Bipolar junction transistors, or BJTs, are three terminal semiconductor devices and are a basic building block for many integrated circuits (although they have mostly been replaced by FETs). They can also be used individually (as discrete devices) and are commonly used as switches or amplifiers. This set of videos below goes through the basic features and characteristics of BJTs and describes how to analyze and design simple BJT circuits. The specific video topics are:

  • BJT Characteristic Curves
  • Base Biased Circuits
  • Fixed Point Biased Circuits
  • Voltage Divider Biased Circuits
  • Switch Circuits
  • Introduction to AC Analysis
  • Modeling BJTs
  • AC Load Lines
  • Common Emitter Amplifiers
  • Common Collector Amplifiers
  • BJT Class A Amplifiers
  • BJT Class B Amplifiers
  • BJT Differential Amplifiers
  • Current Mirrors
  • Constant Current Drivers


BJT Characteristic Curves

A BJT characteristic curve shows the relationship between the current through the collector and the voltage across the Collector-Emitter. Typically, there are several different curves for different values of base current.  To obtain a characteristic curve, apply a particular base current, then sweep VCE through a range of values from 0 up to a maximum while measuring IC at several points. The characteristic curve can tell you a lot about the behaviour of the BJT. Watch the video below to find out more.

BJT Characteristic Curves

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Base Biased Circuits

A base biased circuit is one where the base voltage (and therefore base current) is set by its own individual voltage source. This is an inefficient configuration, but can help with the understanding of what BJT biasing is.

BJT - Base Biased Circuits

Fixed Point Biased Circuits

A fixed bias or fixed point bias circuit has the base connected through a resistor to the same voltage source as the collector/emitter side of the circuit. This configuration reduces the number of required voltage sources to one, but the operating point is very dependent on the beta value of the transistor.

BJT - Fixed Point Bias Circuit

Voltage Divider Biased Circuit

A voltage divider biased circuit sets the base voltage by using a voltage divider at the base. With proper selection of the resistors in the voltage divider circuit, the analysis of these kinds of circuits can be greatly simplified. The operating point of voltage divider bias circuits are not very sensitive to the value of beta for the transistor (this is a good thing).

BJT Voltage Divider Bias Example

Switch Circuits

A BJT can act as a switch and it works by controlling the current to the base. The switch operation is an example of a small amount of current controlling a large amount of current. In this case, a small base current, turns the switch on and allows a large amount of current to flow from the collector to the emitter.

BJT - Switch Circuit

BJT AC Analysis

Up to this point, we’ve only considered BJTs in DC circuits, but all of the biasing we’ve discussed is not very useful (except in the case of switches) unless there is an AC signal as well (typically an AC input that gets amplified by the circuit). The first step to understanding BJT amplifiers is to gain an understanding of the operation of BJT circuits when AC signals are applied.

BJT - Introduction to AC Analysis

AC Load Lines

AC Load Line of BJT Amplifier Circuit

Common Emitter Amplifiers

Common Emitter Amplifier

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Design a Simple Common Emitter Amplifier

Common Collector Amplifiers

Common Collector Amplifier

BJT Class A Amplifiers

BJT Class A Amplifiers
BJT Class A Amplifier Efficiency

Class B Amplifiers

BJT Class B Amplifiers
Class B Amplifier Efficiency Calculation

Differential Amplifiers

BJT Differential Amplifiers Inputs and Outputs
Gain of BJT Differential Amplifiers

Current Mirrors

Current Mirrors