Fiber optic amplifier

Optical fiber transmission of information has played a key role in increasing the capability of communication system to transmit information. Fiber optic communication utilizes optical transmitters, optical receivers and optical fiber, among other components, to transmit light signals through the fiber. A fiber optic amplifier is an optical device for amplifying a plurality of channels of signals so as to compensate for their loss when they propagate through an optical transmission line in an optical communication system. In general, a fiber optic amplifier comprises an optical fiber for amplification, doped with a rare-earth element, and a pumping light supply system for supplying pumping light to the optical fiber for amplification. The pumping light supply system usually includes a semiconductor laser and an optical coupler for guiding the pumping light into the optical fiber for amplification. In fiber-optics communication systems in practice today, repeaters are inserted in the transmission line at regular intervals to compensate for attenuation of the optical signal due to loss in the optical fiber. In a repeater, an optical signal is converted into an electrical signal by a photodiode and amplified by an electronic amplifier, and then converted into an optical signal to be delivered into the fiber-optic transmission line again. Erbium-doped amplifiers are made by doping a segment of the fiber with erbium and then exciting the erbium atoms to a high energy level through the introduction of pumping light. The energy is transferred gradually to signal light passing through the fiber segment during excitation, resulting in an amplification of the signal light upon exit from the amplifier. Fiber optic amplifiers can amplify signal light including one or more wavelengths within a predetermined wavelength band without converting them into electricity.

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Operational amplifier

Operational amplifiers (op-amps) are high-gain DC coupled amplifiers with two inputs and a single output, and have been used as comparators, audio amplifiers, filters, etc. An operational amplifier is characterized by having two inputs, that is, an inverting or negative input and a non-inverting or positive input. The operational amplifier includes an output, that is, a single-ended amplifier, or two outputs, that is, a double-ended amplifier which is also known as a fully differential amplifier. Operational amplifiers are used in many electronic circuits to condition, manipulate and amplify signals. An typical operational amplifier amplifies a voltage difference on the inputs to generate a desired output voltage. The operating characteristics of a particular operational amplifier are dependent upon its circuit topology. Generally, the operational amplifier consists of a number of stages, each containing internal sub-stages. The operating class of the operational amplifiers is defined according to the polarization of the active elements that supply power to the load and can be divided, among the various ones existing, into class A, in which the active elements always operate in a conduction zone and are polarized at about the center of it, class B, in which the active elements are polarized at the locking limit of the conduction zone, class AB, in which the active elements are weakly polarized within the conduction zone, and in class C, in which the active elements operate far from the conduction zone. Single stage differential amplifier circuits are used in many electronic applications, such as programmable logic arrays. For programmable logic arrays differential amplifier circuits are designed to vary the common-mode gain and common-mode rejection ratio utilizing more than one amplifier stage and/or with additional complex electronic circuitry. Two-stage operational amplifiers typically include a first gain stage connected to inputs of the amplifier and a second gain stage driven by the first gain stage. The second gain stage provides the output of the amplifier. Both the first gain stage and the second gain stage are operated at respective bias currents. In metal oxide semiconductor (MOS) amplifiers, the first gain stage is typically operated at bias currents which are comparable in magnitude to the bias currents of the second gain stage so that maximum gain and bandwidth may be achieved. Multiple-stage operational amplifiers typically include a cascade of one or more gain stages and an output driver stage for driving an output load. The output stage is, for example, a Class AB amplifier that provides high low-frequency gain. To achieve an overall high open loop gain (e.g. greater than 150 dB), a multiple-stage opamp normally requires three or more gain stages.

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Variable gain amplifier

A variable gain amplifier (VGA) is a device having a control input that can vary the gain of the device. In the wireless communication industry, particularly for wireless communications, variable gain amplifiers are well known as being used to provide amplification of either intermediate frequency (IF) or radio frequency (RF) signals. Variable gain amplifiers are frequently used in modern radio receivers to amplify or attenuate incoming signals to properly drive an associated analog-to-digital converter (A/D). Typically, the variable gain is distributed among radio frequency (RF), intermediate frequency (IF), and low-frequency or baseband circuits. Radio receivers, or tuners, are widely used in applications requiring the reception of electromagnetic energy. Applications can include broadcast receivers such as radio and television, set top boxes for cable television, receivers in local area networks, test and measurement equipment, radar receivers, air traffic control receivers, and microwave communication links among others. Transmission of the electromagnetic energy may be wirelined over a communication media or wireless by electromagnetic radio waves. In a radio frequency (RF) transceiver, the received signal typically has a high dynamic range. In order to supply a signal of constant amplitude to a baseband section of the transceiver, a variable gain amplifier (VGA) with equivalent or better dynamic range is required. In a variable gain amplifier, a control unit will provide a gain signal to the variable gain amplifier, and, based upon the gain signal, the variable gain amplifier will accordingly amplify an input signal by an amount corresponding to the gain signal, to obtain an amplifier output signal. In order for a signal of a constant level to be supplied to a base band terminal of the received signal, the variable gain amplifier must also have a high dynamic range. Variable gain amplifiers may be based on voltage, current or charge. Voltage mode amplifiers are probably the most widely used. Examples of such include complex circuits where the amplification is provided by discrete transconductance stages. Charge mode amplifiers are one alternative. However, such a circuit utilizes a discrete time technique that is not suitable for high-speed operation. In contrast, current mode amplifiers are less constrained by reduced power supplies and are able to operate at very high speeds.

