Bipolar Junction Transistors (BJT)

What is a Transistor?

A transistor is a three-terminal semiconductor device that can amplify signals or act as an electronic switch. It's arguably the most important invention in electronics — every computer chip contains billions of them. The bipolar junction transistor (BJT) uses a small current at one terminal (base) to control a much larger current between the other two terminals (collector and emitter).

NPN and PNP Types

• •NPN — the most common type. Current flows from collector to emitter when a small current is injected into the base. Think of it as 'Current IN to the base turns it ON.'
• •PNP — current flows from emitter to collector when current is pulled OUT of the base. Used when you need to switch the high side of a load.
• •The arrow on the emitter in the schematic symbol shows the direction of conventional current flow: outward for NPN, inward for PNP.

The Three Terminals

• •Base (B) — the control terminal. A small current here controls the collector-emitter current.
• •Collector (C) — the main current input (for NPN). This connects to the power supply through the load.
• •Emitter (E) — the main current output (for NPN). Usually connected to ground.
• •I_E = I_C + I_B — the emitter current is the sum of collector and base currents.

Current Gain (β or hFE)

The key property of a BJT is current gain — the ratio of collector current to base current, called β (beta) or hFE. If β = 100, then 1 mA of base current allows up to 100 mA of collector current. Typical β values range from 20 to 500 depending on the transistor and operating conditions. β varies widely even between transistors of the same type, so good circuit design should not rely on an exact β value.

Three Operating Regions

• •Cut-off — no base current, transistor is OFF. V_CE ≈ V_CC. Like an open switch.
• •Active (linear) region — base current controls collector current proportionally. Used for amplification. V_CE is between 0 and V_CC.
• •Saturation — maximum base current, transistor is fully ON. V_CE(sat) ≈ 0.2–0.3 V. Like a closed switch. Used for switching.

Using a BJT as a Switch

The most common beginner use of a transistor is as an electronic switch — using a small signal (e.g., from a microcontroller GPIO pin) to switch a larger load (motor, relay, LED strip). To ensure the transistor is fully saturated (ON), you provide more than enough base current: I_B ≥ I_C / β. In practice, use a 'forced beta' of 10–20 (overdrive the base) to guarantee saturation.

• •Calculate the load current I_C (e.g., motor draws 200 mA).
• •Choose a forced beta of ~10: I_B = I_C / 10 = 20 mA.
• •Calculate the base resistor: R_B = (V_drive − V_BE) / I_B = (5 − 0.7) / 0.02 = 215 Ω → use 220 Ω.
• •The V_BE of a silicon BJT is approximately 0.7 V (similar to a diode's forward voltage).

Using a BJT as an Amplifier

In the active region, the transistor amplifies small signal variations at the base into larger variations at the collector. The common emitter configuration is the most basic amplifier: the input signal goes to the base, and the amplified signal is taken from the collector. The voltage gain depends on the collector resistor, the transistor's characteristics, and any emitter resistor used for stability. Amplifier design is a deep topic — for now, understand that transistors can do this.

Popular BJT Types

• •2N2222 / 2N3904 (NPN) — general-purpose, up to ~200 mA. The 'go-to' NPN transistors.
• •2N2907 / 2N3906 (PNP) — complementary to the above. Used for high-side switching.
• •TIP120 / TIP122 (NPN Darlington) — two transistors in one package, very high β (~1000). Can switch loads up to 5 A.
• •BC547 / BC548 (NPN) — European equivalent to 2N3904, very popular in educational circuits.