# Amplifier (electrical engineering)

An amplifier is an electronic assembly with at least one active component (mostly a transistor , occasionally also a tube ), which processes an incoming analog signal in such a way that the output variable is greater than the input variable. The output must usually be able to deliver more power than the input consumes. The additional power is taken from an energy source, e.g. B. a battery or a power supply . There are amplifiers for direct current or direct voltage as well as for alternating current or alternating voltage .

The essential characteristic is usually the linearity : a doubling of the input variable must lead to a doubling of the output variable. Linearity deviations are usually undesirable and are referred to as distortions . Combination frequencies are then also generated that are not contained in the input signal and cause sound distortions in audio amplifiers . For special tasks, instead of the linear behavior, for example, a logarithmic or square root or behavior adapted for aurally correct volume can be appropriate.

Small signal amplifier with bipolar transistor in common emitter circuit with negative current feedback. The input and output variables are alternating voltages. Voltage gain v = Ua / Ue

## Overview

A signal fed in from an external source is shaped by an amplifier in such a way that the temporal course of the input signal is simulated - only with higher power. This can be clearly compared with the enlargement function of a photocopier - this also does not enlarge the original, but only creates a larger image. An amplifier therefore creates a “stronger image” of a weak input signal by essentially working as an electrically controllable resistor : With a low input signal, it opposes the voltage from the energy source with a high resistance, so that it is relatively strongly weakened; with a higher input signal, it represents a lower resistance, so that the energy can flow relatively unhindered.

A distinction is made between a voltage gain a U and a current gain a I , the latter also called a buffer, because the voltage in the input is equal to the voltage in the output. In the case of attenuation, or negative gain, a U becomes less than 1 or less than 0 dB (v), which is a characteristic of a filter in which certain frequency ranges are attenuated. A typical example: the sound setting for the audio preamplifier.

Amplification in the low-frequency range means, for example: The low voltage of a few millivolts that a microphone delivers is increased to a few volts with a small-signal amplifier . In order to operate a loudspeaker , you need a corresponding output stage , which can amplify the audio frequency voltage offered to the required value and, as a large-signal amplifier , can also supply sufficient current.

For example, amplifying in the high-frequency range means: In the receiver , a very low voltage of just a few microvolts, which comes from the antenna, is increased a million times over in several stages, with a frequency selection usually being made at the same time with the help of resonant circuits . In transmission systems , the low output of an oscillator is increased in several amplifier stages to up to a few thousand watts and emitted via antennas. The specific performance depends on the type and purpose of the respective transmission system.

Voltage followers do not increase the voltage, but the current, whereby the input voltage is hardly loaded. The output voltage is approximately equal to the input voltage, it "follows" the input voltage. Voltage followers are used in power amplifiers, in electret microphones and as electrometer amplifiers .

Furthermore, one differentiates between

• Broadband amplifiers with a wide transmission range , e.g. B. Video amplifier with a transmission range of 0 to 100 MHz.
• Selective amplifiers with a narrow transmission range, e.g. B. FM IF amplifier from FM heterodyne receiver with a transmission range of 10.6 to 10.8 MHz.

## Demarcation

Switching amplifiers only have two states and are not discussed further in this article. An essential characteristic is that you can switch on and off a usually considerably larger current (or voltage) with a low power. This is often associated with a potential separation, e.g. B. when mains voltages are switched.

Switching amplifiers can be implemented with active electronic components ( transistors , thyristors , triacs , semiconductor relays ) or with mechanical relays . In contrast to analog amplifiers, they often have positive feedback , which causes hysteresis behavior. They then work like a threshold switch, in particular to avoid imprecise switching behavior and to eliminate interfering signals.

## Components

Amplifier of a hi-fi system, housing open, toroidal transformer at the top right, cooling plate with transistors and cooling slots to the left

The actual amplifying component in amplifiers is a so-called active electronic component . This includes transistors and electron tubes , but also transducers (magnetic amplifiers). In the high-frequency technology also come Maser , IMPATT diodes or tunnel diodes used. In a few cases, SQUIDs are also used as particularly low-noise amplifiers in measurement technology . These components are characterized by the controllability of a large output current or a large output voltage / power with a smaller input signal.

Furthermore, in addition to these active components, an amplifier requires a large number of passive components, which u. a. the energy supply, the stabilization of the parameters, the impedance matching or the protection. These include resistors , capacitors , transformers or transmitters and diodes .