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Preamplifier and Servo amplifier

Preamplifier:

A preamplifier is an electronic component that is connected to a low-level signal source for providing suitable impedances and gain in an amplified signal. A singled ended preamplifier amplifies a single ended input signal by a gain factor such that the output signal is equal to the input signal multiplied by the gain factor. Differential preamplifiers are a particular type of preamplifier wherein the differential input signal comprises a positive rail component and a negative rail component. Preamplifier circuits are used in numerous applications. Typically, sound engineers use preamplifiers to amplify and process sound signals to achieve volume boosting while at the same time manipulating certain frequencies. High performance microphones require immediate preamplification of the signal generated by the microphone capsule. Microphones have low-level outputs, but microphone preamplifiers are generally designed to eliminate hum pick-up through the use of balanced-input circuitry, which electronically cancel any noise that is induced on both leads coming from the microphone. The ability of the output signal of a preamplifier circuit to faithfully reproduce the input signal is a function of many factors, including the bandwidth of the preamplifier, the frequency of the input signal, the impedance of the input system and transmission line impedance that provides the input signal, and the input impedance of the preamplifier.

Servo amplifier:

Servo amplifiers are used to drive servo motors in positioning devices. Multiple, servo-controlled DC motors which are reversible to provide a driving force in either of two rotational directions, are employed in various applications, such as magnetic disk device, for example. The motors are controlled by amplifier circuits which are pulse width modulated to provide precise motor control. In a magnetic disk device, the magnetic head is moved in two seek modes, a forward seek mode and a reverse seek mode, each corresponding to the direction of the motion of the magnetic head, i.e., corresponding to which side of the target track position the magnetic head exists when the target track position is commanded. In a servo system for a magnetic head in a magnetic disk device, a servo amplifier circuit having an amplifier portion and an inverting portion which inverts the polarity of the output of the amplifier, is used. Among the numerous general kinds of amplifiers, a common type is a straight DC amplifier with suitable phase-lead circuits to provide system damping, and output stages with the power capability to drive the motor in either direction. Pulse width modulated servo amplifiers are widely used in control systems for aircraft, marine, industrial and computer applications. Such amplifiers are protected against damage from accidental overload only by fuses or circuit breakers which must be replaced or reset before operation can be resumed after a malfunction. Many servo amplifiers are provided with resistors which generate large amounts of heat such as regenerative resistors which function to consume regenerative current. When installing a resistor which generates large amount of heat in a servo amplifier, the resistor is mounted at location separated from the internal components of the amplifier as well as at the rear of the amplifier in order to eliminate the effects of heat on the internal components of the amplifier.

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Power amplifier and low noise amplifier

Power amplifier:

Amplifiers produce from an input signal, an output signal having an increased magnitude (i.e., gain). Essentially, an amplifier produces a constant output power at a higher level. Radio frequency (RF) power amplifiers are commonly used in numerous applications, such as base stations used in wireless communication systems. Modern wireless communication base stations transmit and receive radio frequency (RF) signals through the use of RF power amplifiers. RF power amplifiers are generally designed to provide maximum efficiency at the maximal output power. A typical radio transmitter uses a radio frequency (RF) power amplifier to amplify outbound signals for transmission by an antenna. A radio frequency power amplifier is typically constructed using a printed circuit board, with various components of the radio frequency power amplifier circuit installed on the printed circuit board. The RF amplifier circuit typically includes an input, an active element, a bias circuit element, an output matching network, and an output. RF power amplifiers characterized by a plurality of operating performance characteristics responsive to a quiescent operating point established by a direct current (DC) bias current. The linear power amplifier is driven by a direct current (DC) input voltage, provided for example by a battery in the transmitter, and the efficiency of the power amplifier is given by the ratio of the output power to the DC input power.

