Basics of power amplifier pdf

After pre-amplification, power amplifier is used to increase the output power by amplifying the current as well as the voltage of the input signal. Audio Amplifier Characteristics. There are many design factors involved in the making of any amplifier circuit like gain, bandwidth, output power and maximum supply voltage. An audio amplifier should be designed considering all these important design factors.

Some of the important design parameters involved in the making of an audio amplifier are as follow —. The gain of an amplifier circuit is expressed as the ratio of the output voltage to the input voltage voltage gain , or ratio of the output current to the input current current gain or ratio of the output power to the input power power gain.

It is expressed in dB decibel. The equation for converting the voltage gain to the gain in dB is as follow —. Where Gv is the voltage gain. In the analysis of the amplifier circuits designed in this series, voltage gain will be taken as the design factor. The voltage gain is expressed as the ratio of the output voltage to the input voltage. Volume and Skew Rate — The volume of an audio is determined by the output amplitude of the audio signal.

As gain of the circuit decides the maximum and minimum amplitude, the volume can be changed only in the range of that amplitude only. By using a potentiometer, the amplitude of the signal can be changed and so the loudness or volume of the audio signal. An amplifier cannot suddenly change the volume of the audio signal. The maximum rate of change of the output signal is called Skew rate of the amplifier. Output Power — The power output of an audio amplifier is equivalent to how loud can be sound output from it.

It is generally expressed in Watts or Milli Watts. The larger will be the speakers, more output power from the amplifier will be required for them. The maximum output power of an amplifier circuit can be calculated as follow —. More will be the linearity, more will be the output audio true representation of the input audio signal. The amplifier circuits in this series are designed to operate within the frequency range from 20 Hz to 20 KHz. So, ideally the output voltage level of this amplifier will not exceed than 50V about the waveform origin.

As voltage level and the power output are related, this also indicates the range of output power that can be delivered by the amplifier. If an attempt is made by the load or speakers at the output of the amplifier to draw more power or voltage levels than the power or voltage levels for which the amplifier has been designed, then the output waveform carrying the audio signals will start to clip.

By clipping, it means that the output voltage level becomes constant and equal to the maximum voltage level it can output for all voltage levels that are beyond the maximum output limit of the amplifier. As the output voltage signal is the audio signal itself, the clipping will cause distortion in the output sound. If the clipping will be severe, the output waveform can become a square wave instead of a sine wave, causing the loss of the audio signal or only noise remaining at the output of the amplifier.

Secondly, the power rating of the square wave is twice of sine wave. The power supply of most of the amplifiers cannot handle to output power twice their rated power for longer period. Also, at the output the voltage will be practically more than the rated voltage which will cause problem at the load that is the speakers. The speakers are designed to have constant impedance. The impedance of the speakers is expressed in ohms and it is typically 2, 4 or 8 ohms.

A low impedance speaker draws more power compared to a high impedance speaker. On clipping a high impedance or low power speaker can practically get damaged. When output signal from an amplifier is clipped, it acts like constant power source or fixed DC input to the speaker inputs. The speakers have an internal coil. On getting a constant input this coil will not get a chance to cool down due to the clipping of soft passage in the audio signal.

It is uncommon to blow the tweeters in the case of extreme clipping. The tweeter is a kind of speaker dome or horn shaped which are meant for generating the high audio frequency in range from 2 kHz to 20 kHz. The risk of damage to a speaker depends on audio signal does it have a large amount of high frequency , the degree of clipping and how sustainable speaker is beyond its rated power.

So, a speaker of high wattage and low impedance can be used with low wattage amplifier but its vice — versa is not true. Suppose if a speaker is rated twice the amplifier output power then there will be no issues with the speaker in case of clipping. However, still the clipping will add distortion to the output audio and sound quality can be horribly reduced that no one can hang around. So it is better to run an amplifier for an occasional clipping only as clipping increases the chances of damage to the speaker, can over stress amplifier or deteriorate the sound quality.

The clipping effect can be observed on a CRO. In the following figure, red waveform represents the supposed audio output and yellow waveform represents the clipped waveform on using an audio amplifier of lower power rating. For increasing the stability of the circuit, negative feedback is used in the design of the amplifier circuits. There are various advantage of negative feedback like Gain stability, Noise reduction, Increase in input impedance, Decrease in output impedance and Increase in bandwidth.

For providing negative feedback, the amplifier circuits in this series are designed in inverting configuration. So, by increasing the output impedance, the power loss of the amplifier can be greatly reduced. This noise is created by the semiconductor components used in the design of the amplifier. Higher is the power output of an amplifier, more is the noise at its output.

An amplifier needs to be designed so that noise at its output remains constant irrespective of the signal. Also, signal to noise ratio should remain high for the entire operating range of the amplifier. If an audio waveform of constant frequency is applied at the input of an amplifier, it is supposed to remain the same at the output of the amplifier as well.

But, frequencies of the integer multiple of the input frequency are added at the output of the amplifier. These frequencies are called Harmonic Distortions and are always the integral multiple of the input frequency. The Total Harmonic Distortion is the ratio of the power of all the harmonic frequencies combined to the power of the original frequency. The harmonic distortions in an amplifier must be occasional and THD for those occasional appearances too should be in tolerable limit.

The THD is usually expressed in percentage. Like if an amplifier has 2 percent THD, it means that the power of all the harmonics combined is just 2 percent of the power of the original frequency. Generally, THD up to 10 percent is tolerable but it should be as low as possible. Standard Audio amplifiers have a THD as less as 1 percent or 0.

The different components of the amplifier are connected at different nodes of the ground. Ideally, the ground should be 0 Volts but due to resistive nature of the ground wire, it has different voltages across its length. The difference in voltage at different nodes of the ground add noise in the output audio signal. For eliminating the problem of ground loops, star topology will be used for ground and the power supply of the amplifier circuit.

Construction of Audio Amplifiers. An amplifier circuit can be designed using transistors or operational amplifiers. Testing Audio Amplifier Circuits. For the testing of the amplifier circuits designed in this series, the function generator is used as the input source.

Questions related to this article? Connect with Engineers Garage. The input capacitor C in allows AC signal, but isolates the signal source from R 2. If this capacitor is not present, the input signal gets directly applied, which changes the bias at R 2. This capacitor is present at the end of one stage and connects it to the other stage. As it couples two stages it is called as coupling capacitor. This capacitor blocks DC of one stage to enter the other but allows AC to pass.

Hence it is also called as blocking capacitor. If this is not present, the bias conditions of the next stage will be drastically changed due to the shunting effect of R C , as it would come in parallel to R 2 of the next stage. This capacitor is employed in parallel to the emitter resistor R E. The amplified AC signal is by passed through this.

If this is not present, that signal will pass through R E which produces a voltage drop across R E that will feedback the input signal reducing the output voltage.

The resistance R L connected at the output is known as Load resistor. When a number of stages are used, then R L represents the input resistance of the next stage.

Let us go through various circuit currents in the complete amplifier circuit. These are already mentioned in the above figure. When no signal is applied in the base circuit, DC base current I B flows due to biasing circuit. When AC signal is applied, AC base current i b also flows.

Therefore, with the application of signal, total base current i B is given by. When no signal is applied, a DC collector current I C flows due to biasing circuit. When AC signal is applied, AC collector current i c also flows. Therefore, the total collector current i C is given by. When no signal is applied, a DC emitter current I E flows. With the application of signal, total emitter current i E is given by. These are the important considerations for the practical circuit of transistor amplifier.

Now let us know about the classification of Amplifiers.