Introduction: Audio Signal and Amplification
In audio electronics, an audio signal means an electrical signal which represent audible wave or sound, regardless of their sources. The source of an audio signal can be microphone, tape or electric guitar pickup head, mp3 player, computer’s sound card, electronic oscillator, or other sources. Amplification, in this context means processing an audio signal to produce bigger or stronger signal to get appropriate level for specific purpose. An electronic device that provide this amplification function is then called amplifier. The amplification is not a magical process that violate the law of energy conservation by producing high energy in the output from nothing (very small energy of the input signal). The small signal of the input is basically only a “driving signal” to transform the power from a power supply into a high power audio signal. It is analog to pressing the throttle or gas pedal of your car with small force to accelerate your heavy car. Here amplification process works: a small signal of force that press the pedal is used as a “driving signal” to produce much stronger signal of force that accelerate the car. Here the power supply comes from the gasoline (converted into force by combustion inside the engine).
Type of Audio Amplifier by Its Power Characteristic
When categorized by its power characteristic, audio amplifier circuits can be categorized into pre-amplifier and power amplifier. Pre-amplifier produces very small power (in the order of several milliwatts), since it is designed mainly to amplify the voltage level, not the output power. With several millivolts signal at the input (such as a signal coming from microphone or other pickups), the amplified signal at the output ranges about 1 – 2 volts (see reference 1). Many types of pre-amplifiers are labelled according to their special application such as dynamic microphone pre-amp, condenser microphone pre-amp, guitar pickup pre-amp, etc. All of these low power pre-amplifiers drive only high impedance loads, which are normally the next stage audio processing devices, such as tone control, audio mixer, equalizer, or the final power amplifier. The power amplifier category has higher power output, ranges from several hundredths milliwats (driving small loudspeaker) to thousands watts (driving large loudspeakers such as in the cinema’s or performance stage’s).
Type of Audio Amplifier by Its Transistor Operation Mode
In electronic circuit, audio amplifier can be designed using vacuum tubes, transistors, or integrated circuits. Theoretically they are the same things, where from the transistor’s point of view the vacuum tube is the obsolete “stuff” providing the same function of the transistor before the invention of semiconductors, and the integrated circuits is nothing more than a package of interconnected transistors inside a chip. One popular amplifier classification is made based on the operation mode of the employed transistor in the circuit design: class-A , class-B, Class-AB, class-C, and class-D amplifiers. Although this classification applies for all types of amplification (not limited to audio, such as video or radio frequency amplifiers), the discussion would be focused on audio amplifier applications.
- Class-A Audio Amplifiers Circuit. The circuit employs transistor to operate fully at its active state for complete cycles of input signal. One popular example of transistor circuit operating at full cycle like this is common emitter circuit shown in the Figure 1.
How to select the proper component values for this amplifier circuit can be found in the reference 2. The drawback of this amplifier class is that it has low power efficiency, but the advantage is its simplicity. The main application for this amplifier class is where power requirement is very small, such as pre-amplifiers, active filters, or other signal processing function. Because of low power efficiency, class-A amplifier should never be used as power amplifier in practical audio application. For experimental, a class-A audio power amplifier could be implemented as shown in figure 2, and it would waste many electrical power into heat. The detail of how to build this experimental power amplifier can be found at reference 3.
- Class-B Audio Amplifier Circuit. The transistor in this circuit conducts the current only at half of the input cycle. To produce complete cycle, two transistors are used complementary, where one transistor is active at positive cycle and the other is active only at negative cycle. A simple 3 transistor audio amplifier circuit in Figure 3 can be classified into this class (see reference 4 for the details). Its biasing circuit is formed by diodes D1 and D2, where it is assumed to have voltage drops similar to the transistor biasing voltage base-emitter voltages of TR3 and TR2, so one transistor starts conducting after the other have just stopped. This configuration is also called push-pull configuration, where the upper transistor TR3 push a charging current into the decoupling capacitor C2 and drive the loudspeaker for the positive cycle, and at the other time the lower transistor pull the current to discharge the decoupling capacitor C2 and drive the loudspeaker for the negative cycle. The drawback of this operation is that they suffer cross-over distortion, at the zero crossing point where the switching between two transistors takes place. A single transistor element class-B amplifier is almost never used in sound producing circuit since reproducing only half-cycle generate very hard distortion, although early IBM personal computer used it to drive a loudspeaker for beeping sound (reference 5). In other audio application where the expected output is not the perfect audio signal, class-B amplifier circuit is still useful for signal detection such as disco lighting control (reference 6) circuit.
- Class-AB Audio Amplifier Circuit. Almost similar with class-B, the transistor in class-AB amplifier conduct a half plus small amount of the other half cycle. When implemented as a push-pull configuration, the circuit is normally different only in the biasing circuit, for example, the diode D1 and D2 in the Figure 3 is connected through a small resistor to add small amount of biasing voltage. With higher biasing level, at zero crossing point both transistor would conducts small short current, with the switching-on of one transistor occurs just before the other transistor is switched-off. This eliminate the cross-over distortion but increase the idle current, the small short current flowing from positive supply terminal into the negative terminal even when there is no output loudspeaker current.
- Class-C Audio Amplifier Circuit. In this class, the transistor is operated in less than the half-cycle. This amplifier is popular in radio frequency for tuned operation, where the hardly distorted signal is used to excite an L-C tank or resonator. In audio frequency application, this amplifier class is never used for reproducing sound or audible signal. However, in the non-audible application, this class of amplifier is still applicable in signal detection, such as audio-controlled lamp controller show in the figure 4 (reference 7). The transistors here are not activated at zero crossing of the input signal, but keep turned off wait until the signal reach the bias point of the transistor’s base-emitter forward bias voltage (around 0.6 – 0.7 volts).
- Class-D Audio Amplifier Circuit. Unlike class-A, -AB, -B, and -C, class-D amplifier works totally in the non-linear operation area of the transistor. The transistor is operated like a switch, by being driven fully -on or fully -off. The block diagram of class-D audio amplifier circuit is shown in the figure 5. The advantage of this amplifier type is its very high efficiency, while the drawback is the higher distortion level of the reproduced audio signal.
- Line Level Standards, https://en.wikipedia.org/wiki/Line_level
- The Common Emitter Amplifier Circuit, http://www.electronics-tutorials.ws/amplifier/amp_2.html
- Class A Darlington Power Amplifier, http://www.techlib.com/electronics/audioamps.html#class-a
- Small audio power amplifier using 3 transistors, http://www.deeptronic.com/small-audio-power-amplifier-using-3-transistors/
- Amplifier Class-B, https://en.wikipedia.org/wiki/Amplifier#Class_B
- Automatic Disco Lighting Control, http://www.deeptronic.com/music-controlled-disco-light-circuit-with-automatic-level-control/
- Hi-Lo Disco Lamp Controller, http://www.deeptronic.com/hi-lo-bass-treble-disco-lamp-controller/