oscillators and signal-processing circuits, where the same device physics is used in different bias conditions.
Transistor as a Switch
A BJT switch is normally driven between cutoff and saturation. Cutoff represents the off state, while saturation represents the on state. Both states are non-ideal: the off device has leakage current, and the on device retains a collector-emitter voltage drop.
Open Switch
In cutoff, the base-emitter junction is not forward biased enough to support the intended collector current. Most of the supply voltage can therefore appear across the collector and emitter. The device presents high effective resistance, but collector leakage is not exactly zero and depends on device voltage and temperature.
Closed Switch
In saturation, both junctions are forward biased and extra base drive produces little additional collector current. The collector-emitter voltage drop falls to VCE(sat), so the device approximates a closed switch. VCE(sat) is not one universal 0.05-to-0.2 V range; it depends on the BJT, collector current, forced current gain and temperature. Designers use the data-sheet test conditions and power limits.
BJT as Amplifier
Single Stage RC Coupled CE Amplifier
The figure shows a single-stage common-emitter amplifier. C1 and C3 are coupling capacitors. They block the surrounding circuits’ DC levels while passing signal frequencies for which their reactance is low. C2 bypasses the emitter resistor R4 resistor at selected AC frequencies, reducing emitter degeneration and increasing gain.
The BJT is biased in the forward-active region around a quiescent point. Keeping enough collector-emitter headroom lets the transistor swing without reaching cutoff or saturation. A small change in base current produces a collector-current change that is often approximated by IC = β × IB, but β varies with device, current and temperature. The changing collector voltage is developed across R3. The signal across R3 represents the output. In the midband small-signal model, the collector output is approximately 180o out of phase with the base input. The voltage across R3 is coupled to the load, while the collector resistance, emitter degeneration and bias point set gain and output swing. A centred Q point can provide roughly symmetrical swing, but low distortion also depends on signal amplitude and transistor linearity.
Several stages can be cascaded when one stage cannot provide the required gain or impedance transformation. Stages may be linked through a capacitor, an electrical transformer or direct coupling. Overall midband voltage gain is the product of individual loaded stage gains, while each coupling network affects bandwidth and bias.
