Note: For clarity, signal periods are not shown to scale.
TO-220-3 N-Channel MOSFET are available at Mouser Electronics. Mouser offers inventory, pricing, & datasheets for TO-220-3 N-Channel MOSFET. Sketch the circuit diagram of a Mosfet d.c. Chopper supplying variable voltage to a resistive load. With the aid of a voltage waveform diagram, obtain an expression for the average load voltage. Draw voltage and current waveforms for a d.c. Chopper driving an R –L load. Assume continuous load current flow between switching. MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. The FET is operated in both depletion and enhancement modes of operation. The following figure shows how a practical MOSFET looks like. D-MOSFET “Depletion MOSFET” Depletion MOSFET or D-MOSFET is a type of MOSFET where the channel is constructed during the process of manufacturing. Therefore, the D-MOSFET can conduct between its drain and gate when the V GS = 0 volts. Therefore, D-MOSFET is also known as normally ON transistor. NTMFS4841N/D NTMFS4841N MOSFET – Power, Single, N-Channel, SO-8FL 30 V, 57 A Features.
A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, and not as linear gain devices as in other amplifiers. They operate by rapidly switching back and forth between the supply rails, being fed by a modulator using pulse width, pulse density, or related techniques to encode the audio input into a pulse train. The audio escapes through a simple low-pass filter into the loudspeaker. The high-frequency pulses are blocked. Since the pairs of output transistors are never conducting at the same time, there is no other path for current flow apart from the low-pass filter/loudspeaker. For this reason, efficiency can exceed 90%.
The first Class-D amplifier was invented by British scientist Alec Reeves in the 1950s and was first called by that name in 1955. The first commercial product was a kit module called the X-10 released by Sinclair Radionics in 1964. However, it had an output power of only 2.5 watts. The Sinclair X-20 in 1966 produced 20 watts, but suffered from the inconsistencies and limitations of the germanium-based BJT (bipolar junction transistor) transistors available at the time. As a result, these early class-D amplifiers were impractical and unsuccessful. Practical class-D amplifiers were later enabled by the development of silicon-based MOSFET (metal-oxide-semiconductor field-effect transistor) technology. In 1978, Sony introduced the TA-N88, the first class-D unit to employ power MOSFETs and a switched-mode power supply. There were subsequently rapid developments in VDMOS (verticalDMOS) technology between 1979 and 1985. The availability of low-cost, fast-switching MOSFETs led to Class-D amplifiers becoming successful in the mid-1980s. The first class-D amplifier based integrated circuit was released by Tripath in 1996, and it saw widespread use.
Class-D amplifiers work by generating a train of rectangular pulses of fixed amplitude but varying width and separation, or varying number per unit time, representing the amplitude variations of the analog audio input signal. The modulator clock can synchronize with an incoming digital audio signal, thus removing the necessity to convert the signal to analog. The output of the modulator is then used to gate the output transistors on and off alternately. Great care is taken to ensure that the pair of transistors are never allowed to conduct together, as this would cause a short circuit between the supply rails through the transistors. Since the transistors are either fully 'on' or fully 'off', they spend very little time in the linear region, and dissipate very little power. This is the main reason for their high efficiency. A simple low-pass filter consisting of an inductor and a capacitor provides a path for the low frequencies of the audio signal, leaving the high-frequency pulses behind. In cost sensitive applications the output filter is sometimes omitted. The circuit then relies on the inductance of the loudspeaker to keep the HF component from heating up the voice coil.
