Amplifier Characteristics and Transistor Models

A set of vocabulary flashcards covering the key characteristics of amplifiers, BJT circuit analysis, and transistor equivalent models including Hybrid-$$\pi$$ and $$r_e$$ models.

"Good" Amplifier

An amplifier with high voltage gain, high input impedance, low output impedance, and high bandwidth.

High Voltage Gain

Ability to produce a large output voltage compared to the input voltage.

High Input Impedance

Allows the amplifier to draw very little current from the input source.

Low Output Impedance

Allows the amplifier to deliver more voltage to the load.

High Bandwidth

Ability to amplify input signals over a wide range of frequencies.

Transistor Equivalent Model

An equivalent circuit that represents the AC characteristics of a transistor.

Equivalent Circuit Model

Uses circuit elements that approximate the behavior of the transistor.

Common-Emitter Amplifier

Amplifier with high voltage and current gain; output signal is $180^{\circ}$ out of phase with the input.

Common-Emitter Amplifier DC Analysis

The DC component only sees the part of the circuit within $C_1$, $C_2$, and $C_3$ because DC does not pass through these capacitors; analysis is similar to a voltage-divider circuit.

Common-Emitter Amplifier AC Equivalent Circuit

Capacitors are replaced by shorts because AC passes through them easily; power supplies are also treated as shorts to ground for AC analysis.

Common-Emitter Amplifier AC Equivalent at Input

The input voltage can be analyzed using the equivalent base circuit to determine the input resistance $R_{in}$ or $Z_{in}$.

Hybrid-$\pi$ Model

A transistor equivalent model most useful for analyzing high-frequency transistor applications.

Hybrid-$\pi$ Model at Lower Frequencies

At lower frequencies, the Hybrid-$\pi$ model closely approximates the $r_e$ parameters and can be replaced by them.

$r_e$ Transistor Model

Uses a diode and a current source to duplicate the behavior of the transistor.

Simplified $r_e$ Model

A simplified version of the Hybrid-$\pi$ model.

Input Resistance

The resistance seen by the input voltage; can be determined using the r-parameter equivalent circuit.

Output Resistance

For practical purposes, the value is $R_c = R_c \parallel R_L$.

Voltage Gain

Determined by dividing the AC output voltage by the AC input voltage.

AC Voltage Gain

Can also be determined using the simplified formula because there is a single current flow ($I_C$) at the output AC equivalent circuit.

Overall Gain

Includes the attenuation caused by the AC supply internal resistance and input resistance.