BJT Basics Quiz: Learn Bipolar Junction Transistor Fundamentals

Study Notes

Bipolar Junction Transistor (BJT)

A BJT is constructed with three doped semiconductor regions separated by two pn junctions: emitter, base, and collector.
There are two types of BJT: npn (two n regions separated by a p region) and pnp (two p regions separated by an n region).
BJT is a current-regulating device that controls the amount of current flowing through it in proportion to the amount of biasing voltage applied to its base terminal.

Operating Conditions

The operating conditions of a BJT need to be set before an input signal is applied, which is achieved with a suitable bias circuit.
A bias circuit allows the operating conditions of a transistor to be defined, so that it will operate over a pre-determined range.
The dc operation of a transistor circuit can be described graphically using a dc load line.

DC Load Line

The load line illustrates the cutoff and saturation of a transistor's operation, with the active region in between.
The point at which the load line intersects a characteristic curve represents the Q-point for that particular value of Ib.

BJT Configurations

Common-Base

The base is the common terminal or grounded base, with the input signal applied between the base and the emitter.
This amplifier is a non-inverting voltage amplifier with high voltage gain and a maximum current gain of 1.
It has a low input resistance, making it suitable for certain applications where sources tend to have very low-resistance outputs.

Common-Emitter

The input is applied at the base, while the output is taken from the collector.
This amplifier is an inverting amplifier circuit with high voltage gain and high current gain, leading to high power gain.
The "voltage divider biasing" method is used to reduce the effect of varying β.

Common-Collector

The input signal is connected directly to the base, while the output is taken from the emitter.
This amplifier is known as an emitter-follower with high input impedance and current gain.
There is no phase inversion from the input to the output, and the output voltage follows the input voltage in both phase and amplitude.

Amplifier Classes

Class B Amplifiers

The advantage of class B and class AB amplifiers over class A amplifiers is the much higher efficiency, reaching 79% for these amplifiers.

Class C Amplifiers

Class C amplifiers are biased so that conduction occurs for much less than 180º, making them more efficient than class A and class B amplifiers.
They are normally limited to applications as tuned amplifiers at radio frequencies (RF) rather than for audio amplifiers due to the severely distorted output waveform.
Class C amplifiers have a very high efficiency that can reach 100% if they are properly tuned.

Multistage Amplifiers

Two or more amplifiers can be connected in a cascaded arrangement to increase the overall voltage gain.
The overall gain is the product of the gains of the individual stages.
There are various ways of coupling signals in amplifier circuits, including direct coupling, RC coupling, impedance coupling, and transformer coupling.
Each coupling method has its advantages and disadvantages, such as frequency response, low-frequency response, and efficiency.

Description

Learn about the fundamental concepts of Bipolar Junction Transistors (BJTs), including their construction, types (npn and pnp), and functionality. Understand the roles of the emitter, base, and collector regions in a BJT. Test your knowledge of BJT basics!