Amplifier

An amplifier is an electronic assembly having at least one active device (usually a transistor, sporadically tube ), which processed an incoming analog signal that the output is greater than the input size. In this case, the output must usually can provide more power than the input receives. The additional power is drawn from a power source, such as a battery or a power adapter. There are amplifiers for both DC and DC as well as AC or AC voltage.

An important characteristic is usually the linearity: Doubling the input size must lead to a doubling of the output variable. Linearity errors are usually undesirable and are referred to as distortion. Then combination frequencies are generated, which are not included in the input signal and cause in audio amplifiers sound distortions. For special tasks may be attached instead of the linear behavior, for example, a logarithmic or radizierendes or adapted for hearing right volume behavior.

  • 8.1 technologies
  • 8.2 Basic Circuits
  • 8.3 applications

Survey

An amplifier not only increases the input voltage, because that can also carry out a transformer (with AC). Rather, a powered from an external power source from the power amplifier is formed so that the waveform of the input signal is replicated - just at a higher current level. One can compare this graphic with the magnification function of a photocopier - this can not also make the original even greater, but it is generated only on new paper a larger image. An amplifier is an "image " of the weak current in stronger by working essentially as an electrically controllable resistance: With a lower input signal it sets the stronger current from the external power source has a high resistance, so that it is relatively strongly attenuated; at higher input signal it represents a lower resistance for higher power, so that current is allowed to flow relatively freely from the external power source.

Increase in the low frequency range is, for example, the low voltage of a few millivolts, which provides a microphone, is raised with a small-signal amplifier to a few volts. In order to operate a speaker, you need a given power, which amplify the audio frequency offered to the required value, while as large-signal amplifier also can supply enough current. The power depends on the intended use, and the range is from a few milliwatts ( room volume ) up to thousands watts ( stadium).

Amplifying the high frequency range is, for example: In the receiver, a very low voltage of only a few micro- volts, which comes from the antenna is increased million times in several stages, is carried out simultaneously as a rule by means of resonant circuits, a frequency selection. In the low power transmitters of an oscillator in a number of amplifier stages is increased up to several thousand watts and transmitted via antennas. The actual performance depends on the type and purpose of the respective transmitter.

A voltage follower, the voltage is not put up but the current, the input voltage is barely charged. The output voltage is approximately equal to the input voltage, it "follows" the input voltage. Voltage followers are used in power amplifiers, in electret and as an electrometer amplifier.

A further distinction between

  • Broadband amplifiers for a wider frequency range, such as broadcasting ( TV ) in the range of 8 to 860 MHz and
  • Selective amplifiers which amplify only a narrow frequency range, for example from 3.5 to 3.55 MHz.

Demarcation

Switching amplifiers have only two states and are not further discussed in this article. An important characteristic is that you can turn on and off at low power one usually considerably larger current (or voltage). Therefore, a potential separation is often associated, for example, when power supply voltages are switched.

Switching amplifier can be realized with active electronic components (transistors, thyristors, triacs, SSR ) or with mechanical relays. In contrast to analog amplifiers often have a positive feedback that causes a hysteresis. Then work as a threshold switch, in particular, to avoid unexaktes switching behavior and to eliminate noise.

Components

The actual reinforcing component in amplifiers is a so-called active electronic component. This includes, among other things fall transistors and vacuum tubes, but also transducers ( magnetic amplifier ). In the high-frequency technology also Maser, IMPATT diodes or tunnel diodes are used. A particularly low-noise amplifier also SQUIDs are used in metrology in a few cases. Characteristic of these devices is the controllability of a large output current or a large output voltage / power with a smaller input signal.

Furthermore, an amplifier requires in addition to these active components, a variety of passive components, which are used, inter alia, the power supply, the stabilization of the parameters of the impedance matching or protection. These include resistors, capacitors, transformers, or transformers and diodes.

Transistor amplifier Discrete are increasingly being replaced by operational amplifiers and integrated power amplifiers that contain almost the entire amplifier circuit and require only a few external components for operation.

