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Electronic Parts

Discrete Components

Transistors

Common Transistor Circuits
BJT low-side switch

An NPN transistor can switch a load connected between the positive supply and its collector. The emitter is connected near ground and a base resistor limits drive current.

RB ≈ (Vdrive - VBE) / IB

For reliable switching, designers commonly force a lower effective current gain than the transistor's small-signal beta rather than depending on a high catalog number.

MOSFET low-side switch

An N-channel MOSFET is often even simpler for logic-controlled loads. The source is near ground, the drain connects to the load, and the gate is driven by the controller. A gate resistor may control ringing and switching speed, while a gate-to-source resistor keeps the device off when the driver is disconnected.

Breadboard example — 2N7000: The 2N7000 is a convenient small through-hole N-channel MOSFET for low-current experiments such as LEDs and timing circuits. In a low-side switch, connect the source to ground, the drain to the load, and the gate to the control voltage. The load itself connects from the positive supply to the drain.

The 2N7000 is used again in the RC timer lesson, where its high-input-resistance gate lets a timing capacitor control an LED without forcing the capacitor to supply the LED current directly. See 2N7000 details and pinout.
Do not confuse threshold with fully ON. The 2N7000 may begin conducting at a relatively low gate voltage, but that does not mean it has reached its lowest drain-source resistance. For switching work, use the datasheet RDS(on) specifications at the gate voltage actually available.
Inductive loads

Relays, solenoids, motors, and coils require a path for current when the transistor switches off. A flyback diode, TVS diode, RC snubber, or active clamp is selected according to current, switching speed, release time, and voltage stress.

Common-emitter amplifier

A BJT common-emitter stage uses collector and emitter resistors to establish a bias point. A small AC input at the base produces a larger inverted voltage swing at the collector.

Emitter follower

An emitter follower provides current gain and low output impedance with voltage gain near one. It is useful for buffering a high-impedance source from a lower- impedance load.

Common-source amplifier

The FET equivalent of the common-emitter stage is the common-source amplifier. A varying gate voltage controls drain current and produces an inverted voltage swing at the drain.

Source follower

The common-drain or source-follower stage provides high input impedance and low output impedance, similar in circuit purpose to the BJT emitter follower.

Differential pair

Two transistors sharing a controlled current source can amplify the difference between two input voltages. Differential pairs form the input stage of many operational amplifiers, comparators, analog ICs, and balanced communication circuits.

Current mirror

Matched transistors can create a current mirror. One device establishes a reference current and one or more related devices reproduce that current in other circuit branches. Current mirrors are basic building blocks of integrated analog circuits.

Push-pull output stage

Complementary NPN/PNP or N-channel/P-channel devices can share output work: one handles positive-going current while the other handles negative-going current. Audio amplifiers, motor drivers, and logic outputs commonly use push-pull structures.

Darlington pair

Two BJTs can be connected so the first drives the second, producing very high overall current gain. The tradeoffs include a larger effective base-emitter voltage and higher saturation voltage.

Bias and feedback

Good transistor circuits rarely depend only on an uncontrolled device parameter. Resistors, current sources, degeneration, and negative feedback are used so gain and operating point depend more on stable circuit values than on one transistor's beta, threshold, or transconductance.