The structure of a class-D power stage is somewhat comparable to that of a synchronously rectified buck converter (a type of non-isolated switched-mode power supply (SMPS)), but works backwards. Whereas buck converters usually function as voltage regulators, delivering a constant DC voltage into a variable load, and can only source current (one-quadrant operation), a class-D amplifier delivers a constantly changing voltage into a fixed load, where current and voltage can independently change sign (four-quadrant operation). A switching amplifier must not be confused with linear amplifiers that use an SMPS as their source of DC power. A switching amplifier may use any type of power supply (e.g., a car battery or an internal SMPS), but the defining characteristic is that the amplification process itself operates by switching. Unlike a SMPS, the amplifier has a much more critical job to do, to keep unwanted artifacts out of the output. Feedback is almost always used, for the same reasons as in traditional analog amplifiers, to reduce noise and distortion.
Theoretical power efficiency of class-D amplifiers is 100%. That is to say, all of the power supplied to it is delivered to the load, none is turned to heat. This is because an ideal switch in its “on” state would conduct all the current but have no voltage loss across it, hence no heat would be dissipated. And when it is off, it would have the full supply voltage across it but no leak current flowing through it, and again no heat would be dissipated. Real-world power MOSFETs are not ideal switches, but practical efficiencies well over 90% are common. By contrast, linear AB-class amplifiers are always operated with both current flowing through and voltage standing across the power devices. An ideal class-B amplifier has a theoretical maximum efficiency of 78%. Class A amplifiers (purely linear, with the devices always 'on') have a theoretical maximum efficiency of 50% and some versions have efficiencies below 20%.
The term 'class D' is sometimes misunderstood as meaning a 'digital' amplifier. While some class-D amplifiers may indeed be controlled by digital circuits or include digital signal processing devices, the power stage deals with voltage and current as a function of non-quantized time. The smallest amount of noise, timing uncertainty, voltage ripple or any other non-ideality immediately results in an irreversible change of the output signal. The same errors in a digital system will only lead to incorrect results when they become so large that a signal representing a digit is distorted beyond recognition. Up to that point, non-idealities have no impact on the transmitted signal. Generally, digital signals are quantized in both amplitude and wavelength, while analog signals are quantized in one (e.g. PWM) or (usually) neither quantity.
The 2-level waveform is derived using pulse-width modulation (PWM), pulse density modulation (sometimes referred to as pulse frequency modulation), sliding mode control (more commonly called 'self-oscillating modulation' in the trade.) or discrete-time forms of modulation such as delta-sigma modulation.
The most basic way of creating the PWM signal is to use a high speed comparator ('C' in the block-diagram above) that compares a high frequency triangular wave with the audio input. This generates a series of pulses of which the duty cycle is directly proportional with the instantaneous value of the audio signal. The comparator then drives a MOS gate driver which in turn drives a pair of high-power switches (usually MOSFETs). This produces an amplified replica of the comparator's PWM signal. The output filter removes the high-frequency switching components of the PWM signal and recovers the audio information that the speaker can use.
DSP-based amplifiers which generate a PWM signal directly from a digital audio signal (e. g. SPDIF) either use a counter to time the pulse length or implement a digital equivalent of a triangle-based modulator. In either case, the time resolution afforded by practical clock frequencies is only a few hundredths of a switching period, which is not enough to ensure low noise. In effect, the pulse length gets quantized, resulting in quantization distortion. In both cases, negative feedback is applied inside the digital domain, forming a noise shaper which has lower noise in the audible frequency range.
Two significant design challenges for MOSFET driver circuits in class-D amplifiers are keeping dead times and linear mode operation as short as possible. 'Dead time' is the period during a switching transition when both output MOSFETs are driven into cut-off mode and both are 'off'. Dead times need to be as short as possible to maintain an accurate low-distortion output signal, but dead times that are too short cause the MOSFET that is switching on to start conducting before the MOSFET that is switching off has stopped conducting. The MOSFETs effectively short the output power supply through themselves in a condition known as 'shoot-through'. Meanwhile, the MOSFET drivers also need to drive the MOSFETs between switching states as fast as possible to minimize the amount of time a MOSFET is in linear mode—the state between cut-off mode and saturation mode where the MOSFET is neither fully on nor fully off and conducts current with a significant resistance, creating significant heat. Driver failures that allow shoot-through and/or too much linear mode operation result in excessive losses and sometimes catastrophic failure of the MOSFETs. There are also problems with using PWM for the modulator; as the audio level approaches 100%, the pulse width can get so narrow as to challenge the ability of the driver circuit and the MOSFET to respond. These pulses can get down to just a few nanoseconds and can result in the above undesired conditions of shoot-through and/or linear mode. This is why other modulation techniques such as pulse density modulation can get closer to the theoretical 100% efficiency than PWM.