Classification of Audio Amplifiers

The gain over a wide frequency range as possible, which is characterized by the lower and upper cutoff frequency should be constant. We distinguish the following modes or amplifier classes:

  • Single-ended Class A: An active component derives However, the current flowing is controlled. Application in a preamp and a tube power amp into a guitar amp. The disadvantage is the low theoretical efficiency at full load of only 25 %.
  • Push-pull amplifier class AB: two active devices operate in a push-pull circuit alternately (german push-pull ). Theoretical efficiency at full load: 50 % ... 78.5 %.
  • In a full-bridge amplifier two push-pull amplifier work against each other on each one of the load ports. The speaker makes a "bridge" between the two amplifiers. They are used, if at a given load impedance, and at a given supply voltage as high performance must be achieved (eg car radios ).
  • Class - C amplifier: These amplifiers operate with a single active component and are for example used in the RF technology ( as a power amp ). They can not be used for all modulation schemes. Class C amplifiers are highly non - linear, however, offer a high efficiency. They are therefore often used for the amplification of signals to transmit antennas.
  • Class D amplifier: analog power amplifier can be constructed using switching amplifiers. In this case an analog signal to a PWM modulator, a pulse width modulated switching signal is converted, the great power on and off at high frequency. Using a filter ( LC low-pass ) the average value of this square wave signal is converted back into a constantly varying voltage. This method is called when audio amplifiers as digital amplifier, whose efficiency is much higher than for Class AB and B amplifiers. They are therefore used in audio amplifiers, high-power and, increasingly, small battery powered devices. Theoretical efficiency at full load: 78.5 % ... 100 %
  • Class E amplifiers combine elements of the Class -D and Class - C amplifier to a high efficiency amplifier. These switching stage operating on a resonance circuit, the voltage passes through a low-pass to the load. The switching stage closes whenever the resonant circuit is reached at the zero crossing, thus the switching losses and noise compared to class D amplifiers reduce dramatically. Field of application of this type are narrow-band high-frequency amplifier.

Function Example

The function of an amplifier will be described below using the example of a small-signal transistor amplifier stage.

The gain of a transistor is in emitter circuit especially large and - when high performance is required - all you need in A-operation, a collector current of about 1 mA. With a current feedback can achieve that the selected operating point is also observed in sample variances of transistor parameters and is almost independent of temperature. For this purpose, the voltage drop across the 1 k-ohm resistor between emitter and ground to ( these are the lowest symbols that can be connected to 0 V ) are about 1 V, because UBE can - depending on specimen and temperature - vary by about 0.06 V.

In the picture, the base voltage is set to a voltage divider on

The cross current Iq of the voltage divider should be large compared to the base current IB. This requirement is fulfilled, as in conventional transistors applies IC / IB ≥ 100 For silicon transistors UBE valid = 0.6V, so are the emitter resistor about 1.5 V and 1.5 mA collector current flows.

The voltage to be amplified by a few millivolts is passed through a capacitor with a low impedance to the base and tapped with an increased amplitude of the collector.

The adjacent upper circuit amplifies indiscriminately all frequencies between about 150 Hz and 20 MHz, the lower only a narrow range. The comparison of the images shows that this shall be determined primarily by the type of collector resistance:

  • In the upper circuit it is independent of frequency and should be chosen so that the collector voltage of the mean value (in this case about 5 V ) can vary symmetrically as possible without falling into the overload (clipping ) and to cause such bias. The frequency range is limited by the coupling capacitances at the input and output down and through the transistor and the switching capacity up.
  • In the lower circuit, only a narrow range around the resonance frequency ω0 of the resonant circuit is enhanced. Only here the parallel resonant circuit is high impedance, that is to be expected with sufficient gain. At lower frequencies, the coil acts as a short circuit, at higher frequencies the capacitor. Gain and bandwidth depends on the quality factor of the resonant circuit.

