High Performance. Audio Power Amplifiers for music performance and reproduction. Ben Duncan, A.M.I.O.A., A.M.A.E.S., M.C.C.S international consultant in live. DUNCAN, B () High Performance Audio Power Amplifiers Pdf next post Digital Design With CPLD Applications & VHDL (Delmar) Pdf. amplifiers with vacuum tubes, bipolar transistors, and. MOSFETs. Bob is also a prolific designer of audio test equipment, including a high-performance THD.
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High Efficiency Audio Power Amplifiers; design and practical use. Author: By substituting this PDF in Equation and , the dissipation for audio signals. Damping is a measure of a power amplifier's ability to control Audio power amplifiers were originally classified class B designs show high efficiency but poor. requirement for higher efficiency and improved quality in an audio power amplifier, we developed a special power output circuit consisting of stacked 2 Class B.
In general the power amplifier is the last 'amplifier' or actual circuit in a signal chain the output stage and is the amplifier stage that requires attention to power efficiency.
Efficiency considerations lead to the various classes of power amplifier based on the biasing of the output transistors or tubes: see power amplifier classes below.
Audio power amplifiers are typically used to drive loudspeakers. They will often have two output channels and deliver equal power to each. An RF power amplifier is found in radio transmitter final stages. A Servo motor controller : amplifies a control voltage to adjust the speed of a motor, or the position of a motorized system. Operational amplifiers op-amps [ edit ] Main articles: Operational amplifier and Instrumentation amplifier 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. Main article: Distributed amplifier 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. Switched mode amplifiers[ edit ] 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.
Compared to other types of amplifiers, this "negative resistance amplifier" will only require a tiny amount of power to achieve very high gain, maintaining a good noise figure at the same time.
Certain requirements for step response and overshoot are necessary for an acceptable TV image. They typically can amplify across a broad spectrum of frequencies; however, they are usually not as tunable as klystrons.
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.
The maser is a non-electronic microwave amplifier. Musical instrument amplifiers[ edit ] Instrument amplifiers are a range of audio power amplifiers used to increase the sound level of musical instruments, for example guitars, during performances. Classification of amplifier stages and systems[ edit ] Common terminal[ edit ] 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. Unilateral or bilateral[ edit ] 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. 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.
Inverting or non-inverting[ edit ] 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. Voltage follower is also non inverting type of amplifier having unity gain. This description can apply to a single stage of an amplifier, or to a complete amplifier system.
Function[ edit ] 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. A servo amplifier indicates an integrated feedback loop to actively control the output at some desired level.
A DC servo indicates use at frequencies down to DC levels, where the rapid fluctuations of an audio or RF signal do not occur. These are often used in mechanical actuators, or devices such as DC motors that must maintain a constant speed or torque.
An AC servo amp. A linear amplifier responds to different frequency components independently, and does not generate harmonic distortion or intermodulation distortion.
No amplifier can provide perfect linearity even the most linear amplifier has some nonlinearities, since the amplifying devices— transistors or vacuum tubes —follow nonlinear power laws such as square-laws and rely on circuitry techniques to reduce those effects.
A nonlinear amplifier generates significant distortion and so changes the harmonic content; there are situations where this is useful. Amplifier circuits intentionally providing a non-linear transfer function include: a device like a silicon controlled rectifier or a transistor used as a switch may be employed to turn either fully on or off a load such as a lamp based on a threshold in a continuously variable input. Following such an amplifier with a so-called tank tuned circuit can reduce unwanted harmonics distortion sufficiently to make it useful in transmitters , or some desired harmonic may be selected by setting the resonant frequency of the tuned circuit to a higher frequency rather than fundamental frequency in frequency multiplier circuits.
Automatic gain control circuits require an amplifier's gain be controlled by the time-averaged amplitude so that the output amplitude varies little when weak stations are being received.
The non-linearities are assumed arranged so the relatively small signal amplitude suffers from little distortion cross-channel interference or intermodulation yet is still modulated by the relatively large gain-control DC voltage. Operational amplifier comparator and detector circuits.
A wideband amplifier has a precise amplification factor over a wide frequency range, and is often used to boost signals for relay in communications systems.
A narrowband amp amplifies a specific narrow range of frequencies, to the exclusion of other frequencies. An RF amplifier amplifies signals in the radio frequency range of the electromagnetic spectrum , and is often used to increase the sensitivity of a receiver or the output power of a transmitter. This category subdivides into small signal amplification, and power amps that are optimised to driving speakers , sometimes with multiple amps grouped together as separate or bridgeable channels to accommodate different audio reproduction requirements.
Frequently used terms within audio amplifiers include: Power amplifier normally drives loudspeakers , headphone amplifiers, and public address amplifiers. Stereo amplifiers imply two channels of output left and right , though the term simply means "solid" sound referring to three-dimensional —so quadraphonic stereo was used for amplifiers with four channels.
