Signal amplifier valves not operating at high power are not affected in this way. Power supply regulation variation of voltage available with current drawn is not an issue, as average current is essentially constant; AB amplifiers, which draw current dependent upon signal level, require attention to supply regulation. Class B and AB amplifiers are more efficient than class A, and can deliver higher power output levels from a given power supply and set of valves. However, the price for this is that they suffer from crossover distortion, of more or less constant amplitude regardless of signal amplitude.
This means that class AB and B amplifiers produce their lowest distortion percentage at near maximum amplitude, with poorer distortion performance at low levels. Measured distortion spectra from such amplifiers [ citation needed ] show that distortion percentage is dramatically reduced by NFB, but the residual distortion is shifted towards higher harmonics. In a class B push—pull amplifier, output valve current which must be provided by the power supply ranges from nearly zero for zero signal to a maximum at maximum signal. Consequently, for linear response to transient signal changes the power supply must have good regulation.
Only class A can be used in single-ended mode, as part of the signal would otherwise be cut off. The driver stage for class AB2 and B valve amplifiers must be capable of supplying some signal current to the power valve grids "driving power". The biasing of a push—pull output stage can be adjusted at the design stage, usually not in a finished amplifier between class A giving best open-loop linearity through classes AB1 and AB2, to class B giving greatest power and efficiency from a given power supply, output valves and output transformer.
Most commercial valve amplifiers operate in Class AB1 typically pentodes in the ultra-linear configuration , trading open-loop linearity against higher power; some run in pure class A. The dominant phase splitter topologies today are the concertina , floating paraphase , and some variation of the long-tail pair.
The gallery shows a modern home-constructed, fully differential, pure class A amplifier of about 15 watts output power without negative feedback, using 6SN7 low-power dual triodes and KT88 power tetrodes. Because of their inability to drive low impedance loads directly, valve audio amplifiers must employ output transformers to step down the impedance to match the loudspeakers. Output transformers are not perfect devices and will always introduce some odd harmonic distortion and amplitude variation with frequency to the output signal. In addition, transformers introduce frequency-dependent phase shifts which limit the overall negative feedback which can be used, to keep within the Nyquist stability criteria at high frequencies and avoid oscillation.
In recent years, however, the development of improved transformer designs and winding techniques greatly reduce these unwanted effects within the desired pass-band, moving them further out to the margins. Following its invention by Harold Stephen Black , negative feedback NFB has been almost universally adopted in amplifiers of all types, to substantially reduce distortion, flatten frequency response, and reduce the effect of component variations.
This is especially needed with non-class-A amplifiers. Feedback very much reduces distortion percentage, but the distortion spectrum becomes more complex, with a far higher contribution from higher harmonics; [1] the high harmonics, if at an audible level, are much more undesirable than lower ones, [1] so that the improvement due to lower overall distortion is partly cancelled by its nature. It is reported that under some circumstances the absolute amplitude of higher harmonics may increase with feedback, although total distortion decreases. NFB reduces output impedance Z out which may vary as a function of frequency in some circuits.
This has two important consequences:. Like any amplifying device, valves add noise to the signal to be amplified. The noise figure is defined as the ratio of the noise power at the output of the amplifier to the noise power that would be present at the output if the amplifier were noiseless due to amplification of thermal noise of the signal source. An equivalent definition is: It is often expressed in decibels dB. The noise properties of valves at audio frequencies can be modelled well by a perfect noiseless valve having a source of voltage noise in series with the grid. For the EF86 low-noise audio pentode valve, for example, this voltage noise is specified see e.
This refers to the integrated noise, see below for the frequency dependence of the noise spectral density. It is not simply double because the noise sources are random and there is some partial cancellation in the combined noise. The noise figure is then 1. To obtain low noise figure, the impedance of the source can be increased by a transformer.
This is eventually limited by the input capacitance of the valve, which sets a limit on how high the signal impedance can be made if a certain bandwidth is desired. The noise voltage density of a given valve is a function of frequency. White noise is often expressed by an equivalent noise resistance, which is defined as the resistance which produces the same voltage noise as present at the valve input. Valves with high g m thus tend to have lower noise at high frequencies. Thus, valves with low noise at high frequency do not necessarily have low noise in the audio frequency range.
It can be reduced by choosing very pure materials for the cathode nickel, and running the valve at an optimized generally low anode current.