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

## Classification of audio amplifiers

Main articles: audio amplifier and power amplifier

Basic
circuit diagram of a full bridge amplifier

The gain within the widest possible frequency range, which is characterized by the lower and upper limit frequency , should be constant. A distinction is made between the following operating modes or amplifier classes:

Power consumption of various (ideal) push-pull power amplifiers
Efficiency of various (ideal) push-pull power amplifiers
• Single-ended amplifier class A : An active component always conducts, the flowing current is controlled. Use in preamplifiers as well as in tube power amplifiers of guitar amplifiers. The disadvantage is the low theoretical efficiency at full modulation of 6.25% (ohmic resistance in the non-active branch) or 25% (coil / transformer or constant current source in the non-active branch).
• Class A push-pull amplifier : There are two active components, both are always conducting. Use in transformerless power amplifiers. The maximum theoretical efficiency at full output is 50%.
• Push-pull amplifier Class B : two active devices operate in a push-pull circuit alternately (Engl. Push-pull ). Theoretical efficiency at full level: 78.5%. Because of the high non-linearities in the transition area, this variant is only used in exceptional cases.
• Push-pull amplifier class AB : two active components work alternately in a push-pull circuit ( push-pull ). Theoretical efficiency at full output: 50%… 78.5%.
• In a full bridge amplifier, two push-pull amplifiers work against each other on one of the load connections. The loudspeaker forms a "bridge" between the two amplifiers. They are used when the highest possible output must be achieved at a given load impedance and with a given supply voltage (e.g. car radios).
Circuit diagram for class E amplifiers
• Class C amplifiers : These amplifiers work with a single active component and are used, for example, in RF technology (as power amplifiers). They cannot be used for all modulation methods . Class C amplifiers are extremely non-linear but offer high efficiency. They are therefore often used to amplify signals at transmission antennas.
• Class D amplifiers : Analog power amplifiers can also be built using switching amplifiers. An analog signal isconvertedinto a pulse-width modulated switchingsignal with a PWM modulator , which switches high power on and off at a high frequency. A low-pass filter removes the unwanted switching frequency components and reconstructs the desired time-continuous useful signal. This method is known as a digital output stagefor audio amplifiers, the efficiency of which is considerably higher than that of class AB and Class B amplifiers. They are therefore used in high-performance audio amplifiers and increasingly also in small battery-operated devices. Theoretical efficiency independent of the modulation: 100%.
• Class E amplifiers combine elements of the class D and class C amplifiers into an amplifier with the highest efficiency. In these, a switching stage works on a resonance circuit, the voltage of which reaches the load via a low-pass filter. The switching stage always closes when the resonant circuit has reached zero , which reduces switching losses and interference compared to class D amplifiers. This type of application is in narrowband high-frequency amplifiers.

## Function example

Small signal broadband amplifier with bipolar transistor in common emitter circuit with negative current feedback
Selective amplifier for about 3 MHz
Dependence of the gain on the excitation frequency and the damping of the resonant circuit

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

The gain of a transistor is particularly high in the emitter circuit and - if no high power is required - a collector current of around 1 mA is sufficient for A operation. With a current negative feedback, one can achieve that the selected operating point is adhered to even if there are variations in the transistor parameters and is almost temperature-independent. For this purpose, the voltage drop at the 1 kΩ resistor between emitter and ground (these are the lowest symbols that are connected to 0 V) ​​should be around 1 V, because U BE can - depending on the model and temperature - by around 0.06 V. vary.

In the picture the base voltage is set to with a voltage divider

${\ displaystyle U_ {B} = {\ frac {12 \, \ mathrm {V} \ cdot 10 \, \ mathrm {k} \ Omega} {10 \, \ mathrm {k} \ Omega +47 \, \ mathrm {k} \ Omega}} = 2 {,} 1 \, \ mathrm {V}}$

The cross current I q of the voltage divider should be large relative to the base current I B . This requirement is met because I C / I B  ≥ 100 applies to conventional transistors. U BE  = 0.6 V applies to silicon transistors, which is why there is around 1.5 V at the emitter resistor and 1.5 mA collector current flows.

The voltage of a few millivolts to be amplified is conducted to the base via a capacitor with low impedance and tapped at the collector with an increased amplitude .

