Introduction Transistors

Introduction

Transistors make our electronics world go ‘round. They’re critical as a control source in just about every modern circuit. Sometimes you see them, but more-often-than-not they’re hidden deep within the die of an integrated circuit. In this tutorial we’ll introduce you to the basics of the most common transistor around: the bi-polar junction transistor (BJT).

In small, discrete quantities, transistors can be used to create simple electronic switches, digital logic, and signal amplifying circuits. In quantities of thousands, millions, and even billions, transistors are interconnected and embedded into tiny chips to create computer memories, microprocessors, and other complex ICs.

Covered In This Tutorial

After reading through this tutorial, we want you to have a broad understanding of how transistors work. We won’t dig too deeply into semiconductor physics or equivalent models, but we’ll get deep enough into the subject that you’ll understand how a transistor can be used as either a switch or amplifier.

This tutorial is split into a series of sections, covering:

Symbols, Pins, and Construction – Explaining the differences between the transistor’s three pins.
Extending the Water Analogy – Going back to the water analogy to explain how a transistor acts like a valve.
Operation Modes – An overview of the four possible operating modes of a transistor.
Applications I: Switches – Application circuits showing how transistors are used as electronically controlled switches.
Applications II: Amplifiers – More application circuits, this time showing how transistors are used to amplify voltage or current.
There are two types of basic transistor out there: bi-polar junction (BJT) and metal-oxide field-effect (MOSFET). In this tutorial we’ll focus on the BJT, because it’s slightly easier to understand. Digging even deeper into transistor types, there are actually two versions of the BJT: NPN and PNP. We’ll turn our focus even sharper by limiting our early discussion to the NPN. By narrowing our focus down – getting a solid understanding of the NPN – it’ll be easier to understand the PNP (or MOSFETS, even) by comparing how it differs from the NPN.

Suggested Reading

Before digging into this tutorial, we’d highly recommend giving these tutorials a look-through:

Voltage, Current, Resistance, and Ohm’s Law – An introduction to the fundamentals of electronics.
Electricity Basics – We’ll talk a bit about electricity as the flow of electrons. Find out how those electrons flow in this tutorial.
Electric Power – One of the transistors main applications is amplifying – increasing the power of a signal. Increasing power means we can increase either current or voltage, find out why in this tutorial.
Diodes – A transistor is a semiconductor device, just like a diode. In a way, it’s what you’d get if you stacked two diodes together, and tied their anodes together. Understanding how a diode works will go a long way towards uncovering the operation of a transistor.

Introduction to Capacitors

Introduction to Capacitors

Just like the Resistor, the Capacitor, sometimes referred to as a Condenser, is a simple passive device that is used to “store electricity”.

The capacitor is a component which has the ability or “capacity” to store energy in the form of an electrical charge producing a potential difference (Static Voltage) across its plates, much like a small rechargeable battery.



There are many different kinds of capacitors available from very small capacitor beads used in resonance circuits to large power factor correction capacitors, but they all do the same thing, they store charge.



In its basic form, a capacitor consists of two or more parallel conductive (metal) plates which are not connected or touching each other, but are electrically separated either by air or by some form of a good insulating material such as waxed paper, mica, ceramic, plastic or some form of a liquid gel as used in electrolytic capacitors. The insulating layer between a capacitors plates is commonly called the Dielectric.

A Typical Capacitor
Due to this insulating layer, DC current can not flow through the capacitor as it blocks it allowing instead a voltage to be present across the plates in the form of an electrical charge.



The conductive metal plates of a capacitor can be either square, circular or rectangular, or they can be of a cylindrical or spherical shape with the general shape, size and construction of a parallel plate capacitor depending on its application and voltage rating.



When used in a direct current or DC circuit, a capacitor charges up to its supply voltage but blocks the flow of current through it because the dielectric of a capacitor is non-conductive and basically an insulator. However, when a capacitor is connected to an alternating current or AC circuit, the flow of the current appears to pass straight through the capacitor with little or no resistance.



There are two types of electrical charge, positive charge in the form of Protons and negative charge in the form of Electrons. When a DC voltage is placed across a capacitor, the positive (+ve) charge quickly accumulates on one plate while a corresponding negative (-ve) charge accumulates on the other plate. For every particle of +ve charge that arrives at one plate a charge of the same sign will depart from the -ve plate.



Then the plates remain charge neutral and a potential difference due to this charge is established between the two plates. Once the capacitor reaches its steady state condition an electrical current is unable to flow through the capacitor itself and around the circuit due to the insulating properties of the dielectric used to separate the plates.



The flow of electrons onto the plates is known as the capacitors Charging Current which continues to flow until the voltage across both plates (and hence the capacitor) is equal to the applied voltage Vc. At this point the capacitor is said to be “fully charged” with electrons. The strength or rate of this charging current is at its maximum value when the plates are fully discharged (initial condition) and slowly reduces in value to zero as the plates charge up to a potential difference across the capacitors plates equal to the source voltage.



The amount of potential difference present across the capacitor depends upon how much charge was deposited onto the plates by the work being done by the source voltage and also by how much capacitance the capacitor has and this is illustrated below.