Large Signal Amplifiers
1.1 Power Amplifiers
All amplifiers are power amplifiers. However those operating in the early stages of the signal processing system deal with small signals. These early stages are designed to give good voltage gain. Since voltage gain is the most important functions of these amplifiers, they are called voltage amplifiers. The set up of a simple audio amplifier large — signal stage and the speaker. The microphone produces a very small signal, in the millivolt range. The small — signal stage amplifiers this audio signal and it becomes larger. The larger signal. It is called a Power Amplifier.
A Power Amplifier is designed for good power gain. It must handle large voltage and current swings. These high voltages and currents, the power is high.
It is very important to have good efficiencies in power amplifier. An efficient power amplifier delivers the signal power for the D.C. power. It takes from the supply since the job of the power amplifier is to change DC power into signal power
1.2 The Preamplifier
A pre-amplifier is used to meet one or move to the following requirements.
1. Impedance Matching:
For optimum performance, different sources of signals have different output impedance. It is not feasible to alter the input impedance of the power amplifier to suit the requirement of the signal source. Generally, a power amplifier has a low to medium input impedance.
2. Signal Amplification:
Raw signal from the source generally lies in the region of 0.15MV to 100MV whereas signal strength required at the input of the power amplifier for rated output is of the order of 350MV to 1V. A preamplifier is used to boost the signal sufficiently to drive the power amplifier.
3. Equalization:
While recording on tapes and discs, certain frequencies are emphasized while others are attenuated. During play back, the amplifier must reverse this deliberately introduced nonlinerity. To achieved this, the amplifier is required to provide different gains at different frequencies. This is termed Equalization.
4. Additional Facilities:
Very often a preamplifier is expected to provide such additional facilities as tone controls and various types to filters to modify the response to make up for deficiencies of the listening area and to cater for the personal taste of the listener.
1.3 The Power Supply
Electronics circuits needs energy to work. In most cases, this energy is provided by a circuit called power supply. A power supply failure will affect all the other circuits. The supply is a key part of any electronics system.
Power supplies use rectifier diodes to convert to direct current. They may also use zener diodes as voltage regulators.
This chapter also covers the features influencing the design of the power supply for this power amplifier circuits.
The preamplifier requires positive and negative po9wer supply. Therefore, the dual power at 3.5A 12vdc and the pre amplifier taking circuit in milli amplifier, the transformer with average D.C. current of 3.5A is chosen to be able to deliver this current tot he load, a rectifier diode IN400 (4 for Bridge rectifier) are used. With 2400 across the transformer's primary 12vac is developed across the secondary.
The impedance that will thus be presented across the power supply by the circuit is: za = VDC IDC = 120 / 3.5 = 3.43 ohms
A full wave rectifier with center; tapped transformer is used in order to achieve a dual polarity power supply for the amplifier unit. The 4 diodes are connected in such a way that during the positive half cycle, D2 and D4 conduct. On the negative alternation, the polarity across t her secondary is reversed. The election leaves the centre tap, flow through D1 and D3. The load current is the same for both alteration. Since the direction never changes the load current is directly current.
Again, the full wave rectifier enables the unit to deliver high power to the amplifier in a given time. It also enables the output wave voltage to be effectively filtered with average sized capacitor.
Smoothing is carried out with capacitors C1 and C2, connected between the position of VCC and ground.
The D.C. output voltage of a filter power supply end to be higher than the output of a non filtered supply. With vims voltage of 12v, after the switch is closed, the capacitor changes to the peak value of the wave form.
VP = 1.414 x vrms
= 1.414 x 12 = 160
This represented a significant change in output voltages. However, as the supply is loaded, the output voltage will drop the required 12V.
Therefore, capacitor C1 and C2 are chosen with values of 330uf / 16v respectively. These capacitors are connected with positive of C1 to VCC and positive off C2 to ground, so as to achieve the dual polarity power supply in addition the secondary function of filtering
Va. C = 0.9 x vrms
= 0.9 x 12 = 10.8 = 11v
Transformer is rated at 240v / 12v gain,
1p = 12 x 3.5 / 240 = 0.175 = 0.200A
The rating of the fuse required is thus
200MA / 240 V.
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