Low and High Frequency Power Amplifiers, Physics tutorial

Amplifier specifications:

The quality of amplifier can be characterized by specifications. Some of the significant specifications that usually detail quality of amplifiers are given below:

Gain:

Gain of the amplifier is ratio of output to input power or output to input amplitude, and is frequently estimated in decibels. When it is estimated in decibels it is logarithmically associated to power ratio:

G(dB) = 10log(Pout/(Pin))

While Radio frequency amplifiers are frequently specified in terms of maximum power gain available, voltage gain of audio amplifiers and instrumentation amplifiers is frequently specified as amplifier's input impedance will be much higher than source impedance, and load impedance higher than amplifier's output impedance.

Bandwidth

Bandwidth of amplifier is range of frequencies for which amplifier gives satisfactory performance and usually accepted metric is half power points. This is frequency where power is decreased to half of its peak value on output versus frequency curve. Bandwidth can be stated as difference between lower and upper half power points. This is thus also called as -3 dB bandwidth or frequency response.

Efficiency:

This is a measure of amount of power from power source that is effectively applied to load at amplifier's output. Class A amplifiers are very ineffective, being typically in range of 10-20% with max efficiency of 25% for direct coupling of output. Inductive coupling of output can increase efficiency of Class A amplifiers to maximum of 50%.

Class B amplifiers have very high efficiency but are unfeasible for linear applications due to high levels of distortion and in practical design, result of the trade-off is class AB design. Modern Class AB amplifiers usually have peak efficiencies between 30-55% in high fidelity applications and 50-70% in radio frequency systems with theoretical maximum of 78.5%.

More efficient amplifiers run cooler, and frequently don't need any cooling fans even in multi-kilowatt designs. Reason for this is that loss of efficiency generates heat as the by-product of energy lost during conversion of power. In more proficient amplifiers there is less loss of energy so in turn less heat.

Linearity:

The ideal amplifier would be the completely linear device but real amplifiers are only linear within limits. When signal drive to amplifier is increased, output also increases until the point is achieved where some part of amplifier turns in saturated and can't produce any more output; this is known as clipping that result in distortion.

In most amplifiers gain reduction occurs before hard clipping takes place that results in compression and for amplifiers, 1 dB compression point is stated as the input power or output the power where the gain is 1 dB less than the small signal gain. Negative feedback is utilized to decrease nonlinearity.

Noise:

Noise is the undesirable but predictable product of electronic devices and components; and much noise results from intentional economies of manufacture and design time. Metric for noise performance of the circuit is noise figure or noise factor this is a comparison between output signal to noise ratio and the thermal noise of the input signal.

Output Dynamic Range:

This is the range between smallest and largest valuable output levels and is generally estimated in dB. Lowest valuable level is restricted by output noise, whereas the largest is restricted most frequently by distortion. Ratio of the two is referred to as amplifier dynamic range.

Slew Rate:

Slew rate is maximum rate of change of output and is generally expressed in volts per second or microsecond. Most amplifiers are slew rate restricted because of circuit capacitive and miller effect that limit full power bandwidth to frequencies well below amplifier's small-signal frequency response.

Rise Time:

This is the time taken by the amplifier for its output to change from 10% to 90% of its final level when driven by the step input which for simple roll-off circuits like RC circuits may be estimated by:

tr * BW = 0.35, where tr is rise time in seconds and BW is bandwidth in Hz.

Setting Time:

Time taken for output to settle to inside certain percentage of final value is known as settling time, and is generally specified for oscilloscope vertical amplifiers and high accuracy measurement systems.

Ringing:

Ringing refers to the output variation which cycle above and below the amplifier's final value and leads to the delay in reaching stable output. Ringing is result of overshoot caused by under damped circuit.

Overshoot:

Overshoot is the amount by which output exceeds its final, steady-state value in response to the step input.

Stability:

Stability is the major concern in Radio Frequency and Microwave amplifiers. Degree of the amplifier's stability can be quantified by stability factor that specifies a condition which should be met for absolute stability of amplifier in terms of two-port parameters.

Power electronic amplifiers:

Modern electronics as it is known today wouldn't have been possible without active devices like vacuum tubes, bipolar junction transistors and field effect transistors. As active devices have regularly are amplifiers, it goes to say that modern electronics would not have been possible without electronic amplifiers.

Major characteristics of the amplifier are Linearity, efficiency, output power, and signal gain and in general, there is tradeoff between the characteristics. Improving amplifier linearity will degrade its efficiency. It is thus significant to know impact of each one of the characteristics as the significant step towards designing the Amplifier. Generally amplifier design is based on application as for example, design the high output power non linear Class C Amplifier for use in transmitter of the transceiver while you design the highly linear class A amplifier to be utilized in receiver side.

Electronic amplifiers act as basis for numerous technologies that have been spawned in recent times. Specific reference is made to Information Technology and Communications and their numerous applications through computers, receivers, transmitters, digital equipment, and instrumentation that all uses amplifiers through active devices.

High frequency power amplifier:

The radio frequency power amplifier as the member of high frequency power amplifiers; that can be described as the electronic amplifier utilized to convert low-power radio-frequency signal in larger signal of significant power that is typically for driving antenna of transmitter.

Radio Frequency:

Power Amplifiers are optimized to get high efficiency and deliver high output Power with minimal loss on input and output while giving impressive gain with optimized heat dissipation.

Applications of RF power amplifier comprise driving to another high power source, driving the transmitting antenna, microwave heating, and exciting resonant cavity structures. Among such applications, driving transmitter antennas is most well known. Transmitter-receivers are utilized not only for voice and data communication but also for weather sensing as in RADAR sensing.

Microwave or Radio Frequency heating is the industrial application that is also visible in homes in form of microwave ovens while Subatomic Particles Accelerators use Radio Frequency sources extensively.

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