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  Types and Classification of Amplifiers, Physics tutorial

Amplifier types:

Of the several amplifier kinds and applications, some of are given below with their properties and characteristics.

Power Amplifier:

Power Amplifiers are frequently designated as last amplifier in the transmission chain that generally is output stage and is amplifier stage which typically needs most attention to power efficiency. This efficiency considerations result in different classes of power amplifiers and power amplification may loosely be associated to amount of power delivered to the load and/or sourced by supply circuitry. Semiconductor amplifiers have to the large extent replaced valve amplifiers in low power applications though; they are little applied in high power applications where valve amplifiers are much more cost effective. Such applications comprise radar, communications equipment and microwave amplifiers that use particularly designed valves, like klystron, gyrotron, travelling wave tube, and crossed-field amplifier that give much greater single-device power output at microwave frequencies than solid-state devices.

Transistor Amplifier:

Transistor amplifiers represent the very rich variety of applications using transistors of aggregations of transistors as may be found on the microchip. Main role of transistor irrespective of it being the Bipolar Junction Transistor of the Field Effect Transistor is to magnify the input signal to yield the considerably larger output signal. Amount of magnification called as forward gain is determined by external circuit design and active device.

Several common active devices in transistor amplifiers are bipolar junction transistors (BJTs) and metal oxide semiconductor field-effect transistors (MOSFETs) in which amplification can be realized utilizing different configurations. With Bipolar Junction Transistors, common base, common collector or common emitter amplifier are realizable while by using the MOSFET you can understand common gate, common source or common drain amplifier; each of which has various characteristic gain, impedance and frequency bandwidth.

Operational Amplifier:

The operational amplifier is characterized by very high open loop gain and differential inputs and uses external feedback for manage of its transfer function.

Operational Amplifiers can be realized using valve as there is general misconception that the Op-Amp implies the solid state Integrated Circuit.

Categorization of amplifiers:

Any amplifier design is always the compromise of several factors, like cost, power consumption, real-world device imperfections, and the multitude of performance specifications that all give many different approaches to categorization of amplifiers.

Common Terminal Classification:

Classifications for amplifiers may be based on which device terminal is common to both input and output circuit. This is suitable as most real world amplifier elements are mainly three terminal circuit elements.

In case of bipolar junction transistors, three classes are common emitter, common base, and common collector, and for field-effect transistors, corresponding configurations are common source, common gate, and common drain

As output voltage of the common plate amplifier is same as input voltage in the arrangement utilized to present the high input impedance so as not to load signal source, common plate amplifier doesn't amplify voltage - but gives the current boost. This configuration of Triode Tube is referred to as cathode follower and by analogy terms emitter follower and source follower are at times utilized for same configuration in BJT and FET transistors circuits.

Output and Input Variable Classification:

Electronic amplifiers utilize two variables: current and voltage and either can be utilized as input or output leading to four kinds of amplifiers. In idealized form they are signified by each of four kinds of dependent source utilized in linear analysis, that is: current amplifier, transresistance amplifier, transconductance amplifier and voltage amplifier. In real world ideal impedances are only estimated. For any specific circuit, the small-signal analysis is frequently utilized to find impedance really attained. A small-signal AC test current Ix is applied to input or output node, all external sources are set to AC zero, and corresponding alternating voltage Vx across test current source finds out impedance seen at that node as R = Vx / Ix.

Unilateral or Bilateral Classification:

By definition the unilateral amplifier has the output which shows no feedback to its input and as such input impedance of the unilateral amplifier is independent of load, and output impedance is independent of signal source impedance. If feedback connects part of output back to input of amplifier it is known as bilateral amplifier. Input impedance of the bilateral amplifier is dependent on load, and output impedance is dependent on signal source impedance.

All amplifiers are bilateral to some degree; though they may frequently be modeled as unilateral under operating conditions where feedback is small adequate to neglect for most purposes, simplifying analysis.

When negative feedback is applied deliberately, it is to modify amplifier behaviour. Few feedback that may be positive or negative, is inevitable and frequently undesirable, introduced, for instance, by parasitic elements like inherent capacitance between input and output of the device like transistor and capacitive coupling because of external wiring.

Power amplifiers:

Power amplifier circuits that are frequently output stages are categorized as A, B, AB and C for analogue designs and applications. Extra classes D and E are for switching designs and are based on conduction angle of input signal through output amplifying device. This is portion of input signal cycle during which amplifying device conducts.

Image of conduction angle is derived from amplifying the sinusoidal signal. Angle of flow is closely associated to amplifier power efficiency. There are numerous other amplifier classes, though they are mostly variations of previous classes.

For instance, class G and class H amplifiers are marked by variation of supply rails either in discrete steps or continuous following input signal and wasted heat on output devices can be decreased as excess voltage is kept to minimum. Amplifiers fed with the rails can be of any class.

Such extra classes of amplifiers are more complex, and are generally utilized for specialized applications, like very high-power units. Also, class E and class F amplifiers are usually described for radio frequencies applications where efficiency of traditional classes are significant. Other classes utilize harmonic tuning of their output networks to get higher efficiency and can be considered the subset of Class C because of their conduction angle characteristics.

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