The adjacent upper circuit amplifies all frequencies between about 150 Hz and 20 MHz indiscriminately , the lower only a narrow range. The comparison of the pictures shows that this is primarily determined 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 (5 V here approx) can vary symmetrically as possible, without going into the overload ( clipping to fall), and so as to cause distortion. The frequency range is limited downwards by the coupling capacitances at the input and output and upwards by the transistor and the switching capacitances.
• In the circuit below, only a narrow range around the resonance frequency ω 0 of the resonant circuit is amplified. Only here is the parallel resonant circuit so high that sufficient amplification can be expected. At lower frequencies the coil acts as a short circuit, at higher frequencies the capacitor. The gain factor and bandwidth depend on the quality factor of the resonant circuit.

Both amplifiers are fed back with direct current with the emitter resistor 1 kΩ, which ensures a stable operating point of the transistor. Assuming that U BE drops by 40 mV due to the temperature, the voltage at the emitter resistor increases to 1.54 V and the planned collector current increases so slightly that there are no significant effects on gain or distortion factor. Without this negative feedback, the operating point could reach the saturation area, where both change drastically.

This desired and necessary direct current negative feedback also reduces the gain factor for alternating voltage in the circuit above to the very low value of 4.7, which results from the quotient of collector and emitter resistance. This can be avoided by connecting 100 Ω and 10 μF in series in parallel. The capacitor now determines the lower limit frequency . If the impedance of the capacitor is sufficiently low (in the circuit above, e.g. at frequencies in the kHz range), the gain is now calculated from the quotient of the collector resistance and the emitter resistance effective for alternating voltage (parallel connection 1 kOhm and 100 Ohm) and increases to the value 4700/91 = 52.

If you do without the 100 Ω resistor and connect the 10 μF capacitor directly from the emitter to ground, the gain does not increase indefinitely, but to around 200 - this is limited by internal effects in the transistor. In return, there are audible distortions because the non-linear characteristic of the transistor is no longer linearized by negative feedback .

## Parameters of analog amplifiers

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

The gain factor (in short: the gain) indicates the ratio between the input and output variables (voltage, current or power). It is given by a factor or logarithmically (in decibels ).

### Signal-to-noise ratio

Disturbances when amplifying analog signals are noise (see also: signal-to-noise ratio ) as well as external voltages such as residues of the supplying AC voltage. They are by the signal to noise ratio or the signal to noise ratio described and mostly in decibels relative to full scale indicated the amplifier.

The electromagnetic compatibility (EMC) describes u. a. the sensitivity of an amplifier to external electromagnetic fields (e.g. from radio transmitters, switching sparks or cell phones).

### Distortions

A distinction is made between linear and non-linear distortions.

• Linear distortions relate to the frequency dependency of the amplification and the associated phase angle deviations. With music, the amplification is often intentionally adjusted to suit individual tastes using tone controls . Linear distortion can be recognized by the fact that when several frequencies are amplified at the same time, no new combination frequencies arise that are not contained in the original signal.
• Nonlinear distortion occurs when the output voltage does not change proportionally to the input voltage, for example when overdriving . Then one speaks of the distortion factor of the amplifier, which is produced by the inadequate amplitude linearity . This produces always new frequencies that are not included in the original signal. If the amplifier is fed with a single frequency, the newly created frequency components are called harmonics . If several frequencies are fed in at the same time (frequency mixture), the intermodulation distortions always lead to combination frequencies, for example the sum or difference of the original frequencies. This is desirable with a mixer or guitar amplifier , with a hi-fi amplifier it is a lack of quality.

Quantization errors also occur with class D amplifiers . In addition, in accordance with the Nyquist-Shannon sampling theorem , errors can also occur due to insufficient sampling or working frequency ( aliasing , sub-harmonics).

Transfer distortion in the area of ​​the zero crossing in a class B amplifier stage

Non-linear distortions occur in the event of overdrive (exceeding the maximum amplitude of the output voltage) or in the case of class B amplifiers due to so-called transfer distortion . These are caused by insufficiently fast current flow takeover of the two alternately conductive output stages.

At measurement particularly high demands on the will and audio amplifier noise and signal to noise ratio, the stability and the response provided.

