Amplifiers are used to increase the voltage or power amplitude of signals. They have many applications.

Amplifier - Input - Output - Diagram

AUDIO VOLTAGE amplifiers boost the amplitude of signals between the frequency range 20 Hz to 20 KHz. This is the range of human hearing.
They are often used as PRE-AMPLIFIERS before the main amplifier.

AUDIO POWER amplifiers provide the power necessary to drive loudspeakers. They also amplify a frequency range from 20Hz to 20 KHz.


Circuit Symbols Tutorial

The diagram below shows passive components, resistors, capacitors and inductors, and also electrical components such as switches, relays, motors and lamps. Also shown are the symbols for wires that are not joined ( no physical electrical connection ) and wires that are joined ( a physical electrical connection ).


This next diagram depicts active components, the difference between active and passive is that active components require a power source to work, whereas passive components do not. The top symbols represent vacuum tube or thermionic devices. Although at one time, these were being replaced by the smaller transistor and integrated circuits, they are finding their way back into electronics for use in professional audio equipment and some radio receivers



Filters Tutorial

A filter circuit is like a sieve. It allows some things through and holds back others. In this case we are talking about AC frequencies. Some frequencies pass through the filter while others are rejected.

Filter Circuit - Input Selected Frequencies out Diagram

The characteristics of a filter can be shown on a graph called a FREQUENCY RESPONSE CURVE.


VOLTAGE OUT is plotted against FREQUENCY.

Figure 2 shows a LOW PASS filter response curve giving output at low frequencies but none at higher frequencies.

Filters - Low Pass - High Pass - Band Pass - Band Stop Diagram

Figure 3 shows a selection of filter characteristics.

Simple filters can be made from capacitors and resistors

Capacitors and Resistors - Low Pass Filter - Hight Pass Filter Diagram

Filters have many applications.

Block Diagram Symbols of Filters - A Filter - Low Pass - Hight Pass - Band Pass - band Stop - Interference Filter

In audio frequency amplifiers, CROSSOVER filters to direct low frequencies to the WOOFER and high frequencies to the TWEETER speakers.

In SCRATCH filters to remove unwanted high frequency noise.

In NOTCH filters to remove whistles due to two radio stations being too close together in frequency.

In Hum filters to remove low frequency noise due to the mains supply.


RMS and Peak to Peak

 If someone measures the value of the AC voltage coming out of a transformer using an oscilloscope and says it is 20 volts peak to peak and we use a voltmeter to confirm this we will find that the meter reads only 7.07 volts.

This is because the scope measures peak to peak values and the meter measures RMS values.

In figure 1 the 'scope displays the peak value. The peak to peak voltage is twice this. For example if the peak is 10 volts then the peak to peak is 20 volts.

When using a meter to measure the same AC voltage a different value is obtained. This is because, as we said, meters measure RMS values.

A Root Mean Square (RMS) voltage gives the same heating effect as a DC voltage of the same value.  See figures 2 and 3. Both thermometers show the same temperature when the resistors are heated by the current passing through them.

RMS values can be converted to peak to peak values and vice-versa.

RMS values times 1.414 equals the Peak value. Peak to Peak is twice this. 7.07 volts RMS times 1.414 and then doubled is 20 volts, the Peak to Peak value.


Peak values times 0.707 gives the RMS value. Don't forget that Peak is half the Peak to Peak.
20 volts Peak to Peak is 10 volts Peak.  10 volts Peak times 0.707 equals 7.07 volts RMS.

Feedback Tutorial

It is best if you read the page on the PHASE first.

Feedback is when some of the output signal from a circuit is fed back to the input and combined with the input signal.

If input and output signals are in phase then the feedback is POSITIVE.

If the two signals are out of phase then it is NEGATIVE FEEDBACK.

 Feedback Amplifier Input Output Diagram

Positive feedback in an amplifier increases the gain and reduces the bandwidth of the amplifier.
If there is sufficient positive feedback then the amplifier will oscillate.

If a microphone is too near to a loudspeaker then you will get positive feedback causing "howl round".

Negative feedback reduces gain and increases bandwidth.



Oscillators are amplifiers with such a large amount of positive feedback that they produces an output signal with no signal applied to the input.

Oscillator Feedback Amplifier Diagram

The output amplitude is determined by the gain of the amplifier and the feedback circuit.

Oscillators can produce sine waves, the frequency of which is determined by TUNED CIRCUITS.
Tuned circuits consist of a capacitor and inductance.

Square wave oscillators use resistors and capacitors to determine the frequency of oscillation.

Ideally the frequency of an oscillator should be stable, but due to temperature variations and mechanical vibration this may not be so.

Precautions are taken against frequency DRIFT.

Circuit Diagrams Tutorial

Circuit diagrams are one method of describing electronic equipment.
They are made up of BS3939 standard circuit symbols.


READING a circuit diagram is the ability to look at the diagram and understand how the circuitry works.

Be aware that the layout of the circuit diagram may be nothing like the physical layout of the actual equipment.

Although the circuit diagram shows all capacitors the same size and shape, in reality they will be of assorted sizes, shapes and color.

This applies to other components.

9 Volts DC Stabilised Power Supply UNit CIRCUIT DIAGRAMS


Amplitude Modulation

If you connect a long wire to the output terminals of your Hi-Fi amplifier and another long wire to the input of another amplifier, you can transmit music over a short distance. DON'T try this. You could blow up your amplifier.


A radio wave can be transmitted long distances. To get our audio signal to travel long distances we piggyback it onto a radio wave. This process is called MODULATION.

The radio wave is called the CARRIER.

The audio signal is called the MODULATION.

At the receiving end the audio is recovered by a process called DEMODULATION.

From the diagram below, it can be seen that when the carrier is modulated, its amplitude goes above and below its unmodulated amplitude.

It is about 50% modulated in the diagram.
The maximum percentage modulation possible is 100%.
Going above this causes distortion.

AMPLITUDE MODULATION - Unmodulated Peak Trough Diagram

Most broadcasters limit modulation to 80%.

Modulating the carrier frequency with an audio frequency produces two new frequencies.
At this point it would be a good idea to read the page on MIXERS.
These new frequencies are called the upper and lower SIDEBANDS.
The upper sideband is the carrier frequency plus the audio frequency.
The lower side band is the carrier frequency minus the audio frequency.

Since the audio signal is not a single frequency but a range of signals (usually 20 Hz to 20 KHz) the sidebands are each 20Hz to 20 KHz wide.

AMPLITUDE MODULATION - Carrier - Lower SideBand - Upper Side Band Diagram

If you tune across a station in the Medium Wave Band you will find that it takes up space in the band.
This is called the signal BANDWIDTH.
This is the space taken by the upper and lower sidebands.
In the the example given above it would be 40 KHz.
Since the Medium Wave is only 500 KHZ wide there would only be space for about 12 stations.
Therefore the bandwidth of stations is limited to 9 KHz, which limits the audio quality.

If there are two stations too close together, their sidebands mix and produce HETERODYNE whistles.

Since both sidebands carry the same information, one side can be removed to save bandwidth.
This is SSB, single sideband transmission.


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