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Unlike solid-state devices, valves are assemblies of mechanical parts whose arrangement determines their functioning, and which cannot be totally rigid. If a valve is jarred, either by the equipment being moved or by acoustic vibrations from the loudspeakers, or any sound source, it will produce an output signal, as if it were some sort of microphone the effect is consequently called microphony. All valves are subject to this to some extent; low-level voltage amplifier valves for audio are designed to be resistant to this effect, with extra internal supports.
The EF86 mentioned in the context of noise is also designed for low microphony, though its high gain makes it particularly susceptible. For high-end audio , where cost is not the primary consideration, valve amplifiers have remained popular and indeed during the s made a commercial resurgence. Circuits designed since then in most cases remain similar to circuits from the valve age, but benefit from advances in ancillary component quality including capacitors as well as general progress across the electronics industry which gives designers increasingly powerful insight into circuit operation.
Solid-state power supplies are more compact, efficient, and can have very good regulation. Semiconductor power amplifiers do not have the severe limitations on output power imposed by thermionic devices; accordingly loudspeaker design has evolved in the direction of smaller. In response, many modern valve push—pull amplifiers are more powerful than earlier designs, reflecting the need to drive inefficient speakers.
When valve amplifiers were the norm, user-adjustable "tone controls" a simple two-band non-graphic equaliser and electronic filters were used to allow the listener to change frequency response according to taste and room acoustics; this has become uncommon.
Some modern equipment uses graphic equalisers, but valve preamplifiers tend not to supply these facilities except for RIAA and similar equalisation needed for vinyl and shellac discs. Modern signal sources, unlike vinyl discs, supply line level signals without need for equalisation. It is common to drive valve power amps directly from such source, using passive volume and input source switching integrated into the amplifier, or with a minimalist "line level" control amplifier which is little more than passive volume and switching, plus a buffer amplifier stage to drive the interconnects.
However, there is some small demand for valve preamps and filter circuits for studio microphone amplifiers, equalising preamplifiers for vinyl discs, and exceptionally for active crossovers. When valve amplifiers were the norm, SETs more-or-less disappeared from western products except for low-power designs up to 5 watts , with push—pull indirectly heated triodes or triode-connected valves such as EL84 becoming the norm.
However, the far east never abandoned valves, and especially the SET circuit; indeed the extreme interest in all things audiophile in Japan and other far eastern countries sustained great interest in this approach. Since the s a niche market has developed again in the west for low-power commercial SET amplifies up to 7 watts , notably using the B valve in recent years, which has become fashionable and expensive. Lower-power amplifiers based on other vintage valve types such as 2A3 and 45 are also made.
Even more rarely, higher powered SETs are produced commercially, usually using the or transmitting valves, which are able to deliver 20 watts, operating at V. Notable amplifiers in this class are those from Audio Note corporation designed in Japan , including the "Ongaku", voted amplifier of the year during the late s. The Wavac may be the world's most expensive hi-fi amplifier, delivering around watts using an A valve.
Aside from this Wavac and a very few other high-power SETs, SET amplifiers usually need to be carefully paired with very efficient speakers, notably horn and transmission-line enclosures and full-range drivers such as those made by Klipsch and Lowther , which invariably have their own quirks, offsetting their advantages of very high efficiency and minimalism.
This is made possible by an output transformer design which does not saturate at high levels and has high efficiency. Mainstream modern loudspeakers give good sound quality in a compact size, but are much less power-efficient than older designs and require powerful amplifiers to drive them. This makes them unsuitable for use with valve amplifiers, particularly lower-power single-ended designs. Valve hi-fi power amplifier designs since the s have had to move mainly to class AB1 push—pull PP circuits. Tetrodes and pentodes, sometimes in ultra-linear configuration, with significant negative feedback, are the usual configuration.
Some class A push—pull amplifiers are made commercially. Some amplifiers can be switched between classes A and AB; some can be switched into triode mode. The simplicity of valve amplifiers, especially single-ended designs, makes them viable for home construction. This has some advantages:. Point-to-point hand-wiring tends to be used rather than circuit boards in low-volume high-end commercial constructions as well as by hobbyists.
This construction style is satisfactory due to ease of construction, adapted to the number of physically large and chassis mounted components valve sockets, large supply capacitors, transformers , the need to twist heater wiring to minimise hum, and as a side effect benefiting from the fact that "flying" wiring minimises capacitive effects. One advantage a hobbyist has over a commercial producer is the ability to use higher quality parts that are not reliably available in production volumes or at a commercially viable cost price.