The switching power stage generates both high dV/dt and dI/dt, which give rise to radiated emission whenever any part of the circuit is large enough to act as an antenna. In practice, this means the connecting wires and cables will be the most efficient radiators so most effort should go into preventing high-frequency signals reaching those:
- Avoid capacitive coupling from switching signals into the wiring.
- Avoid inductive coupling from various current loops in the power stage into the wiring.
- Use one unbroken ground plane and group all connectors together, in order to have a common RF reference for decoupling capacitors
- Include the equivalent series inductance of filter capacitors and the parasitic capacitance of filter inductors in the circuit model before selecting components.
- Wherever ringing is encountered, locate the inductive and capacitive parts of the resonant circuit that causes it, and use parallel RC or series RL snubbers to reduce the Q of the resonance.
- Do not make the MOSFETs switch any faster than needed to fulfil efficiency or distortion requirements. Distortion is more easily reduced using negative feedback than by speeding up switching.
Power supply design
Class-D amplifiers place an additional requirement on their power supply, namely that it be able to sink energy returning from the load. Reactive (capacitive or inductive) loads store energy during part of a cycle and release some of this energy back later. Linear amplifiers will dissipate this energy, class-D amplifiers return it to the power supply which should somehow be able to store it. In addition, half-bridge class D amplifiers transfer energy from one supply rail (e.g. the positive rail) to the other (e.g. the negative) depending on the sign of the output current. This happens regardless of whether the load is resistive or not. The supply should either have enough capacitive storage on both rails, or be able to transfer this energy back.
Active device selection
The active devices in a Class D amplifier need only act as controlled switches, and need not have particularly linear response to the control input. Bipolar transistors or field effect transistors are usually used. Vacuum tubes can be used as power switching devices in Class-D power audio amplifiers. 
The actual output of the amplifier is not just dependent on the content of the modulated PWM signal. The power supply voltage directly amplitude-modulates the output voltage, dead time errors make the output impedance non-linear and the output filter has a strongly load-dependent frequency response. An effective way to combat errors, regardless of their source, is negative feedback. A feedback loop including the output stage can be made using a simple integrator. To include the output filter, a PID controller is used, sometimes with additional integrating terms. The need to feed the actual output signal back into the modulator makes the direct generation of PWM from a SPDIF source unattractive. Mitigating the same issues in an amplifier without feedback requires addressing each separately at the source. Power supply modulation can be partially canceled by measuring the supply voltage to adjust signal gain before calculating the PWM and distortion can be reduced by switching faster. The output impedance cannot be controlled other than through feedback.
The major advantage of a class-D amplifier is that it can be more efficient than a linear amplifier, with less power dissipated as heat in the active devices. Given that large heat sinks are not required, Class-D amplifiers are much lighter weight than class A, B, or AB amplifiers, an important consideration with portable sound reinforcement system equipment and bass amplifiers. Output stages such as those used in pulse generators are examples of class-D amplifiers. However, the term mostly applies to power amplifiers intended to reproduce audio signals with a bandwidth well below the switching frequency.
- Home theater in a box systems. These economical home cinema systems are almost universally equipped with class-D amplifiers. On account of modest performance requirements and straightforward design, direct conversion from digital audio to PWM without feedback is most common.
- Mobile phones. The internal loudspeaker is driven by up to 1 W. Class D is used to preserve battery lifetime.
