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Transistors Explained: The Switches and Amplifiers Inside Everything

The transistor is the device the modern world is built from. Billions of them switch inside every processor, and single ones switch motors, amplify sensor signals and regulate power supplies in equipment all around you. Yet the idea is simple: a small control signal governs a much larger current. This guide explains the two families you will actually use, the bipolar junction transistor (BJT) and the metal-oxide-semiconductor field-effect transistor (MOSFET), how each one switches and amplifies, what biasing means, the common circuits they appear in, and how to choose the right device for a job.

The fundamentals behind every calculation here are covered in our electronics circuits basics guide, and the transistor is what makes the amplifier stages in op-amp basics possible. Transistors are also the switches inside switch-mode power supplies and the output drivers in 555 timer circuits. All practical work in this guide stays at low, safe voltages on a bench or breadboard: the same principles scale to industrial drives, but mains-powered work follows its own safety rules.

What a transistor does: a small signal controls a large current

Every transistor has three terminals and works as a controlled valve for current. A small signal at the control terminal determines how much current flows through the other two terminals, and that current can be many times larger than the control signal itself. Used as a switch, the transistor is either fully on, passing current with little voltage drop, or fully off, blocking current almost completely. Used as an amplifier, it sits part-way on, and small changes in the control signal produce larger, faithful changes in the output current. Those two modes, switching and linear amplification, cover nearly every transistor application, and the design rules for the two modes are different, so the first question about any transistor circuit is always which mode it is in.

Transistors come in two polarities, NPN and PNP for BJTs, N-channel and P-channel for MOSFETs, which are mirror images of each other with supply rails and current directions reversed. Beginners usually start with NPN BJTs and N-channel MOSFETs, switched on the low side, meaning the transistor sits between the load and ground. That arrangement is the easiest to drive from a microcontroller or logic output, and it is the arrangement used in the examples below. High-side switching, where the transistor sits between the supply and the load, is sometimes necessary but needs extra drive circuitry, which is worth knowing before you wonder why a circuit looks more complicated than expected.

BJTs: current-controlled devices

A bipolar junction transistor has three terminals: base, collector and emitter. It is current-controlled: a small base current allows a much larger collector current to flow, and the ratio between them is the current gain, beta (β) or hFE, typically somewhere between 50 and a few hundred for general-purpose parts. The collector current is roughly beta times the base current, so a 1 mA base current with a beta of 100 gives about 100 mA of collector current. That relationship is the basis of both switching and amplification with BJTs, but it comes with a warning: beta varies between individual devices, with temperature, and with collector current, so good designs never depend on an exact beta value.

The base-emitter junction behaves like a diode, dropping about 0.6–0.7 V for silicon devices when conducting. That is why a BJT switch always needs a base resistor: without one, the driving source would try to force far more base current than the transistor or the source can handle. The base resistor value follows from Ohm's law: subtract the 0.7 V base-emitter drop from the drive voltage, and divide by the base current you want. For switching, the standard practice is to supply considerably more base current than beta alone would suggest, deliberately overdriving the transistor into saturation, where the collector-emitter voltage falls to a few tenths of a volt and the transistor behaves like a closed switch. Saturated switching wastes a little base current but gives a clean, low-loss on-state.

MOSFETs: voltage-controlled devices

A MOSFET has three terminals too: gate, drain and source. It is voltage-controlled: the voltage between gate and source determines how much current flows between drain and source, and the gate itself draws essentially no steady current, because it is insulated from the channel by a thin oxide layer. To turn an N-channel MOSFET on, you raise the gate several volts above the source; the threshold voltage at which it starts to conduct is typically 1–4 V, but full turn-on, the low on-resistance quoted in datasheets, usually needs 10 V for standard parts or about 5 V for logic-level parts. That distinction is the single most common beginner trap: a MOSFET that "turns on" at 3 V on a meter may still drop significant voltage at your load current if its gate is not driven high enough.

The key MOSFET specification for switching is R_DS(on), the drain-source resistance when fully on. Power lost in the switch is current squared times this resistance, so a 10 mΩ device passing 5 A loses only 0.25 W, while a 1 Ω device passing the same current loses 25 W and will quickly overheat. That is why MOSFETs dominate power switching: their on-state losses can be made extremely small. The gate behaves like a capacitor, so switching it on and off means charging and discharging that capacitance, and a small series gate resistor limits the current spike and damps ringing on the gate waveform. MOSFET gates are also sensitive to static electricity, so handle loose devices with the usual anti-static care.