Low noise amplifier:

A low noise amplifier (LNA) is utilized in various aspects of wireless communications, including wireless LANs, cellular communications, and satellite communications. A typical receiver for a radio frequency signal (RF signal) comprises a combination of an amplifier and a mixer for signal amplification and frequency conversion. The amplifier, usually a low-noise amplifier (LNA), receives the RF signal, amplifies the RF signal and feeds the amplified RF signal to the mixer which in addition receives a local signal from a local oscillator (LO). A critical building block in a radio receiver is the low noise amplifier (LNA). The LNA amplifies the received signal and boosts its power above the noise level produced by subsequent circuits. An LNA provides a steady gain over a specified frequency bandwidth. One common application is the use of a LNA as the input stage of a receiving circuit, such as in a mobile communication device. In a radio frequency (RF) signal receiving apparatus such as a cellular phone and a base station of a wireless communication system, a received signal has very weak intensity and includes considerable noise mixed therein. As such, the performance of the LNA greatly affects the sensitivity of the radio receiver. The low noise amplifier is capable of decreasing most of the incoming noise and amplifying a desired signal within a certain frequency range to increase the signal to noise ratio (SNR) of the communication system and improve the quality of received signal as well. Depending on signal frequency, an LNA can be implemented as an open loop or closed-loop amplifier and may also have a requirement to match a specific source impedance.

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power divider and filter

Power divider:

A large class of microwave components can be formed by combining two phase shifters and two fixed power dividers (combiners). A power divider and a power combiner may be made to operate over broad frequency bands at relatively high RF power levels. In the field of RF transmission, power requirements may exceed practical levels for a single amplifier, making it desirable to amplify a signal with multiple RF power amplifiers in parallel. An input power divider circuit is used to divide the signal and provide it to the inputs of the multiple amplifiers, while an output combiner provides the combined outputs of the multiple amplifiers to the next stage load. Many applications involve circuits requiring two input power signals to be combined into one output signal to be operated on by following circuits. Radio frequency power combiners are used for combining a number of RF inputs into a single output in a transmission system. The combiner also prevent a high current flow in a short circuited failed radio frequency power source, this current being derived from the remaining operative radio frequency power sources. Power combining techniques for radio frequency signals, including millimeter wavelength signals, have been accomplished in either a waveguide circuit or in a microstrip circuit.

Filter:

A radio frequency (RF) filter is designed to be tuned to pass energy at a specified resonant frequency. In microwave communications systems, it is often necessary to filter a relatively broadband microwave signal into its component sub-bands. In order to combine a number of RF transmitters, the RF signals from each transmitter must be isolated from one another to prevent intermodulation and possible damage to the transmitters. RF filters of the air-filled cavity type may be utilized to provide isolation between the RF transmitters. High-frequency filters, such as RF filters, are used to implement high-frequency circuits in the base stations of mobile networks, mobile phones and other radio transceivers. Other RF filter applications include the adapter circuits and filter circuits of transmitter and receiver amplifiers. RF cavity filters may be used in linear power amplifiers and radio equipment such as cellular base stations, among other things, to, for example, reduce undesired frequencies in an RF signal, or to delay an RF signal by a predetermined amount of time. Radio frequency (RF) filters and multiplexers having filters typically employ a plurality of resonators.

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RF Microwave oscillator and microwave connector

MIcrowave oscillator:

Oscillator circuits generate periodic electrical signals by converting a fraction of the constant polarity power supply thereto into a periodic signal output without requiring a period signal input. Radio frequency (RF) oscillators are widely used for generating, tracking, cleaning, amplifying, and distributing RF carriers. Microwave oscillators are employed in wireless telecommunication equipment, such as for instance radio links or satellite transponders, as local oscillators for frequency converters. In particular, voltage-controlled oscillators with phase-locked loops are used for clock recovery, carrier recovery, signal modulation and demodulation, and frequency synthesizing. A microwave VCO is a microwave oscillator whose oscillation frequency is controllable by means of a voltage. RF oscillators and modulators typically must meet certain requirements in power and frequency output.

