0.1 Discrete components  (Page 7/6)

Discrete components

It is almost impossible to complete a circuit design without including discrete components. Most instruction at university courses about these devices focus on the theory of their operation. You will learn about Ohm’s law and all of the theory that ties the law to resistors, capacitors and inductors. You’ll learn transistor theory and all of the aspects that it includes. And as I remember, the concepts of a diode seem to have fallen into the same discussion.

This section will be split into two subsections:

• Passive components.
• Active components.

Passive components

Passive components are resistors, capacitors and inductors. The surprise that most circuit designers encounter when first using passive components can be shown in Figure 1: the circuit equivalent for a resistor, capacitor or inductor.

Put into words, every resistor has an element of inductance and capacitance. Every capacitor has an element of resistance and inductance. And every inductor has an element of resistance and capacitance. In each case, their influence on the actual value of the component will be a function of the frequency of the signal applied to the device.

Here are a couple of examples to get you thinking:

• A wire-wound resistor has an element of inductance that may significantly alter the performance of the resistor over the frequency range of the circuit design.
• In a power supply, a small ceramic capacitor may be required to eliminate the high frequencies passed by the larger electrolytic capacitor due to its significantly lower inductance component as a result of how it is made.
• An inductor that has been made by winding wire around a core will have the DC resistance component of that wire.

Active components

I’ll cover two basic types of active components: diodes and transistors. A diode in its most simple application allows current to flow in only one direction. A transistor in its most simple application can act as a signal amplifier or as a switch.

Since both of these active elements can be easily integrated onto a silicon substrate, they are the workhorses found in all integrated circuits available today, both analog and digital. The only time you will need to use discrete diodes and transistors is when you need to do something that is considered beyond the capability of an IC. (Generally, what is considered beyond the capability of an IC is too high of a voltage on the device or too high of a current flowing through the device.)

In each of the discussions on these discrete components, I will assume that you already understand the theory behind them and are looking for some basic ideas on how to use them in your senior project design.

Resistors