For example, the "silver top getter" Sylvania brown base 6SN7s in use in the external picture date from the s. Another picture shows exactly the same circuit constructed using Russian military production Teflon capacitors and non-inductive planar film resistors, of the same values.
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Very occasionally, very-high-power valves usually designed for use in radio transmitters from decades ago are pressed into service to create one-off SET designs usually at very high cost. Examples include valves and The main problem with these designs is constructing output transformers able to sustain the plate current and resultant flux density without core saturation over the full audio-frequency spectrum.
This problem increases with power level. Many modern commercial amplifiers and some hobbyist constructions place multiple pairs of output valves of readily obtainable types in parallel to increase power, operating from the same voltage required by a single pair. A beneficial side effect is that the output impedance of the valves, and thus the transformer turns ratio needed, is reduced, making it easier to construct a wide bandwidth transformer.
Some high-power commercial amplifiers use arrays of standard valves e. Some home-constructed amplifiers use pairs of high-power transmitting valves e.
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The output transformer OPT is a major component in all mainstream valve power amplifiers, accounting for significant cost, size, and weight. It is a compromise, balancing the needs for low stray capacitance, low losses in iron and copper, operation without saturation at the required direct current, good linearity, etc. One approach to avoid the problems of OPTs is to avoid the OPT entirely, and directly couple the amplifier to the loudspeaker, as is done with most solid-state amplifiers. Some designs without output transformers OTLs were produced by Julius Futterman in the s and '70s, and more recently in different embodiments by others.
Valves normally match much higher impedances than that of a loudspeaker. Low-impedance valve types and purpose-designed circuits are required. Reasonable efficiency and moderate Z out damping factor can be achieved. These effects mean that OTLs have selective speaker load requirements, just like any other amplifier. Certain signal processing applications use exponential gain amplifiers. Amplifiers are usually designed to function well in a specific application, for example: Every amplifier includes at least one active device , such as a vacuum tube or transistor.
Negative feedback feeds the difference of the input and part of the output back to the input in a way that cancels out part of the input. The main effect is to reduce the overall gain of the system. However, the unwanted signals introduced by the amplifier 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 acts as a technique to reduce errors at the expense of gain.
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. With negative feedback , 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. 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: 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: 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: 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: 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. This allows a higher bandwidth to be achieved than could otherwise be realised even with the same gain stage elements. These nonlinear amplifiers have much higher efficiencies than linear amps, and are used where the power saving justifies the extra complexity. Class-D amplifiers are the main example of this type of amplification. Certain requirements for step response and overshoot are necessary for an acceptable TV image. Traveling wave tube amplifiers TWTAs are used for high power amplification at low microwave frequencies.
They typically can amplify across a broad spectrum of frequencies; however, they are usually not as tunable as klystrons. Klystrons are specialized linear-beam vacuum-devices, designed to provide high power, widely tunable amplification of millimetre and sub-millimetre waves. Klystrons are designed for large scale operations and despite having a narrower bandwidth than TWTAs, they have the advantage of coherently amplifying a reference signal so its output may be precisely controlled in amplitude, frequency and phase.
Instrument amplifiers are a range of audio power amplifiers used to increase the sound level of musical instruments, for example guitars, during performances. One set of classifications for amplifiers is based on which device terminal is common to both the input and the output circuit. In the case of bipolar junction transistors , the three classes are common emitter, common base, and common collector. For field-effect transistors , the corresponding configurations are common source, common gate, and common drain; for vacuum tubes , common cathode, common grid, and common plate.
The common emitter or common source, common cathode, etc. The common collector arrangement applies the input voltage between base and collector, and to take the output voltage between emitter and collector. This causes negative feedback, and the output voltage tends to follow the input voltage.
This arrangement is also used as the input presents a high impedance and does not load the signal source, though the voltage amplification is less than one. The common-collector circuit is, therefore, better known as an emitter follower, source follower, or cathode follower. An amplifier whose output exhibits no feedback to its input side is described as 'unilateral'. The input impedance of a unilateral amplifier is independent of load, and output impedance is independent of signal source impedance.
