# Practical Analog Circuits

In the previous volumes of this study guide, we have discussed the laws and theories pertaining to electronic circuits and the rules governing various electronic components. Now, we will learn how these components interact with each other to make useful circuits and how circuits interact with other circuits.

An analog circuit operates on a continuously variable signal. This means the voltage may vary anywhere between certain limits. This is as opposed to digital circuits where only two discrete voltages may be used, such as 0 and 5 volts or +12 volts and -12 volts (refer to the class on digital circuits). If a digital signal is converted to an analog signal, the resulting analog signal will have discrete steps. For example, an analog circuit may be confined to a range between 0 and +12 volts. A natively analog signal may use any voltage in that range without limit on how close any two levels may be. A digital signal converted to analog may use 65,536 steps in that range. Nevertheless, the signal with the steps is still considered an analog signal and works with analog circuits.

An analog circuit operates on a continuously variable signal. This means the voltage may vary anywhere between certain limits. Analog circuits fall into three main categories: DC, audio frequency (AF), and radio frequency (RF).

## DC Circuits

DC circuits are those where the voltages or currents remain constant or do not vary periodically. Any variations in voltage or current are not repetitive.

## Audio Frequency Circuits

AF circuits work with AC signals that have frequencies between 20Hz and 20kHz. This is the nominal range of human hearing. Humans perceive musical pitch according to frequency. The higher the frequency, the higher the pitch.

At first impression, you may think that the middle of the human range of hearing is about 10 kHz (about halfway between 20Hz and 20kHz). However, the musical pitch of an oscillating signal is perceived logarithmically. This means that what sounds like an even increase in pitch is not an even increase in frequency. To understand what this means, think of a piano keyboard. Each octave is about a hand-span apart, evenly spaced. This is how we hear pitch; each octave is an even increase in perceived pitch. However, each octave is actually double the frequency of the next lower octave. For example, the musical pitch notated as a', (called A-prime, middle A or A above middle C) has a frequency of 440Hz. The next-higher octave, a'' (A-double-prime), is 880 Hz. Another octave higher, a''', is 1760 Hz. Therefore, each octave is twice the frequency of the next-lower octave. This puts the middle of the range of human hearing near 1 kHz (a bit of a flat C'''). For this reason, 1 kHz is the standard frequency for testing AF circuits.

RF circuits work at frequencies from 30kHz to 300GHz. Because of the very wide range of radio frequencies, the RF range is broken up into several sub-ranges called bands. RF circuits are usually designed to work in only one of the RF bands and often only a small part of a band. Designing a circuit that works well at all RF frequencies is challenging. This is because as you increase a circuit's operating frequency, the output level will drop. This problem is more pronounced in circuits that operate over wider ranges.

Another issue with radio frequency circuits is stray capacitance. Any time you have two conductors separated by an insulator, you have a capacitor. Stray capacitance is the capacitance between traces on a circuit board, adjacent wires, the junctions of transistors, etc. As frequency increases, capacitive reactance decreases, meaning more and more signals will leak through the stray capacitance in a circuit. High-frequency RF circuits use special components with reduced capacitance, and measures are taken to reduce stray capacitance elsewhere in circuits.

## Other Frequency Ranges

There are no widely accepted names for circuits that work at frequencies below 20Hz or from 20 kHz to 30 kHz. Frequencies below 20Hz are often referred to as infrasonic. Frequencies between 20kHz and 30kHz are in the range often referred to as ultrasonic. However, frequencies as high as 100kHz may be referred to as ultrasonic.

Frequencies above 300GHz are in the infrared range. These frequencies are considered light rather than radio. Such high frequencies are generally beyond what electronic devices and circuits can treat as alternating current. They are handled by devices and circuits that handle

Practical Analog Circuits