Bipolar Junction Transistors
The bipolar junction transistor (BJT) is the original electronic amplifier. Think of it like a water tap controlled by a small trickle: a tiny current into one terminal controls a much larger current flowing between the other two. This ability to amplify — to use a small signal to control a large one — is the foundation of every audio amplifier, sensor preamplifier, and switching circuit built with discrete transistors. In practice, BJTs appear in audio stages, oscillators, motor and relay drivers, and as switching elements throughout electronic equipment.
- BJT construction: NPN and PNP
- The three terminals: base, collector, emitter
- Current gain: β (hFE)
- BJT as a switch
- BJT as an amplifier
- Biasing the transistor
- BJTs in practice
BJT Construction: NPN and PNP
A bipolar junction transistor is made from three layers of semiconductor material. In an NPN transistor, a thin P-type layer (the base) is sandwiched between two N-type layers (the collector and emitter). In a PNP transistor, the arrangement is reversed: a thin N-type base between two P-type layers. The name "bipolar" comes from the fact that both electrons and holes participate in the current flow, unlike FETs which use only one type of carrier.
NPN (left) and PNP (right) transistor symbols. The arrow on the emitter indicates the direction of conventional current flow — outward for NPN, inward for PNP. The base is the control terminal.
View LargerThe Three Terminals: Base, Collector, Emitter
Every BJT has three terminals:
- Base (B): The control terminal. A small current into (NPN) or out of (PNP) the base controls the transistor. The base-emitter junction behaves like a forward-biased diode when the transistor is on — approximately 0.6–0.7 V for silicon.
- Collector (C): The terminal through which the main controlled current enters (NPN) or exits (PNP). This is typically connected to the load and the supply voltage.
- Emitter (E): The terminal where the combined base and collector currents exit (NPN) or enter (PNP). The emitter is often connected to ground (NPN) or the positive supply (PNP).
- IE = IB + IC
- IC = β × IB
- β (hFE) = current gain = IC / IB
- Typical β values: 20–1000 (commonly 50–300 for small-signal transistors)
Current Gain: β (hFE)
The most important parameter of a BJT in switching and simple amplifier applications is its DC current gain, given the symbol β (beta) or hFE. A transistor with β = 100 means that 1 mA of base current allows 100 mA of collector current. The transistor multiplies the base current by its gain factor.
Current gain is never perfectly predictable — it varies between individual transistors of the same type, changes with temperature, and varies with collector current. Designers either use negative feedback to make circuit performance independent of β, or they choose circuits that are tolerant of a wide β range.
BJT Current Gain Calculator
Calculate collector current from base current and gain, or find the base current needed to achieve a required collector current.
BJT as a Switch
When used as a switch, the transistor is driven either fully on (saturation) or fully off (cut-off). In cut-off, no base current flows, so no collector current flows — the transistor is an open circuit between collector and emitter. In saturation, enough base current is supplied to drive the transistor fully on — the collector-emitter voltage drops to a low saturation voltage (typically 0.1–0.3 V) and the collector current is determined by the external circuit.
For reliable saturation, you drive the base with about 10 times the minimum base current needed (β × IC / 10 rule of thumb — forcing the transistor into deep saturation to ensure it is fully on despite β variation).
⚖ Experiment: BJT as a Switch — Control an LED from Logic
Use a small NPN transistor to switch an LED on and off from a low-current control signal. This is the same principle used to drive relays, buzzers, and motors from microcontroller outputs.
- 9 V battery
- 2N2222A or BC547 NPN transistor
- Red LED
- 470 Ω resistor (LED current limiter)
- 10 kΩ resistor (base resistor)
- Breadboard and wires
- Connect the LED anode to +9 V through the 470 Ω resistor. Connect the LED cathode to the transistor collector.
- Connect the transistor emitter to the 0 V rail (ground).
- Connect the 10 kΩ resistor between the +9 V rail and the transistor base.
- Observe: the LED lights because the base is pulled high, turning the transistor on.
- Now connect the base end of the 10 kΩ resistor to ground instead of +9 V. The LED should go out.
Base connected to +9 V through 10 kΩ: base current = (9 V − 0.7 V) / 10 kΩ ≈ 0.83 mA. With β of at least 50, IC can be 41 mA — well above the LED current of about 15 mA, so the transistor is fully saturated. LED glows brightly. Base connected to ground: zero base current, transistor in cut-off, LED off. This is the basis of every relay driver, buzzer driver, and switching power circuit using BJTs.
BJT as an Amplifier
When used as an amplifier, the transistor is biased at a linear operating point (between cut-off and saturation). A small AC signal on the base causes proportional changes in the collector current, producing an amplified version of the input signal across a load resistor in the collector circuit. This is the common-emitter configuration — the emitter is common to both input and output.
The voltage gain of a common-emitter amplifier is approximately Av = −RC / re, where re is the transistor's small-signal emitter resistance (approximately 26 mV / IC). The negative sign indicates signal inversion — when the input goes positive, the output goes negative.
Biasing the Transistor
For linear amplification, the transistor must be held at the correct DC operating point (the Q-point) so that the signal can swing both positive and negative without hitting saturation or cut-off. The most stable biasing method for a common-emitter amplifier is voltage-divider bias, where two resistors form a voltage divider to set the base voltage, and an emitter resistor provides negative feedback that stabilizes the operating point against temperature changes and β variation.
Voltage-divider bias: R1 and R2 set the base voltage, RE stabilizes the emitter current against temperature variations, and RC is the collector load resistor where the amplified signal appears.
View LargerBJTs in Practice
- Audio amplifiers: Common-emitter stages amplify microphone, guitar and sensor signals. The 2N3904 and BC547 are common small-signal audio transistors.
- Output stages: Complementary NPN/PNP pairs drive loudspeakers in audio power amplifiers.
- Sensor interfaces: A transistor turns the tiny current from a phototransistor, thermistor divider or other sensor into a clean logic signal.
- Oscillators: BJTs in Colpitts and Hartley oscillator circuits generate the sine waves used in signal generators and timing circuits.
- Relay and motor drivers: NPN transistors switch relay coils, buzzers, small motors and LED strips under microcontroller or logic control.
- Level shifting: A transistor switch lets a 3.3 V microcontroller control a 5 V or 12 V circuit without loading the microcontroller output.
Frequently Asked Questions
What is the difference between NPN and PNP and when do I use each?
In an NPN transistor, a positive voltage on the base relative to the emitter turns it on, and current flows from collector to emitter. In a PNP transistor, a negative voltage on the base relative to the emitter turns it on, and current flows from emitter to collector. In practice, NPN transistors with the emitter connected to ground (low-side switching) are far more common because most logic circuits provide positive control signals. PNP transistors are used for high-side switching, where you need to switch the positive supply rail.
Why does β vary so much between transistors of the same type?
Beta (hFE) depends on the exact thickness of the base region during manufacturing, which is difficult to control precisely at the nanometre scale. Even transistors from the same production batch can have β varying by a factor of three to five. Good circuit design accounts for this by either using feedback to make gain independent of β, or by biasing the transistor so it operates well into saturation (for switching circuits) where β variation does not matter as long as it is above a minimum value.
What does VCE(sat) mean on a transistor datasheet?
VCE(sat) is the collector-emitter saturation voltage — the residual voltage across the transistor when it is fully turned on (saturated). A typical value is 0.1–0.3 V for a small-signal transistor. This voltage is important in switching applications because it represents a power loss: P = VCE(sat) × IC. In high-current applications this can become significant, which is why power MOSFETs (which have very low on-resistance) often replace BJTs in high-current switching circuits.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.