Audio amplifiers not only have to ensure a large frequency range , which should include the audible range , a linear frequency response and low distortion ( THD ) of the signal, but also the smallest possible internal resistance , a short rise time , impulse fidelity and channel separation . The subject of hearing-correct volume is dealt with in Psychoacoustics .

### Negative feedback

Negative feedback is the in-phase feedback of part of the output signal to the input of the amplifier with the aim of reducing the gain . The disadvantage of the reduced output power can be easily compensated for with additional amplifier stages. There is no other way to achieve the benefits:

• The working point is stabilized and hardly affected by manufacturing tolerances, temperature changes and the like. a. influenced. Negative coupling has also proven itself in other areas of technology .
• Distortion from amplifiers can only be reduced by means of negative feedback.

There are two different approaches:

• With negative voltage feedback , a fraction of the output voltage is subtracted from the input voltage and only the difference is amplified. Consequence: With increasing negative feedback , the output resistance (also called source resistance or internal resistance) of the amplifier decreases (electrical behavior of a constant voltage source ). With audio amplifiers, the unwanted natural resonances of the loudspeakers are strongly attenuated.
• With current negative feedback, the output current flows through a resistor with a comparatively low value (through the load resistor), at which the required negative feedback voltage can be tapped. In this case , the higher the negative feedback, the greater the output resistance (electrical behavior of a constant current source ). This side effect is desirable with selective amplifiers because it increases the quality factor of the connected resonant circuits.

A strong negative feedback requires a higher number of amplifier stages due to the reduced gain. Because electron tubes are significantly more expensive (~ 10 € each) and more bulky than transistors (~ 0.1 € for single transistors, ~ 0.001 € in operational amplifiers ), negative feedback is used sparingly in tube amplifiers and the low fidelity is accepted. In addition, the output transformer generates phase shifts in the vicinity of the natural resonance of its windings, which can transform the negative feedback into a very disruptive positive feedback.

Semiconductor technology, with its smaller dimensions and component prices as well as the ability to be integrated, offers the possibility of extremely increasing the loop gain (for example in operational amplifiers ) and of achieving a linearization of excellent quality with very high negative feedback factors.

However, negative feedback can negatively affect the frequency range and the time response of an amplifier: If a pulse (one-off, possibly steep-edged process) reaches the input of an amplifier, the output signal only appears after a certain time; the negative feedback signal reaches the input even later. During this period the negative feedback has no effect, the loop is "open". This leads, in particular at high negative feedback factors and insufficient circuit design to transient signal variations (so-called. "Overshoots" or transient), and this until the output signal calmed (engl. Settling ). These deviations are greater, the closer the amplifier works to its instability limit. The load also influences the phase behavior, which is why audio amplifiers are particularly affected, as the loudspeaker boxes operated on them have a strongly frequency-dependent impedance curve.

Electron tube and transistor amplifiers differ both in the ratio between even and odd harmonics (distortion spectrum) and in the transient distortions. Tube amplifiers are characterized by the softer use of overdrive distortion ( soft clipping ), but the higher source resistance compared to transistor amplifiers leads to poorer impulse fidelity, because the loudspeaker natural resonances are hardly attenuated. With its unavoidable leakage inductance, the output transformer ensures a low bandwidth.

Audio transistor amplifiers, on the other hand, have more unpleasant distortions when overloaded. Transfer distortions can be avoided by increasing the quiescent current and sufficiently fast transistors.

## Areas of application

Amplifier of a hi-fi system

Amplifiers are used in almost all areas of electrical engineering and electronics.

Examples are communications engineering , entertainment electronics ( effect devices , electronic musical instruments , synthesizers , audio amplifiers , microphone amplifiers), measuring amplifiers, amplifiers for controlling actuators ( motors , piezo elements , pull magnets). In communications technology, they are also called repeaters .

In hard drives and tape recorders , amplifiers work with magnetic heads when reading and writing . In fiber optic networks and CD and DVD players, electrical amplifiers are required to operate laser diodes and to amplify the signals from photodiodes .

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

In cell phones , radios , satellites and broadcast transmitters , high frequency amplifiers are required to transmit and receive radio waves .

Switching amplifiers work z. B. to operate the signal lamps and the window lifters in motor vehicles or in impulse circuits and button circuits . They drive pull magnets and solenoid valves in automation systems and machines.

## literature

• Jean Pütz: Introduction to electronics , vgs, 3rd edition 1972, chapter The transistor in amplifier operation