Switching versus amplification: two different design rules

In switching service, the transistor spends its time fully on or fully off and passes through the in-between region as quickly as possible, because that middle region is where power dissipation peaks: significant voltage across the device at the same time as significant current through it. Fast, clean switching therefore matters, and gate or base drive must be strong enough to switch quickly. Driving a relay, a motor or an LED strip from a microcontroller is switching service, and the transistor choice is governed by voltage rating, current rating and on-state loss.

In linear service, the transistor deliberately sits in the middle region, and the design goal is faithfulness: the output must be a scaled copy of the input, not a clipped or distorted version. That requires biasing, setting a resting operating point so the signal can swing both ways. A common-emitter BJT amplifier stage, for example, biases the base with a resistor divider so the collector sits near half the supply voltage with no signal, giving the output room to swing up and down. Emitter degeneration, a small unbypassed resistor in the emitter lead, trades a little gain for much better stability against beta variation and temperature, and is one of the most useful tricks in discrete amplifier design. Linear stages are also where heat matters most, because a transistor passing current with voltage across it continuously dissipates real power, and heatsinks enter the picture.

Biasing concepts: setting the operating point

Biasing means establishing the DC conditions, voltages and currents, around which the signal varies. Get it wrong and the amplifier clips: if the operating point sits too close to saturation, positive peaks flatten; too close to cut-off, negative peaks flatten. The classic stable bias arrangement for a BJT uses a voltage divider to set the base voltage, an emitter resistor to provide negative feedback against drift, and a collector resistor across which the output is developed. The divider current is chosen to be much larger than the base current, so the base voltage stays put even as beta varies between devices, and the emitter resistor automatically reduces base current if collector current tries to rise with temperature. That self-correcting behaviour is why the circuit is standard practice rather than a curiosity.

Thermal runaway is the failure mode biasing exists to prevent. In a BJT, rising temperature increases base-emitter conduction, which increases collector current, which increases heating, which increases conduction further. Without the stabilising effect of an emitter resistor, that loop can destroy the device. MOSFETs behave differently: their on-resistance rises with temperature, which naturally limits current in parallel devices, one reason MOSFETs parallel more gracefully than BJTs. Either way, the lesson is the same: a transistor circuit is not finished when it works on the bench at room temperature; it is finished when it still works across the temperature range it will actually meet.

Common circuits you will meet

The low-side switch is the circuit most beginners build first: an NPN transistor or N-channel MOSFET between load and ground, driven from a logic output, with a flyback diode across any inductive load such as a relay coil or motor. That diode is not optional: when the switch turns off, the inductor tries to keep its current flowing and generates a voltage spike that can destroy the transistor in a single event. The common-emitter amplifier, with its voltage gain and phase inversion, is the basic linear stage, and the emitter follower, where the output is taken from the emitter, gives a gain of about one but transforms a high-impedance source into a low-impedance output, the same buffering role as the voltage follower in op-amp basics.

Push-pull output stages pair complementary devices so one handles the positive half of the waveform and the other the negative half, which is how audio amplifiers and motor drivers deliver real power efficiently. Darlington pairs stack two BJTs so their current gains multiply, giving enormous overall gain for driving heavy loads from a feeble source, at the cost of a higher saturation voltage and slower switching. And in digital logic, complementary MOSFET pairs, CMOS, are the reason modern chips can pack billions of transistors: a CMOS gate draws significant current only while switching, which is what makes dense, low-power computing possible at all.

Choosing the right transistor: a practical checklist

Start with the ratings, and always leave margin. The maximum collector or drain voltage must exceed the worst-case supply, including spikes from inductive loads. The maximum continuous current must exceed the load current, and for switching, check the pulsed ratings and the safe operating area, which limits combinations of voltage and current that are safe together. Then check the on-state loss: saturation voltage for a BJT, R_DS(on) for a MOSFET, and work out the power dissipation at your load current. If that power needs a heatsink, the thermal resistance figures in the datasheet tell you how big.

Next, check the drive requirements. Can your driving source supply the base current a BJT switch needs, or the gate voltage a MOSFET needs for full turn-on? Logic-level MOSFETs exist precisely for 3.3 V and 5 V drive, and using a standard MOSFET from a 3.3 V microcontroller is a classic cause of a hot, underperforming switch. Finally, check speed and package: switching frequency, turn-on and turn-off times, and whether the package can shed the heat you calculated. A part that looks perfect electrically but overheats in its package is the wrong part. Work through that checklist in order, ratings, losses, drive, speed, package, and most transistor selections become routine rather than guesswork.

Frequently asked questions

Next steps: see transistors at work inside amplifier circuits in op-amp basics, as the switches inside switch-mode power supplies, and as output drivers in 555 timer circuits. For the circuit laws behind every calculation here, see electronics circuits basics.