- Bibliographic Information.
- The Enigma of Comparative Law: Variations on a Theme for the Twenty-first Century.
- Coming Out Atheist: How to Do It, How to Help Each Other, and Why.
The first practical version of such devices was the Audion triode , invented in by Lee De Forest ,    which led to the first amplifiers around The amplifying vacuum tube revolutionized electrical technology, creating the new field of electronics , the technology of active electrical devices. For 50 years virtually all consumer electronic devices used vacuum tubes. Early tube amplifiers often had positive feedback regeneration , which could increase gain but also make the amplifier unstable and prone to oscillation.
Much of the mathematical theory of amplifiers was developed at Bell Telephone Laboratories during the s to s.
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The vacuum tube was virtually the only amplifying device, other than specialized power devices such as the magnetic amplifier and amplidyne , for 40 years. Power control circuitry used magnetic amplifiers until the latter half of the twentieth century when power semiconductor devices became more economical, with higher operating speeds. The old Shreeve electroacoustic carbon repeaters were used in adjustable amplifiers in telephone subscriber sets for the hearing impaired until the transistor provided smaller and higher quality amplifiers in the s.
The replacement of bulky electron tubes with transistors during the s and s created another revolution in electronics, making possible a large class of portable electronic devices, such as the transistor radio developed in Today, use of vacuum tubes is limited for some high power applications, such as radio transmitters.
Beginning in the s, more and more transistors were connected on a single chip thereby creating higher scales of integration small-scale, medium-scale, large-scale, etc. Many amplifiers commercially available today are based on integrated circuits. For special purposes, other active elements have been used.
For example, in the early days of the satellite communication , parametric amplifiers were used. The core circuit was a diode whose capacitance was changed by an RF signal created locally. Under certain conditions, this RF signal provided energy that was modulated by the extremely weak satellite signal received at the earth station.
Advances in digital electronics since the late 20th century provided new alternatives to the traditional linear-gain amplifiers by using digital switching to vary the pulse-shape of fixed amplitude signals, resulting in devices such as the Class-D amplifier. In principle, an amplifier is an electrical two-port network that produces a signal at the output port that is a replica of the signal applied to the input port, but increased in magnitude.
The input port can be idealized as either being a voltage input, which takes no current, with the output proportional to the voltage across the port; or a current input, with no voltage across it, in which the output is proportional to the current through the port. The output port can be idealized as being either a dependent voltage source , with zero source resistance and its output voltage dependent on the input; or a dependent current source , with infinite source resistance and the output current dependent on the input.
Combinations of these choices lead to four types of ideal amplifiers. Each type of amplifier in its ideal form has an ideal input and output resistance that is the same as that of the corresponding dependent source: . In real amplifiers the ideal impedances are not possible to achieve, but these ideal elements can be used to construct equivalent circuits of real amplifiers by adding impedances resistance, capacitance and inductance to the input and output.
For any particular circuit, a small-signal analysis is often used to find the actual impedance. Amplifiers designed to attach to a transmission line at input and output, especially RF amplifiers , do not fit into this classification approach. Rather than dealing with voltage or current individually, they ideally couple with an input or output impedance matched to the transmission line impedance, that is, match ratios of voltage to current. Many real RF amplifiers come close to this ideal. Although, for a given appropriate source and load impedance, RF amplifiers can be characterized as amplifying voltage or current, they fundamentally are amplifying power.
Amplifiers are described according to the properties of their inputs, their outputs, and how they relate. The gain may be specified as the ratio of output voltage to input voltage voltage gain , output power to input power power gain , or some combination of current, voltage, and power. In many cases the property of the output that varies is dependent on the same property of the input, making the gain unitless though often expressed in decibels dB. Most amplifiers are designed to be linear.
RF CMOS Power Amplifiers Theory,Design and Implementation.
That is, they provide constant gain for any normal input level and output signal. If an amplifier's gain is not linear, the output signal can become distorted. There are, however, cases where variable gain is useful. Certain signal processing applications use exponential gain amplifiers.
Amplifiers are usually designed to function well in a specific application, for example: radio and television transmitters and receivers , high-fidelity "hi-fi" stereo equipment, microcomputers and other digital equipment, and guitar and other instrument amplifiers. Every amplifier includes at least one active device , such as a vacuum tube or transistor.
Negative feedback is a technique used in most modern amplifiers to improve bandwidth and distortion and control gain. In a negative feedback amplifier part of the output is fed back and added to the input in opposite phase, subtracting from the input. The main effect is to reduce the overall gain of the system. However, any unwanted signals introduced by the amplifier, such as distortion are also fed back.
Since they are not part of the original input, they are added to the input in opposite phase, subtracting them from the input. In this way, negative feedback also reduces nonlinearity, distortion and other errors introduced by the amplifier. Large amounts of negative feedback can reduce errors to the point that the response of the amplifier itself becomes almost irrelevant as long as it has a large gain, and the output performance of the system the "closed loop performance " is defined entirely by the components in the feedback loop.