- Hearing aids. The miniature loudspeaker (known as the receiver) is directly driven by a class-D amplifier to maximise battery life and can provide saturation levels of 130 dB SPL or more.
- High-end audio is generally conservative with regards to adopting new technologies but class-D amplifiers have made an appearance
- Active subwoofers
- Sound reinforcement systems. For very high power amplification the power loss of AB amplifiers is unacceptable. Amplifiers with several kilowatts of output power are available as class-D. Class-D power amplifiers are available that are rated at 1500 W per channel, yet weigh only 21 kg (46 lb).
- Radio frequency amplifiers may use Class D or other switch-mode classes to provide high efficiency RF power amplification in communications systems. 
- Class-A amplifier (a linear, non-PWM amplifier class)
- Class-AB amplifier (a linear, non-PWM amplifier class)
- Class-B amplifier (a linear, non-PWM amplifier class)
- Class-C amplifier (a non-PWM amplifier class)
- Class-T amplifier (a proprietary implementation of class D)
- Sinclair Radionics, which sold one of the first commercial Class-D amplifiers in 1964
- ^Duncan, Ben (1996). High Performance Audio Power Amplifiers. Newnes. pp. 147–148. ISBN9780750626293.
- ^'Class-D Audio: The Power and the Glory'. IEEE Spectrum.
- ^The generic analysis of sliding mode control is quite math heavy. The specific case of 2-state self-oscillating class-D amplifiers is much more intuitive and can be found in Globally Modulated Self-Oscillating Amplifier with Improved Linearity, 37th AES Conference
- ^The Analog DevicesAD1990 class-D audio power amplifier is an example.
- ^Sandler et al., Ultra-Low Distortion Digital Power Amplification, Presented at the 91st AES convention
- ^Analytical and numerical analysis of dead-time distortion in power inverters
- ^'IRAUDAMP7S, 25W-500W Scalable Output Power Class D Audio Power Amplifier Reference Design, Using the IRS2092S Protected Digital Audio Driver'(PDF). irf.com. October 28, 2009. p. 26.
- ^Rampin M., 2015. AmpDiVa White Paper - On the use of vacuum tubes as switching devices in Class-D power audio amplifiers
- ^Putzeys et al. All Amplifiers etc., Presented at the AES 120th conventionArchived 2011-07-24 at the Wayback Machine
- ^Boudreaux, Randy, Real-Time Power Supply Feedback Reduces Power Conversion Requirements For Digital Class D Amplifiers
- ^'Group review of 'high end' class D offerings and round-table discussion with amplifier designers'.
- ^'Home > Products > CD 3000(r)'. Crest Audio. Archived from the original on 2012-11-09. Retrieved 2013-07-16.
- ^Andrei Grebennikov, Nathan O. Sokal, Marc J Franco, Switchmode RF Power Amplifiers, Newnes, 2011, ISBN0080550649, page vii
- Sánchez Moreno, Sergio (June 2005). 'Class-D Audio Amplifiers - Theory and Design'
- Haber, Eric Designing With class-D amplifier ICs – some IC-oriented Class D design considerations
- Harden, Paul Introduction to Class C,D,E and F, The Handiman's Guide to MOSFET 'Switched Mode' Amplifiers, Part 1 – an article on basic digital RF amplifier design intended for ham radio operators but applicable to audio class-D amplifiers
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FETs have a few disadvantages like high drain resistance, moderate input impedance and slower operation. To overcome these disadvantages, the MOSFET which is an advanced FET is invented.
MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. The FET is operated in both depletion and enhancement modes of operation. The following figure shows how a practical MOSFET looks like.
Construction of a MOSFET
The construction of a MOSFET is a bit similar to the FET. An oxide layer is deposited on the substrate to which the gate terminal is connected. This oxide layer acts as an insulator (sio2 insulates from the substrate), and hence the MOSFET has another name as IGFET. In the construction of MOSFET, a lightly doped substrate, is diffused with a heavily doped region. Depending upon the substrate used, they are called as P-type and N-type MOSFETs.
