Class -A Power Amplifiers

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Large Signal Amplifier or Power Amplifier Class A Power amplifier. Basics , Efficiency , Transformer Coupled and it's Operation


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Class-A Power Amplifier


Contents Introduction of Power Amplifier Power and Efficiency Amplifier Classification Basic Class A Amplifier Transformer Coupled Class A Amplifier


Introduction Power amplifiers are used to deliver a relatively high amount of power , usually to a low resistance load . Typical load values range from 300W (for transmission antennas) to 8W (for audio speaker). Typical output power rating of a power amplifier will be 1W or higher . Ideal power amplifier will deliver 100% of the power it draws from the supply to load. In practice , this can never occur. The reason for this is the fact that the components in the amplifier will all dissipate some of the power that is being drawn form the supply.

Amplifier Power Dissipation :

Amplifier Power Dissipation The total amount of power being dissipated by the amplifier, P tot , is P tot = P 1 + P 2 + P C + P T + P E The difference between this total value and the total power being drawn from the supply is the power that actually goes to the load – i.e. output power .

Amplifier Efficiency h :

Amplifier Efficiency h A figure of merit for the power amplifier is its efficiency, h . Efficiency ( h ) of an amplifier is defined as the ratio of ac output power (power delivered to load) to dc input power . By formula : As we will see, certain amplifier configurations have much higher efficiency ratings than others. This is primary consideration when deciding which type of power amplifier to use for a specific application.

Amplifier Classifications :

Amplifier Classifications Power amplifiers are classified according to the percent of time that collector current is nonzero . The amount the output signal varies over one cycle of operation for a full cycle of input signal.

Efficiency Ratings :

Efficiency Ratings The maximum theoretical efficiency ratings of class-A, B, and C amplifiers are: Amplifier Maximum Theoretical Efficiency,  max Class A 25% Class B 78.5% Class C 99%

Class A Amplifier:

Class A Amplifier  output waveform  same shape   input waveform +  phase shift. The collector current is nonzero 100% of the time.  inefficient , since even with zero input signal, I CQ is nonzero (i.e. transistor dissipates power in the rest, or quiescent, condition)

Basic Operation:

Basic Operation Common-emitter (voltage-divider) configuration (RC-coupled amplifier)

Typical Characteristic Curves for Class-A Operation :

Typical Characteristic Curves for Class-A Operation

Typical Characteristic:

Typical Characteristic Previous figure shows an example of a sinusoidal input and the resulting collector current at the output. The current, I CQ , is usually set to be in the center of the ac load line.

DC Input Power:

DC Input Power The total dc power, P i (dc) , that an amplifier draws from the power supply : Note that this equation is valid for most amplifier power analyses. We can rewrite for the above equation for the ideal amplifier as

AC Output Power :

AC Output Power AC output (or load) power, P o (ac) Above equations can be used to calculate the maximum possible value of ac load power. HOW?? Disadvantage of using class-A amplifiers is the fact that their efficiency ratings are so low,  max  25% . Why?? A majority of the power that is drawn from the supply by a class-A amplifier is used up by the amplifier itself.



Transformer-Coupled Class-A Amplifier:

Transformer-Coupled Class-A Amplifier A transformer-coupled class-A amplifier uses a transformer to couple the output signal from the amplifier to the load. The relationship between the primary and secondary values of voltage, current and impedance are summarized as: N 1 , N 2 = the number of turns in the primary and secondary V 1 , V 2 = the primary and secondary voltages I 1 , I 2 = the primary and secondary currents Z 1 , Z 2 = the primary and seconadary impedance ( Z 2 = R L )

Transformer-Coupled Class-A Amplifier:

Transformer-Coupled Class-A Amplifier An important characteristic of the transformer is the ability to produce a counter emf , or kick emf. When an inductor experiences a rapid change in supply voltage, it will produce a voltage with a polarity that is opposite to the original voltage polarity. The counter emf is caused by the electromagnetic field that surrounds the inductor.

Counter emf:

Counter emf This counter emf will be present only for an instant. As the field collapses into the inductor the voltage decreases in value until it eventually reaches 0V.

DC Operating Characteristics :

DC Operating Characteristics The dc biasing of a transformer-coupled class-A amplifier is very similar to any other class-A amplifier with one important exception :  the value of V CEQ is designed to be as close as possible to V CC . The dc load line is very close to being a vertical line indicating that V CEQ will be approximately equal to V CC for all the values of I C . The nearly vertical load line of the transformer-coupled amplifier is caused by the extremely low dc resistance of the transformer primary. V CEQ = V CC – I CQ (R C + R E ) The value of R L is ignored in the dc analysis of the transformer-coupled class-A amplifier. The reason for this is the fact that transformer provides dc isolation between the primary and secondary. Since the load resistance is in the secondary of the transformer it dose not affect the dc analysis of the primary circuitry.

AC Operating Characteristics:

AC Operating Characteristics 1. Determine the maximum possible change in V CE Since V CE cannot change by an amount greater than (V CEQ – 0V) , v ce = V CEQ . 2. Determine the corresponding change in I C Find the value of Z 1 for the transformer: Z 1 = (N 1 /N 2 ) 2 Z 2 and i c = v ce / Z 1 3. Plot a line that passes through the Q -point and the value of I C(max) . I C(max) = I CQ + i c 4. Locate the two points where the load line passes through the lies representing the minimum and maximum values of I B . These two points are then used to find the maximum and minimum values of I C and V CE

PowerPoint Presentation:

There are several reasons for the difference between the practical and theoretical efficiency ratings for the amplifier: The derivation of the  = 50% value assumes that V CEQ = V CC . In practice, V CEQ will always be some value that is less the V CC . The transformer is subject to various power losses. Among these losses are couple loss and hysteresis loss. These transformer power losses are not considered in the derivation of the  = 50% value.

PowerPoint Presentation:

One of the advantages of using the transformer-coupled class-A amplifier is the increased efficiency over the RC-coupled class-A circuit. Another advantage is the fact that the transformer-coupled amplifier is easily converted into a type of amplifier that is used extensively in communications :- the tuned amplifier (A tuned amplifier has a circuit that is designed to have a specific value of power gain over a specific range of frequency ). . Advantages of Transformer Coupled Class-A Amplifier

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