ECE, ME – I SEM BASIC ELECTRONICS Amplifiers Its Classification Based on Transistor Configuration A) Common Base Amplifier B) Common Emitter Amplifier c) Common collector Amplifier Based on Active device used BJT Amplifier FET Amplifier On the basis Of Output Voltage Amplifier: It raises the voltage level of the signal. It is designed to achieve the largest possible gain. Only very little power can be drawn from its output. Here the Load current is less to draw the Less Power. Power Amplifier: The power amplifiers are large signal amplifiers which raise the power level of signals. The power amplifier may also be defined as a device which converts dc power to ac power and whose action is controlled by the input signal. In fact, the power amplifier draws power from dc power supply connected to the output circuit and converts it into useful ac signal power. Here the Load current is high. Important Points for Voltage & Power Amplifier The voltage gain of an amplifier is given by the expression AV = ß Rc/Rin . So in order to achieve high voltage amplification (i) transistors having thin base i.e. the transistors with high ß (exceeding 100) are used (ii) input resistance RIN is kept quite low in comparison to RC and (iii) to permit high collector load, which is necessary to give high voltage gain, the voltage amplifiers are always operated at low collector current (of the order of 1mA). A power amplifier is required to deliver a large amount of power and such it has to handle large currents. So in order to achieve high power amplification (i) large sized power transistors are used in order to dissipate the heat produced in transistor during operation (ii) transistors having thick base (i.e. transistors with comparatively smaller ß ) are used in order to handle large currents (iii) transformer coupling is used for impedance matching (iv) collector resistance is made low. On the basis of frequency range of Operation DC Amplifier( 0- 10 Hz) Audio frequency Amplifier( 20 Hz- 20 kHz): It is an electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 hertz to 20,000 hertz, the human range of hearing) to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain. While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens, hundreds, or thousands of watts. Radio frequency Amplifier( few kHz to MHz) An RF amplifier is distinguished by its ability to tune over the desired range of input frequencies. A tuned amplifier that amplifies the high-frequency signals commonly used in radio communications. The frequency at which maximum gain occurs in a radio-frequency (RF) amplifier is made variable by changing either the capacitance or the inductance of the tuned circuit. On the basis of input Small Signal Amplifiers: Small signal amplifiers are generally referred to as "Voltage" amplifiers as they convert a small input voltage into a much larger output voltage Large Signal Amplifiers: Large signal amplifiers are generally referred to as “Power" amplifiers. They deals with large signals. On the basis of number of stages Single Stage Multi Stage further multi stage is divided on the basis of coupling: A) R-C coupled B) Transformer coupled C) Direct Coupled On the basis of operation A) Class A: Class A Amplifier operation is were the entire input signal waveform is faithfully reproduced at the amplifiers output as the transistor is perfectly biased within its active region, thereby never reaching either of its Cut-off or Saturation regions. This then results in the AC input signal being perfectly "centred" between the amplifiers upper and lower signal limits as shown below : Class A Output Waveform B) Class B Amplifier Class B Amplifier uses two complimentary transistors (an NPN and a PNP) for each half of the output waveform. One transistors for the positive half of the waveform and another for the negative half of the waveform. This means that each transistor spends half of its time in the Active region and half its time in the Cut-off region. Class B operation has no DC bias voltage instead the transistor only conducts when the input signal is greater than the base-emitter voltage and for silicon devices is about 0.7v. Therefore, at zero input there is zero output. This then results in only half the input signal being presented at the amplifiers output giving a greater efficiency as shown below. Class B Output Waveform As the output transistors for each half of the waveform, both positive and negative, requires a base-emitter voltage greater than the 0.7v required for the bipolar transistor to start conducting, the lower part of the output waveform which is below this 0.7v window will not be reproduced accurately resulting in a distorted area of the output waveform as one transistor turns "OFF" waiting for the other to turn back "ON". This type of distortion is called Class AB Amplifier The Class AB Amplifier is a compromise between the Class A and the Class B configurations above. While Class AB operation still uses two complementary transistors in its output stage a very small biasing voltage is applied to the Base of the transistor to bias it close to the Cut-off region when no input signal is present. An input signal will cause the transistor to operate as normal in its Active region thereby eliminating any crossover distortion. A small Collector current will flow when there is no input signal but it is much less than that for the Class A amplifier configuration. This means then that the transistor will be "ON" for more than half a cycle of the waveform. This type of amplifier configuration improves both the efficiency and linearity of the amplifier circuit compared to Class A. Class AB Output Waveform Class AB Amplifier (Cont’d) The class of operation for an amplifier is very important as it determines both the efficiency and the amount of power that the amplifier consumes and dissipates in the form of wasted heat, which may also require larger power transistors, more expensive heat sinks, cooling fans, or even an increase in the size of the power supply required to deliver the extra power required by the amplifier. Power converted into heat from transistors, resistors or any other component makes any electronic circuit inefficient and will result in premature failure of the device. So why use a Class A amplifier if its efficiency is less than 40% compared to a Class B amplifier that has a higher efficiency rating of nearly 70%. Basically, a Class A amplifier gives a more linear output meaning that it has, Linearity over a larger frequency response. Class C Amplifier In class C operation, collector current flows for less than one half cycle of the input signal,. Less than 50% of the input signal is used (conduction angle Θ < 180°). The advantage is potentially high efficiency, but a disadvantage is high distortion. The class C operation is achieved by reverse biasing the emitter-base junction, which sets the dc operating point below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow The class C operated amplifier is used as a radio- frequency amplifier in transmitters. Class D Amplifier A Class D amplifier or switching amplifier is an electronic amplifier where all power devices (usually MOSFETs) are operated as binary switches. They are either fully on or fully off. Ideally, zero time is spent transitioning between those two states. Theoretical power efficiency of class D amplifiers is 100%. That is all of the power supplied to it is delivered to the load, none is turned to heat. This is because a switch in its on state will conduct all current but has no voltage across it, hence no heat is dissipated. And when it is off, it will have the full supply voltage standing across it, but no current flows through it. Again, no heat is dissipated. Real life power MOSFETs are not ideal switches, but practical efficiencies well over 90% are common On the basis Of Bandwidth A) Narrow Band Amplifier( RF or Tuned Amplifier) An amplifier which increases the magnitude of signals over a band of frequencies whose bandwidth is small compared to the average frequency of the band A) Wide band Amplifier (Video Amplifier) These deal with video signals and have varying bandwidths depending on whether the video signal is for SDTV, EDTV, HDTV 720p or 1080i/p etc..