Both amplifiers are negatively fed back for direct current to the emitter resistor 1 k, which provides for a stable operating point of the transistor. Suppose UBE decreases due to the temperature by 40 mV, then the voltage at the emitter resistor to 1.54 V and the planned collector current increases slightly so that there are no significant effect on gain or distortion. Without this feedback, the operating point could come in the saturation region, where both changes drastically.

But this desirable and necessary DC negative feedback reduces the gain for AC voltage at the upper circuit on the very low value 4.7, resulting from the ratio of collector and emitter resistor. This can be avoided by a parallel series of 100 Ω and 10 uF. The capacitor now determined the lower limit frequency. When the impedance of the capacitor is sufficiently small ( in the upper circuit, for example, at frequencies in the kHz range ), the gain is now calculated from the ratio of the collector and the active, alternating voltage emitter resistance ( parallel connection of 1 kOhm and 100 ohms) and increases to the value of 4700/91 = 52

When not using the 100 Ω resistor and a 10 - uF capacitor inserted directly from the emitter to ground, but the gain does not increase indefinitely but at about 200 - which is delimited by internal feedback effects in the transistor. But this comes at a audible distortion, since the non-linear characteristic of the transistor is no longer linearized by negative feedback.

Characteristics of analog amplifier

The power at the output of amplifiers ranges from a few uW in hearing aids up to several hundred kilowatts in output stages of amplitude-modulated radio stations on medium wave and short wave. Amplifiers are specified for a given load impedance ( 4 ... 8 ohms in audio amplifiers ) or in the case of switching amplifiers for a maximum output current and a maximum output voltage.

The amplification factor (in short: the gain ) is the ratio between input and output (voltage, current or power ) on. It is given by a factor or logarithmic ( decibel ).

Signal to noise ratio

Disturbances in amplifying analog signals are the noise ( see also: Signal - to-noise ratio ) and external voltages, such as residues of the supplying mains AC voltage. They are described by the signal to noise ratio or the noise ratio or mostly expressed in decibels with respect to full scale of the amplifier.

The electromagnetic compatibility ( EMC) describes, inter alia, the sensitivity of an amplifier to external electromagnetic fields (such as radio transmitters, electrical spark or mobile phones ).

Distortions

Can be linear and nonlinear distortions.

  • Linear distortion relating to the frequency dependency of the gain, as well as associated phase angle deviations. For music, the gain is often intentionally adjusted by tone controls to individual tastes. Linear distortion is recognized by the fact that in the simultaneous amplification of multiple frequencies no new combination frequencies arise, which are not included in the original signal.
  • Nonlinear distortions occur when the output voltage is not proportional to the input voltage changes, for example when oversteer. Then one speaks of the distortion of the amplifier, which is generated by the lack of amplitude linearity. Always, new frequencies are not included in the original signal. If the amplifier is fed with a single frequency, refers to the newly created frequency components as harmonics. If several frequencies fed ( frequency mix ), the intermodulation distortion always lead to combination frequencies, such as the sum or difference of the original frequencies. This is at a mixer or guitar amp desirable in a hi-fi amplifier, a quality defect.

In Class D amplifiers additionally quantization occur. In addition, errors due to insufficient sampling or working frequency occur (aliasing, sub- harmonic) in these according to the Nyquist -Shannon sampling theorem.

Nonlinear distortions occur at overdrive (exceeding the maximum amplitude of the output voltage ) or Class B amplifiers by the so-called crossover distortion. These are caused by not fast enough current flow takeover of the two alternately conducting output stages.

On measuring and audio amplifier particularly high demands on the signal to noise and signal to noise ratio, the stability and frequency response will be provided.

In audio amplifiers need not only for a wide frequency range to include the listening area, for a linear frequency response and low distortion ( THD ) of the signal are taken care of, but it is also the smallest possible internal resistance, a short rise time, pulse fidelity and channel separation required. The issue right ear volume is treated in psychoacoustics.

