A transistor can amplify both current and voltage, depending on its configuration and how it is biased in a circuit. In common emitter configuration, for example, a transistor primarily amplifies current. The input signal controls the base-emitter current, which in turn controls the larger collector-emitter current, thus amplifying the current signal. This configuration is typical for applications where current amplification is required, such as in signal amplifiers.
Transistors can also amplify AC signals, DC signals, or both, depending on their design and circuit configuration. AC signals are typically amplified by transistors in various amplifier circuits, including audio amplifiers, radio frequency amplifiers, and others. DC amplification can also be achieved in certain transistor circuits, where the DC biasing allows the transistor to operate in its active region and amplify DC voltages.
Transistors themselves do not add voltage in the sense of creating additional voltage levels. Instead, they can amplify an input voltage signal to a higher output voltage signal. This amplification process is achieved by controlling the transistor’s base current (for bipolar junction transistors) or gate voltage (for field-effect transistors), which in turn modulates the collector current or drain current, thus amplifying the voltage across the load resistor in the circuit.
To amplify voltage with a transistor, you typically use it in a common emitter or common source configuration, where the input signal (voltage) is applied to the base (for BJTs) or gate (for FETs) through appropriate biasing resistors or circuits. The transistor’s amplification occurs as the input voltage signal modulates the transistor’s conductivity, resulting in a larger output voltage signal across the load resistor connected in the collector (for BJTs) or drain (for FETs) circuit.
Whether a transistor turns on (or amplifies) primarily depends on the base current (for BJTs) or gate voltage (for FETs). For a bipolar junction transistor (BJT), a sufficient base current causes the transistor to turn on, allowing current to flow from collector to emitter and thus amplifying the signal. For field-effect transistors (FETs), applying a suitable gate voltage relative to the source allows current to flow from drain to source, amplifying the input signal. Proper biasing and circuit design are critical to ensuring transistors operate in their intended amplification mode effectively.