Bipolar Junction Transistor Part 1

Sources

1. Bipolar Junction Transistor (BJT) (Lecture Slides)
2. Class Lecture

Introduction

Types of Transistors

1. Junction Field Effect Transistor (JFET)

• Voltage driven devices

1. Bipolar Junction Transistor (BJT)

• A current triggering device

A bipolar junction transistor is a three-terminal electronic device composed of either the following:

1. two P-type materials and one N-type material.
   • It has a negative base activation because of the P at the center.
   • In the electronic symbol, the arrow is pointing towards the base.
2. or two N-type materials and one P-type material;
   • It has a positive base activation because of the P at the center.
   • In the electronic symbol, the arrow is pointing away from the base.

• The input is located at the base activation. Conversely, the output is located at the collector and emitter.

Operation

Because a BJT is a three-terminal device, there are three ways of biasing it.

Base-Emitter Junction	Base-Collector Junction	Region of Operation
forward bias	reverse bias	Active region
reverse bias	reverse bias	Cut-off region
forward bias	forward bias	Saturation region

• The base can either be paired with the emitter or the collector because it is at the neutral side.
• There is no current at the cut-off region.

Forward Bias and Reverse Bias

Forward bias implies that there is a high current. In contrast, reverse bias implies that there is a high resistance.

The Eber’s Moll model is used to visualize the behavior of a BJT transistor. It contains two diodes: a Base-Emitter PN junction representation and a Base-Collector PN junction. In addition, it also contains a dependent current source between the base and collector—a function of the current gain, or .

• • Used for common base design (base is at ground)
• • Used for common emitter design (emitter is at ground)
• • Used for common collector design (collector is at ground)

1. PNP - both output has currents going up, while the input is leaving through the base.
2. NPN - both output has currents going down, while the input is entering through the base.

Active Region

• base-emitter is forward biased
• base-collector is reverse biased
• Plenty of majority carriers (electrons) will diffuse around the base-emitter junction. When these electrons go into the base region, they may leave through the base resistance to the positive terminal of , or they may flow towards the collector region (usually the latter).
• The base region is very thin.
• Few amount of current, flow through the transistor due to the minority carriers.

Cutoff Region

• both base-emitter junction and base-collector junction are reverse biased.
• Small amount of leakage current resulting from a widening of the depletion region (usually neglected due to minimal impact)
• Two diodes act like open circuits (with current through them equal to 0 and max voltage across them ) in the Eber’s Moll model since it is reverse biased.
• =

Saturation Region

• Both base-emitter junction and base-collector junction are forward biased.
• Net voltage drop present between the collector and emitter would be equal to 0 since base-emitter voltage and base-collector voltage are opposing and are equal.
• Transistor acts like a short circuit. Its maximum current flows through the transistor.

BJT Output Characteristic Curve

• Active region is between the saturation region and the cut-off region.
• The saturation region has the maximum amount of collector current but has no collector-emitter voltage .
• The cut-off region has the minimum amount of collector current but has the maximum collector-emitter voltage .
• The best and most stable value is located at the Q-point or the quiescent point.

Basic Transistor Formulas

BJT Amplifier Configuration

To identify the configuration (whether it is Common Base, Common Emitter, or Common Collector), check where the input source and output source is. The one without the voltage source indicates the common type (because of the ground).

DC Analysis

Notation for operating currents and voltages of the BJT under DC conditions:

Parameter	Notation
DC emitter current	or
DC collector current	or
DC base current	or
DC collector-emitter voltage	or
DC base-emitter voltage	or
DC bias supply voltages	, ,

• Before solving for the emitter or collector current, solve for the base current first.
• The in the subscript stands for quiescent.
• The collector-emitter voltage should be solved for, while the base-emitter voltage should be given.
• The bias supply voltages should be given.
• All capacitors are replaced with an open circuit to isolate the AC signal.

Fixed Bias Circuit

• This type of bias circuit configuration is contingent on only the base resistance .
• No .
• Is the simplex bias configuration.
• Use KVL to solve for
• At saturation region, is at minimum, while is at maximum
• At cut-off region, there is no current at output () because the transistor is open. The base current and emitter current are also equal to 0.
• Get equations from the outer loop equation when at saturated region or cut-off region.