Feedback

With feedback is defined as the correct phase returning a part of the output signal to the input of the amplifier, with the aim to reduce the gain. The disadvantage of the reduced output power can be easily compensated for by additional amplifier stages. The benefits, however, can achieve in any other way:

  • The operating point is stabilized and hardly by manufacturing tolerances, temperature variations, inter alia, affected. Negative feedback has been proven in other fields of technology.
  • Only through feedback can reduce distortion of amplifiers.

There are two different approaches:

  • With voltage feedback of a fraction of the output voltage from the input voltage is subtracted, and only the difference is amplified. Result: With increasing negative feedback, the output impedance of the amplifier is reduced (also called source resistance or internal resistance ) ( electrical behavior of a constant voltage source). In audio amplifiers, the unwanted resonances of the speaker are strongly damped.
  • For current feedback of the output current flows ( through load resistance ) has a resistance with a comparatively low value when the negative feedback voltage required can be taken. In this case, increases with increasing negative feedback of the output resistance ( electrical behavior of a constant current source). This side effect is at selective amplifiers desirable because it increases the quality factor of the connected resonant circuits.

A strong negative feedback due to the reduced gain requires a greater number of amplification stages. Because electron tubes significantly more expensive ( ~ 10 € per piece ) and are bulkier than transistors (~ € 0.1 for single transistors, operational amplifiers ~ € 0.001 in ), use negative feedback rather sparingly in tube amplifiers and accepted the low fidelity. In addition, the output transformers produced in the vicinity of the resonances of its windings phase shifts, which can transform the negative feedback in a very disturbing positive feedback.

The semiconductor technology offers the possibility of the loop gain ( for example, operational amplifiers ) to increase extremely with very high feedback factors to achieve a linearization of excellent quality with their smaller dimensions and component prices as well as the integrability.

A negative feedback, however, can adversely affect the frequency response and the time response of an amplifier may: Achieved a pulse (one, possibly fast rise- operation ) the input of an amplifier, the output signal will only appear after a certain time; the feedback signal reaches the input later. During this period, the negative feedback has no effect, the loop is "open". This leads, in particular at high feedback factors and inadequate circuit design to transient signal variations (so-called " overshoot " or transient ) until the output has settled (English settling ). These variations are greater, the amplifier operates closer to its instability limit. Also, the load affects the phase behavior, which is why audio amplifiers are particularly affected because the operated on them speakers have a strongly frequency-dependent impedance curve.

Electron tube and transistor amplifiers differ both between even and odd harmonics (distortion spectrum) as well as in the transient distortions. Tube amplifiers are characterized by the use of softer overdrive distortion ( soft clipping ), but the results compared with transistor amplifiers, higher swelling resistance to a poorer signal fidelity because the speaker resonances are hardly attenuated. The output transformer makes its inevitable leakage inductance for a small bandwidth.

Contrast, audio transistor amplifiers have unpleasant distortion at clipping. Crossover distortion can be avoided by increased quiescent current and sufficiently fast transistors.

Areas of application

Amplifier used in almost all areas of electrical engineering and electronics.

Examples include the telecommunications, consumer electronics ( effects pedals, electronic musical instruments, synthesizers, audio amplifier, microphone amplifier ), measuring amplifier, amplifier for controlling actuators (motors, piezo elements, pull magnets ). In communication engineering, they are also called English repeaters.

In hard disks and tape recorders amplifiers work in reading and writing by magnetic head. In fiber-optic networks and CD and DVD players electric drive for operation of laser diodes and for amplifying the signals of photodiodes are required.

CD and DVD drives also have analog amplifier to drive the galvanometer drives for position control of the optical head for reading / burning.

In cell phones, radios, satellite and radio stations high-frequency amplifier for transmission and reception of radio waves are necessary.

Work switching amplifier, for example, to operate the signal lights and the power windows in motor vehicles or in rush current circuits and switch circuits. You push and pull type solenoids and solenoid valves in automation equipment and machinery.

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