Buffer amplifiers , which may include emitter followers , provide a high impedance input for a device perhaps another amplifier, or perhaps an energy-hungry load such as lights that would otherwise draw too much current from the source. Line drivers are a type of buffer that feeds long or interference-prone interconnect cables, possibly with differential outputs through twisted pair cables. Interstage coupling method[ edit ] See also: multistage amplifiers Amplifiers are sometimes classified by the coupling method of the signal at the input, output, or between stages.
Different types of these include: Resistive-capacitive RC coupled amplifier, using a network of resistors and capacitors By design these amplifiers cannot amplify DC signals as the capacitors block the DC component of the input signal.
RC-coupled amplifiers were used very often in circuits with vacuum tubes or discrete transistors. In the days of the integrated circuit a few more transistors on a chip are much cheaper and smaller than a capacitor. Inductive-capacitive LC coupled amplifier, using a network of inductors and capacitors This kind of amplifier is most often used in selective radio-frequency circuits.
When a power transistor is driven below a collector current of about 15mA the amplification falls dramatically. If one could prevent the current in the output power-transistors from ever going below about 15mA and into this non-linear region, it would considerably improve things.
This change of amplification causes the crossover distortion characteristic of class-B amplifiers. Note that this crossover distortion should not be confused with the audio distortion, often also referred to as crossover distortion, which arises when audio signals are separated into frequency bands, as in loudspeaker circuits to feed the appropriate frequency range to each discrete driver unit. Transient Intermodulation Distortion Modern amplifier distortion is controlled by negative feedback, which reduces the distortion in proportion to the feedback.
Amplification is cheaply available so the apparent non-linearity can be reduced to arbitrarily low levels by sufficient feedback. But the feedback signal takes time to get through the amplifier and back to the input negatively to quash the distortion.
So when sudden changes transients occur there is a period during which the naked amplifier is exposed to the world, and the non-linearity adds intermodulation rogue signals to the original, which are not entirely canceled by the feedback. This is transient intermodulation distortion.
One of the reasons that modern amplifiers sound better than their older counterparts using essentially the same class A or B circuits as always is the increase in speed of the components. Multi-gigahertz discrete components are freely available, and even cheap power transistors have an ft of many MHz. This means that the feedback time is now very short indeed. Clipping Distortion and How to Soften It Transistor amplifiers driven into saturation sound horrible because the tops of the waveform are very sharply clipped off, leading to square corners and a huge explosion of unpleasant harmonics.
I find myself waiting to wince when a conventional amplifier is driven hard. A simple circuit invented by Carl F Wheatley, Jr. The complete Blomley Amplifier with complementary output and clipping protection. Note that the 0. It protects the output transistors from most abuse.
The very clever bit of the amplifier he describes is that Blomley split the incoming signal into top and bottom halves before applying the separate signals to the output transistors.
Additionally, he made the observation that with voltage signals, diodes are very non-linear, but if you use a current source, the diode is so close to the theoretical ideal that one can really call it perfect difference between forward and backward current in cheap diodes.
As shown in the schematic above, he used a constant-current source Tr6 and had a varying current sink Tr3. This effectively transfers signal splitting from the sub-amplifiers to a separate part of the circuit. The end result of this difficult-to-understand circuitry we are used to voltage circuits is a Class B amplifier that has a distortion lower than 0. And on an oscilloscope, there is no discernible crossover distortion with no feedback.
After a little feedback is applied, there is unmeasurable intermodulation distortion, transient intermodulation distortion, and harmonic distortion.
The resultant output of this amplifier is so clear that a recorded voice can easily be mistaken for a live person. And yet Peter Blomley and his amplifier have gone virtually unrecognized in the audio world for more than 40 years.
I suggest two reasons for this. First, his design was so original and so unexpected that few people understood it or took it seriously. Second, Blomley never put his design into commercial production because Plessey held the patent, so even fewer people were able to listen to it or review its performance.
Most of the audio hobbyists who constructed their own Blomley amplifier modified the design and in doing so introduced distortions. I suggest you build the original design with perhaps just the minor modifications afforded by modern components and listen to it.
This will give you a reference sound to check any further modifications with which you might like to experiment. Unfortunately, in ignoring the Blomley design for so long, the audio world has deprived itself of a fundamentally better amplifier.
We have instead put all our efforts over the last forty years in trying to mitigate what we thought were unavoidable inherent characteristics of electronic amplifiers, particularly Class B amplifiers. Today, superb high-voltage, high-speed transistors are available which makes the Blomley amplifier even better than his version. The original amplifier design was for a 30W amplifier with a 60V power-rail, and because of the purity, this is more than adequate for normal home use.
In , V small-signal transistors were rare, but this is no longer so and an 80V power-rail can now be used, with different transistors, increasing the power to 50W. However, high sound volumes are not needed as the sound is so exceptionally clean.
The huge headroom provided with most amplifiers is there so you can play them at high volume and bury the crossover-caused intermodulation distortion in the high sound-level quite sad really.
Conclusion 40 years ago I fell in love with the clarity and purity of the sound from the Blomley amplifier, but it took a long time to understand the circuit and to appreciate the brilliance of Peter Blomley. Now, I have built several of these amplifiers and, provided I stick to the original Blomley design, they all sounded better than superb.