Microwave connector:

Electrical and electronic systems make use of connectors for coupling different portions of the system together. Such connectors are capable of being engaged and disengaged so that the different parts of the system may be separated. Radio frequency (RF) connectors are generally used to connect various components of RF equipment. Such RF connectors interconnect various components including coaxial cable and printed circuit boards. Connectors associated with RF communication systems typically use coaxial transmission line systems to conduct RF signals from one point to another. Coaxial radio frequency cables having hollow center conductors are generally used for many applications including land mobile, microwave broadcast and radar band frequencies. These coaxial transmission line systems employ connectors at their ends to connect the transmission line system to additional coaxial transmission line systems or various RF circuit assemblies. The technological advancement has been calling for broader bandwidths for the radio frequency and microwave equipment. As a result, the RF coaxial connectors, either on the coaxial cable ends or on the PC boards of signal devices, play a more and more important role in signal input and output. The RF connector has an inner conductor and tube-shaped outer conductor, which connect to the respective conductors of the cable. Radio frequency connectors typically comprise a solid, straight center pin, an extruded dielectric material, and a matable housing.

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Dielectric resonator

Dielectric resonators are key elements for filters, low phase noise oscillators and frequency standards in current microwave communication technology. Applications of dielectric resonators in filter design have become more and more popular due to impressive advantages, such as small size, low weight, low loss (high Q), and common commercial availability. Dielectric resonators are used in microwave circuits for concentrating electric fields. They can be used to form filters, oscillators, triplexers and other circuits. The higher the dielectric constant of the dielectric material out of which the resonator is formed, the smaller the space within which the electric fields are concentrated. Dielectric resonators possess resonator quality factors (Q) comparable to cavity resonators, strong linearity at high power levels, weak temperature coefficients, high mechanical stability and small size. Microwave oscillators are used in transmission systems and more particularly close to the antenna in order to carry out a frequency transposition between an intermediate frequency band and a transmission frequency band. Dielectric resonator oscillators are commonly used in high-precision RF and microwave systems to generate high-frequency signals of extremely good spectral purity. A typical dielectric resonator for use in the microwave band is formed using a rectangular or cylindrical dielectric block having a coaxial through-hole wherein an inner conductor is formed on the inner surface of the through-hole and an outer conductor is formed on the outer surface of the dielectric block.

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RF circulator

A radio frequency (RF) or microwave circulator is used to pass RF signals and block returned signals. The circulator is a three-terminal device which passes signals input to one port to the next port in a rotational fashion without allowing signals to pass in the opposite rotation. Circulators generally contain a microwave circuit comprising an arrangement of conductors and ferrite blocks, and a magnetic circuit providing a magnetic biasing field applied to the ferrite blocks that act as a non-reciprocal media for propagating radio frequency signals throughout the device. RF circulators are suitable for essentially any radio frequency (RF) application, including communications. RF circulators are also useful as isolators, easily made by tying the third circulator port to ground through a resistor. RF circulators can be built in resonant structures such as radio frequency resonant cavities and in waveguide at higher frequencies. Circulators may also be realized in planar configuration using stripline or microstrip technology which employ a planar resonating element between two ground plane conductors (stripline) or coupled to a single ground plane conductor (microstrip). Radio frequency and microwave circulators employ a DC-biasing magnetic field generated in ferrite material enveloping a conductor to provide at least one non-reciprocal transmission path between signal ports on a network. In a radar or communications system it is often advantageous to couple multiple devices, such as a transmitter and a receiver, to the same antenna. A common approach uses a radio frequency circulator to isolate the transmitted signal from the received signal to avoid overloading the receiver front end. An array antenna can include a plurality of radio frequency (RF) circulators disposed in an array in a manner in which RF signals can be received from or transmitted to the same individual radiator.

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Circulator

A circulator is a passive non-reciprocal three- or four-port device, in which microwave or radio frequency power entering any port is transmitted to the next port in rotation (only). Thus, to within a phase-factor, the scattering matrix for an ideal three-port circulator is

When one port of a three-port circulator is terminated in a matched load, it can be used as an isolator, since a signal can travel in only one direction between the remaining ports.[1]

There are circulators for LF, VHF, UHF, microwave frequencies and for light, the latter being used in optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetised material, and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetised material. Waveguide circulators may be of either type, while more compact devices based on striplines are of the 3-port type. Sometimes two or more Y-junctions are combined in a single component to give four or more ports, but these differ in behaviour from a true 4-port circulator.

In radar, circulators are used to route outgoing and incoming signals between the antenna, the transmitter and the receiver. In a simple system, this function could be performed by a switch that alternates between connecting the antenna to the transmitter and to the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulses, however, requiring a circulator for this function.

Radio frequency circulators are composed of magnetised ferrite materials. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators.

There have also been investigations into making "active circulators" which are based on electronics rather than passive materials. However, the power handling capability and linearity and signal to noise ratio of transistors is not as high as those made from ferrites. It seems that transistors are the only (space efficient) solution for low frequencies.

A waveguide circulator used as an isolator by placing a matched load on port 3. The label on the permanent magnet indicates the direction of circulation

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