An amplifier that uses feedback to connect part of the output back to the input is a bilateral amplifier. Bilateral amplifier input impedance depends on the load, and output impedance on the signal source impedance. All amplifiers are bilateral to some degree; however they may often be modeled as unilateral under operating conditions where feedback is small enough to neglect for most purposes, simplifying analysis see the common base article for an example.
Another way to classify amplifiers is by the phase relationship of the input signal to the output signal. An 'inverting' amplifier produces an output degrees out of phase with the input signal that is, a polarity inversion or mirror image of the input as seen on an oscilloscope. A 'non-inverting' amplifier maintains the phase of the input signal waveforms.
An emitter follower is a type of non-inverting amplifier, indicating that the signal at the emitter of a transistor is following that is, matching with unity gain but perhaps an offset the input signal.
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Voltage follower is also non inverting type of amplifier having unity gain. Other amplifiers may be classified by their function or output characteristics. These functional descriptions usually apply to complete amplifier systems or sub-systems and rarely to individual stages. Amplifiers are sometimes classified by the coupling method of the signal at the input, output, or between stages. Different types of these include:.
Depending on the frequency range and other properties amplifiers are designed according to different principles. Frequency ranges down to DC are only used when this property is needed. Amplifiers for direct current signals are vulnerable to minor variations in the properties of components with time. Special methods, such as chopper stabilized amplifiers are used to prevent objectionable drift in the amplifier's properties for DC.
Depending on the frequency range specified different design principles must be used. Up to the MHz range only "discrete" properties need be considered; e. For example, a specified length and width of a PCB trace can be used as a selective or impedance-matching entity. Above a few hundred MHz, it gets difficult to use discrete elements, especially inductors. In most cases, PCB traces of very closely defined shapes are used instead stripline techniques.
The power amplifier classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current. The angle of flow is closely related to the amplifier power efficiency. The practical amplifier circuit to the right could be the basis for a moderate-power audio amplifier. It features a typical though substantially simplified design as found in modern amplifiers, with a class-AB push—pull output stage, and uses some overall negative feedback. Bipolar transistors are shown, but this design would also be realizable with FETs or valves.
The input signal is coupled through capacitor C1 to the base of transistor Q1. The capacitor allows the AC signal to pass, but blocks the DC bias voltage established by resistors R1 and R2 so that any preceding circuit is not affected by it. Q1 and Q2 form a differential amplifier an amplifier that multiplies the difference between two inputs by some constant , in an arrangement known as a long-tailed pair. This arrangement is used to conveniently allow the use of negative feedback, which is fed from the output to Q2 via R7 and R8.
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The negative feedback into the difference amplifier allows the amplifier to compare the input to the actual output. The amplified signal from Q1 is directly fed to the second stage, Q3, which is a common emitter stage that provides further amplification of the signal and the DC bias for the output stages, Q4 and Q5. R6 provides the load for Q3 a better design would probably use some form of active load here, such as a constant-current sink.
So far, all of the amplifier is operating in class A. The output pair are arranged in class-AB push—pull, also called a complementary pair. They provide the majority of the current amplification while consuming low quiescent current and directly drive the load, connected via DC-blocking capacitor C2.
The diodes D1 and D2 provide a small amount of constant voltage bias for the output pair, just biasing them into the conducting state so that crossover distortion is minimized. That is, the diodes push the output stage firmly into class-AB mode assuming that the base-emitter drop of the output transistors is reduced by heat dissipation. This design is simple, but a good basis for a practical design because it automatically stabilises its operating point, since feedback internally operates from DC up through the audio range and beyond.
Further circuit elements would probably be found in a real design that would roll-off the frequency response above the needed range to prevent the possibility of unwanted oscillation. A common solution to help stabilise the output devices is to include some emitter resistors, typically one ohm or so. Calculating the values of the circuit's resistors and capacitors is done based on the components employed and the intended use of the amp.
Any real amplifier is an imperfect realization of an ideal amplifier. An important limitation of a real amplifier is that the output it generates is ultimately limited by the power available from the power supply. An amplifier saturates and clips the output if the input signal becomes too large for the amplifier to reproduce or exceeds operational limits for the device. The power supply may influence the output, so must be considered in the design. The power output from an amplifier cannot exceed its input power.
The amplifier circuit has an "open loop" performance. This is described by various parameters gain, slew rate , output impedance , distortion , bandwidth , signal-to-noise ratio , etc. Many modern amplifiers use negative feedback techniques to hold the gain at the desired value and reduce distortion.