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The Usage of Capacitors and Resistors in Single-Stage Common Emitter Amplifiers

A simple electronic amplifier circuit uses a single transistor in a configuration called common-emitter. The amplifier̵7;s input signal drives the transistor̵7;s base and emitter and the emitter and collector produce the amplified output. Though the transistor is the active component in the circuit, a few resistors and capacitors have supporting roles, improving the amplifier̵7;s fidelity.

Class A Amplifier
- The single-stage common emitter amplifier exemplifies what electronics engineers call a Class A design. A Class A amplifier has excellent linearity, meaning the waveshape of the output closely resembles the input. Class A amplifiers are simple but lack energy efficiency: if it consumes 10 watts of power, it produces only a watt or two of output power.
Amplifier Gain
- Two resistors, one at the transistor̵7;s collector and one at the emitter, determine the circuit̵7;s voltage gain. The collector resistor manages the current coming into the transistor from the circuit̵7;s power supply. The resistor at the emitter biases the transistor̵7;s voltage higher than it otherwise would be. You calculate the amplifier̵7;s voltage gain by dividing the ohm value of the collector resistor by that of the emitter and multiplying the result by -1. For example, if the collector resistor̵7;s value is 1,000 ohms, and the emitter resistor̵7;s is 500 ohms, 1,000 / 500 * -1 = -2. With an input signal of 1 volt, the amplifier produces a -2 volt output.
Bias and Linearity
- The emitter resistor̵7;s bias voltage allows the circuit to amplify normal signals that alternate between positive and negative values. The transistor makes a good DC amplifier but cannot work on bipolar signals; the transistor cuts off one-half of the signal. The bias voltage adds to the input signal, turning it into an alternating wave that never crosses zero. The transistor can amplify this, although the circuit needs other components to remove the bias before passing the signal to other circuits.
Decoupling
- Two capacitors, one at the amplifier̵7;s input and one at the output, remove voltage bias in a process called decoupling. A capacitor consists of a pair of metal foils separated by an thin insulator. It permits AC signals to flow in a circuit but blocks DC. The first capacitor removes any DC from the input signal, allowing the amplifier̵7;s internal bias to work without interference. The amplifier̵7;s output has a bias on it, which the second capacitor removes. The circuit̵7;s output is an AC signal free of DC bias.

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