The transistor is a semiconductor device that can amplify or switch electronic signals and electrical power. The two principal families are the Bipolar Junction Transistor (BJT) and the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). A BJT has three terminals: the base, collector, and emitter, and exists in two polarities, NPN and PNP. The defining characteristic of a BJT is its current gain, \(\beta = I_C / I_B\), which represents the ratio of collector current to base current and typically ranges from 50 to 300.
A MOSFET also has three terminals: the gate, drain, and source. Unlike the current-controlled BJT, the MOSFET is voltage-controlled: the gate-to-source voltage modulates the current flowing between drain and source. The minimum gate-to-source voltage required to create a conducting channel is called the threshold voltage, \(V_{th}\). MOSFETs operate in three regions: cutoff (off), triode or linear (acting as a variable resistor), and saturation (delivering a nearly constant current, useful for amplification).
The operational amplifier, or op-amp, is a high-gain differential voltage amplifier with two inputs (inverting and non-inverting) and a single output. When configured with negative feedback, an ideal op-amp obeys two golden rules: no current flows into either input (implying infinite input impedance), and the voltage difference between the inputs is zero (\(V^+ = V^-\)). The inverting configuration has a gain of \(A_v = -R_f / R_{in}\), while the non-inverting configuration has a gain of \(A_v = 1 + R_f / R_{in}\). A special case, the voltage follower or buffer, ties the output directly to the inverting input, giving unity gain and providing impedance matching between stages. A practical limitation is the gain-bandwidth product, a constant equal to the open-loop gain multiplied by the bandwidth, meaning that higher closed-loop gains reduce available bandwidth. Other useful op-amp circuits include the summing amplifier, which produces a weighted sum of inputs, and the comparator, used without feedback to detect which of two inputs is greater.