This technique is particularly used with operational amplifiers op-amps. With negative feedback , distortion can typically be reduced to 0. Noise, even crossover distortion, can be practically eliminated. Negative feedback also compensates for changing temperatures, and degrading or nonlinear components in the gain stage, but any change or nonlinearity in the components in the feedback loop will affect the output. Indeed, the ability of the feedback loop to define the output is used to make active filter circuits. Another advantage of negative feedback is that it extends the bandwidth of the amplifier.
The concept of feedback is used in operational amplifiers to precisely define gain, bandwidth, and other parameters entirely based on the components in the feedback loop. Negative feedback can be applied at each stage of an amplifier to stabilize the operating point of active devices against minor changes in power-supply voltage or device characteristics. Some feedback, positive or negative, is unavoidable and often undesirable—introduced, for example, by parasitic elements , such as inherent capacitance between input and output of devices such as transistors, and capacitive coupling of external wiring.
Excessive frequency-dependent positive feedback can produce parasitic oscillation and turn an amplifier into an oscillator. All amplifiers include some form of active device: this is the device that does the actual amplification. The active device can be a vacuum tube , discrete solid state component, such as a single transistor , or part of an integrated circuit , as in an op-amp. Transistor amplifiers or solid state amplifiers are the most common type of amplifier in use today. A transistor is used as the active element. The gain of the amplifier is determined by the properties of the transistor itself as well as the circuit it is contained within.
Applications are numerous, some common examples are audio amplifiers in a home stereo or public address system , RF high power generation for semiconductor equipment, to RF and microwave applications such as radio transmitters. Transistor-based amplification can be realized using various configurations: for example a bipolar junction transistor can realize common base , common collector or common emitter amplification; a MOSFET can realize common gate , common source or common drain amplification.
Each configuration has different characteristics. Vacuum-tube amplifiers also known as tube amplifiers or valve amplifiers use a vacuum tube as the active device. While semiconductor amplifiers have largely displaced valve amplifiers for low-power applications, valve amplifiers can be much more cost effective in high power applications such as radar, countermeasures equipment, and communications equipment. Many microwave amplifiers are specially designed valve amplifiers, such as the klystron , gyrotron , traveling wave tube , and crossed-field amplifier , and these microwave valves provide much greater single-device power output at microwave frequencies than solid-state devices.
Magnetic amplifiers are devices somewhat similar to a transformer where one winding is used to control the saturation of a magnetic core and hence alter the impedance of the other winding. They have largely fallen out of use due to development in semiconductor amplifiers but are still useful in HVDC control, and in nuclear power control circuitry due to not being affected by radioactivity. Negative resistances can be used as amplifiers, such as the tunnel diode amplifier. A power amplifier is an amplifier designed primarily to increase the power available to a load.
In practice, amplifier power gain depends on the source and load impedances , as well as the inherent voltage and current gain. A radio frequency RF amplifier design typically optimizes impedances for power transfer, while audio and instrumentation amplifier designs normally optimize input and output impedance for least loading and highest signal integrity. In general the power amplifier is the last 'amplifier' or actual circuit in a signal chain the output stage and is the amplifier stage that requires attention to power efficiency.
Efficiency considerations lead to the various classes of power amplifier based on the biasing of the output transistors or tubes: see power amplifier classes below. Audio power amplifiers are typically used to drive loudspeakers. They will often have two output channels and deliver equal power to each.
An RF power amplifier is found in radio transmitter final stages. A Servo motor controller : amplifies a control voltage to adjust the speed of a motor, or the position of a motorized system. An operational amplifier is an amplifier circuit which typically has very high open loop gain and differential inputs. Op amps have become very widely used as standardized "gain blocks" in circuits due to their versatility; their gain, bandwidth and other characteristics can be controlled by feedback through an external circuit.
Though the term today commonly applies to integrated circuits, the original operational amplifier design used valves, and later designs used discrete transistor circuits. A fully differential amplifier is similar to the operational amplifier, but also has differential outputs. These use balanced transmission lines to separate individual single stage amplifiers, the outputs of which are summed by the same transmission line. The transmission line is a balanced type with the input at one end and on one side only of the balanced transmission line and the output at the opposite end is also the opposite side of the balanced transmission line.
The gain of each stage adds linearly to the output rather than multiplies one on the other as in a cascade configuration.
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See the help page for more details. Want to get more out of the basic search box? Read about Search Operators for some powerful new tools. The authors also focus on design guidelines for the inductor s geometrical characteristics for power applications and cover their measurement and characterization. Additionally, a model is proposed which facilitates design in terms of transistor sizing, required inductor quality factors or minimum supply voltage. The model considers the limitations that CMOS processes can impose on implementation. A low-noise amplifier is the first stage of the receiver front-end and it is used to increase the signal power coming from the antenna while introducing less noise by the same LNA.
Figure 6 shows the block diagram of LNA. Matching networks account for performing part of filtering, optimum noise performance, and provides stability at the input as well as output. The matching elements are passive, consisting of strip lines, inductors, capacitors, and resistors. R S and R L represent the source and load impedances, respectively. The cascode structure is popularly used in LNA for narrow-band wireless applications.