Discrete Semiconductor and Transistor Circuits

Discrete transistor circuits are built from individual semiconductor devices rather than integrated chips, and they remain the clearest way to understand how amplification and switching work. The two main device families are the bipolar junction transistor (BJT), which is controlled by a base current, and the field-effect transistor such as the MOSFET, which is controlled by a gate voltage. In both, a small input signal controls a much larger current, and this current gain is what makes amplification possible.

A transistor used as an amplifier is biased so that it operates in its active region, where the output current tracks the input signal. The way the device is connected defines its configuration: a common-emitter stage gives both voltage and current gain and inverts the signal, a common-collector (emitter-follower) stage provides current gain with a low output impedance and unity voltage gain, and a common-base stage is used at high frequencies. Each configuration suits different needs for gain, input and output impedance, and bandwidth.

The same transistors can be operated as switches, fully on or fully off, which underlies pulse and timing circuits. A multivibrator is a two-stage switching circuit that alternates between states. The astable multivibrator (AMV) has no stable state and oscillates continuously to generate a square wave, the monostable produces a single timed pulse, and the bistable holds either state until triggered. Built from a pair of cross-coupled transistors with resistors and capacitors, these circuits show how discrete devices create timing and waveform generation.

Frequently asked questions

How is a BJT different from a MOSFET?

A bipolar junction transistor is controlled by the current into its base, while a MOSFET is controlled by the voltage on its gate, which draws almost no steady current.

What does a common-emitter amplifier do?

It provides both voltage and current gain and inverts the signal, making it one of the most common general-purpose amplifying configurations for a bipolar transistor.

What is an astable multivibrator?

It is a switching circuit with no stable state that continuously flips between two conditions, producing a repeating square-wave output without any external trigger.