Audio Power Amplifier With Speaker AGC Load With AGC Circuit_docx

Audio Power Amplifier with speaker AGC circuit 1

Audioa PowerP AmplifierA

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speakers AGCA loadl withw

AGCa circuitt

-Project by

Ashok Kumar.E Ashok.N

Ganesh Balaji.N Hari Prassana


INDEX

1. Stages of an audio power amplifier-------- 3 2. Conventional amplifier design-------------- 3 3. What is an AGC?------------------------------ 4 4. AGC circuit schematic----------------------- 5 5. Working of the AGC circuit----------------- 6 6. Ideal transfer function of AGC------------- 7 7. Block diagram of AGC----------------------- 7 8. Power amplifier schematic------------------ 8 9. Working of the power amplifier----------- 9

10. Need of an AGC circuit---------------------- 9 11. Over-all circuit-------------------------------- 11


Stages of a audio amplifier

� Transconductance stage

� Transimpedence stage(voltage-amplifier stage or VAS)

� Unity–voltage-gain output stage

THREE STAGE AUDIO AMPLIFIER (CONVENTIONAL DESIGN)


What is AGC?

In the early years of radio circuits, fading (defined as slow variations in the amplitude of the received signals) required continuing adjustments in the receiver’s gain in order to maintain a relative constant output signal. Such situation led to the design of circuits, which primary ideal function was to maintain a constant signal level at the output, regardless of the signal’s variations at the input of the system Originally, those circuits were described as automatic volume control circuits, few years later they were generalized under the name of Automatic Gain Control (AGC) circuits. Now that we roughly know what an AGC means, we make a definition of AGC. AGC can be defined as “a process or means by which gain is automatically adjusted in a specified manner as a function of a specified parameter, such as received signal level”. Automatic gain control (AGC) is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels. For example, without AGC the sound emitted from an AM radio receiver would vary to an extreme extent from a weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it when it is weaker.


AGC circuit schematic


Working of the AGC circuit The AGC circuit consists of an OP-Amp (LM358). The OP-Amp acts as an amplifier with positive feedback. The LM358 is a single supply OP-Amp. Hence the input that is given to the inverting input of the OP-Amp is biased about the mean value of the supply Vcc. The output of the OP-Amp is fed back to the non-inverting input of the OP-Amp. Also the output is fed back to the inverting input coupled with the audio input by means of an active network. The active network consists of a BJT and a P-channel JFET. The BJT acts as a switch which sources or sinks the collector current based on the base voltage.

The P-channel JFET acts as a linear resistor dependent on the gate voltage. The resistance is directly proportional to the gate voltage, which is in-turn proportional to the collector current of the BJT. The drain current is then fed back to the inverting input of the OP-Amp coupled with the audio input.

When the input amplitude is given it is biased about the mean value of the supply. The OP-Amp amplifies the value by the ratio (1+R9/R10). The output of the OP-Amp is then given to the base of the BJT. The collector current is inversely proportional to the base voltage. Hence when the output goes high the collector current goes low. When the collector current goes low the resistance of the JFET also goes low. Hence the voltage at the inverting input also goes low. Thus the output is controlled automatically depending on the input.

The output of the AGC is then given to the power amplifier via a variable resistor. First the output is coupled to the variable resistor with the help of a coupling capacitor. It is used for scaling the output.


Ideal transfer function of an AGC circuit

The transfer function of the AGC gives a good idea of what the AGC does. The x-axis is the input amplitude and the y-axis is the output voltage. The output is linearly dependent on the input till the voltage V1.

After V1 the output remains fairly constant for change in the input till V2. After V2 the AGC breaks down. The AGC is demonstrated in the region between V1 and V2.

Block Diagram of an AGC circuit


Power Amplifier circuit schematic


The power amplifier is an IC LM380. It is a 14 pin IC. The input of the power amplifier is the output of the AGC circuit after suitable scaling. The scaling is done by dual volume control passive circuitry. The circuit consists of two variable resistors. The output of the power amplifier is then taken across a resistor and a capacitor. The output is connected to the speaker.

Need for an AGC circuit The automatic maintenance of a nearly constant output level of an amplifying circuit by adjusting the amplification in inverse proportion to the input field strength, also called automatic volume control (AVC). Almost all radio receivers in use employ AGC. In broadcast receivers, AGC makes it possible to receive incoming signals of widely varying strength, yet have the sound remain at nearly the same volume. In communications receivers a type of AGC circuit called a squelch circuit is used to prevent noise during periods of no transmission, such as in the reception of on-off keying, frequency-shift keying (FSK), and phone. AGC is also useful in accelerating the switching action between receivers in diversity connection. See also Radio receiver. AGC action depends on the characteristic, possessed by most electronic tubes and transistors, of adjustment of gain by the variation of the applied bias voltage. If the dc voltage applied to the control grid of a vacuum tube is made more negative, the amplification of that stage will be reduced. In most broadcast receivers the AGC voltage is taken from the detector. This dc voltage, proportional to the average level of the carrier, adjusts


the gain of the radio-frequency (RF) and intermediate-frequency (I-F) amplifiers and the converter, as shown in the illustration. AGC tends to keep the input signal to the audio frequency (AF) amplifier constant despite variations in RF signal strength. There are several modifications of this basic circuit.

Block diagram of broadcast receiver using AGC. AGC circuits are also used in dictation recording equipment, public address systems, and similar equipment where a constant output level is desirable.


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Audio Power Amplifier With Speaker AGC Load With AGC Circuit_docx