Electronic Devices & Circuits ITM UNIVERSE, VADODARA Electronic Devices & Circuits TOPIC NAME POWER AMPLIFIERS PREPARED BY : TIRTH PANDYA ENROLLMENT NO : 140950111006

What is amplifier? An amplifier is an electronic device that increases the voltage, current, or power of a signal. Amplifiers are used in wireless communication and broadcasting and in audio equipments of all kinds. They can be categorized as either weak signal amplifiers or power amplifiers.

Power Amplifier A power amplifier is an electronic device that receives an electrical signal and reprocesses it to amplify, or increase, its power. The boost in power is achieved by significantly increasing the input signal’s voltage. A power amplifier is used to power an output source, such as a stereo speaker, a relay or a motor.

Classification of Power Amplifiers Class A Amplifiers Class B Amplifiers Class AB Amplifiers Class C Amplifiers

Class A Amplifiers The VDB amplifier of the fig is a class A amplifier as long as the output signal is not clipped. With this kind of amplifier, collector current flows throughout the cycle.

The most commonly used type of power amplifier configuration is the Class A Amplifier. It uses the switching transistor in the standard common emitter circuit configuration. The transistor is always biased “ON” so that it conducts during one complete cycle of the input signal waveform producing minimum distortion and maximum amplitude to the output. Class A Amplifier configuration is the ideal operating mode, because there can be no crossover or switch-off distortion to the output waveform even during the negative half of the cycle. Its output stages may use a single power transistor or pairs of transistors connected together to share the high load current.

Class B Amplifiers Class B operation means that the collector current flows for only 180 degree of the ac cycle. For this to occur, the Q point is located at cutoff on both the dc and the ac load lines.

To improve the full power efficiency of Class A amplifier by reducing the wasted power in the form of heat, it is possible to design the power amplifier circuit with two transistors in its output stage producing what is commonly termed as a Class B Amplifier, also known as a push-pull amplifier configuration. The conduction angle for this type of amplifier circuit is only 180o or 50% of the input signal. This pushing and pulling effect of the alternating half cycles by the transistors.

Class AB Amplifier

The class AB push-pull output circuit is slightly less efficient than class B because it uses a small quiescent current flowing, to bias the transistors just above cut off, but the crossover distortion created by the non-linear section of the transistor’s input characteristic curve, near to cut off in class B is overcome. In class AB each of the push-pull transistors is conducting for slightly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A. As each cycle of the waveform crosses zero volts, both transistors are conducting momentarily and the bend in the characteristic of each one cancels out.

Class C Amplifier Class-C amplifiers conduct less than 50% of the input signal and the distortion at the output is high, but high efficiencies (up to 90%) are possible.

The Class C Amplifier design has the greatest efficiency but the poorest linearity of the classes of amplifier. The  A, B and AB are considered linear amplifiers, as the output signals amplitude and phase are linearly related to the input signals amplitude and phase. However, the class C amplifier is heavily biased so that the output current is zero for more than one half of an input sinusoidal signal cycle with the transistor idling at its cut-off point. In other words, the conduction angle for the transistor is significantly less than 180 degrees, and is generally around the 90 degrees area. While this form of transistor biasing gives a much improved efficiency of around 80% to the amplifier, it introduces a very heavy distortion of the output signal. Therefore, class C amplifiers are not suitable for use as audio amplifiers.

Push-Pull Amplifier When a transistor operates as class B, it clips off half a cycle. Push-pull means that one transistor conducts when the other is off, & vice versa.

On the positive half cycle of input voltage, the secondary winding of T1 has voltage V1 and V2 as shown in the fig. Therefore the upper transistor conducts and the lower one cuts off. The collector current through Q1 flows through the upper half of the output primary winding. This produces an amplified and inverted voltage, which is transformer coupled to the loudspeaker. On the next half cycle, the polarities reverse. The lower transistor turns on and the upper transistor turns off. The lower transistor amplifies the signal, and the alternate half cycle appears across the loudspeaker. Since each transistor amplifies one-half of the input cycle, the loudspeaker receives a complete cycle of the amplified signal.

Class B as Push-Pull Amplifier Advantages - Possible to obtain greater power output - efficiency is higher - Negligible power loss at no signal Disadvantages Harmonic distortion in higher Supply voltage must have good regulation

Cross Over Distortion The figure shows the ac equivalent circuit of a class B push-pull emitter follower.

Suppose that no bias is applied to the emitter diodes Suppose that no bias is applied to the emitter diodes. Then, the incoming ac voltage has to rise to about 0.7 V to overcome barrier potential of the emitter diodes. Because of this, no current flows through Q1 when signal is less than 0.7 V. No current flows through Q2 until the ac input voltage is more negative than -0.7 V. For this reason, if no bias is applied to the emitter diodes, the output looks like the figure shown below.

Because of clipping between half cycles, the output is distorted Because of clipping between half cycles, the output is distorted. Since the clipping occurs between the time, one transistor cuts off and the other one comes on, it is called cross over distortion. To eliminate cross over distortion, we need to apply a slide forward bias to each emitter diode. This means locating the Q point slightly above cut -off.

Parameters Class A Class B Class C Class AB Collector current waveform Conduction angle of collector current 360⁰ or full cycle 180⁰ or half cycle Less than 180⁰ Between 180⁰ & 360⁰ Position of Q point on the load line At the centre On the axis Below the axis Above the X axis but below the mid-point

Parameters Class A Class B Class C Class AB Distortion in output voltage No distortion More than class A (Cross over) More than A, B and AB Low Efficiency Lowest 25% to 50% Higher 78.5% Very high 95% Between 50% to 78.5% Power dissipation in transistor Very low Moderate

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