The following figure shows the construction of a MOSFET.
The voltage at gate controls the operation of the MOSFET. In this case, both positive and negative voltages can be applied on the gate as it is insulated from the channel. With negative gate bias voltage, it acts as depletion MOSFET while with positive gate bias voltage it acts as an Enhancement MOSFET.
Classification of MOSFETs
Depending upon the type of materials used in the construction, and the type of operation, the MOSFETs are classified as in the following figure.
After the classification, let us go through the symbols of MOSFET.
The N-channel MOSFETs are simply called as NMOS. The symbols for N-channel MOSFET are as given below.
The P-channel MOSFETs are simply called as PMOS. The symbols for P-channel MOSFET are as given below.
Now, let us go through the constructional details of an N-channel MOSFET. Usually an NChannel MOSFET is considered for explanation as this one is mostly used. Also, there is no need to mention that the study of one type explains the other too.
Construction of N- Channel MOSFET
Let us consider an N-channel MOSFET to understand its working. A lightly doped P-type substrate is taken into which two heavily doped N-type regions are diffused, which act as source and drain. Between these two N+ regions, there occurs diffusion to form an Nchannel, connecting drain and source.
A thin layer of Silicon dioxide (SiO2) is grown over the entire surface and holes are made to draw ohmic contacts for drain and source terminals. A conducting layer of aluminum is laid over the entire channel, upon this SiO2 layer from source to drain which constitutes the gate. The SiO2 substrate is connected to the common or ground terminals.
Because of its construction, the MOSFET has a very less chip area than BJT, which is 5% of the occupancy when compared to bipolar junction transistor. This device can be operated in modes. They are depletion and enhancement modes. Let us try to get into the details.
Working of N - Channel (depletion mode) MOSFET
For now, we have an idea that there is no PN junction present between gate and channel in this, unlike a FET. We can also observe that, the diffused channel N (between two N+ regions), the insulating dielectric SiO2 and the aluminum metal layer of the gate together form a parallel plate capacitor.
If the NMOS has to be worked in depletion mode, the gate terminal should be at negative potential while drain is at positive potential, as shown in the following figure.
When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some negative voltage is applied at VGG. Then the minority carriers i.e. holes, get attracted and settle near SiO2 layer. But the majority carriers, i.e., electrons get repelled.
With some amount of negative potential at VGG a certain amount of drain current ID flows through source to drain. When this negative potential is further increased, the electrons get depleted and the current ID decreases. Hence the more negative the applied VGG, the lesser the value of drain current ID will be.
The channel nearer to drain gets more depleted than at source (like in FET) and the current flow decreases due to this effect. Hence it is called as depletion mode MOSFET.
Working of N-Channel MOSFET (Enhancement Mode)
Mosfet De Potencia
The same MOSFET can be worked in enhancement mode, if we can change the polarities of the voltage VGG. So, let us consider the MOSFET with gate source voltage VGG being positive as shown in the following figure.
When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some positive voltage is applied at VGG. Then the minority carriers i.e. holes, get repelled and the majority carriers i.e. electrons gets attracted towards the SiO2 layer.
With some amount of positive potential at VGG a certain amount of drain current ID flows through source to drain. When this positive potential is further increased, the current ID increases due to the flow of electrons from source and these are pushed further due to the voltage applied at VGG. Hence the more positive the applied VGG, the more the value of drain current ID will be. The current flow gets enhanced due to the increase in electron flow better than in depletion mode. Hence this mode is termed as Enhanced Mode MOSFET.
P - Channel MOSFET
The construction and working of a PMOS is same as NMOS. A lightly doped n-substrate is taken into which two heavily doped P+ regions are diffused. These two P+ regions act as source and drain. A thin layer of SiO2 is grown over the surface. Holes are cut through this layer to make contacts with P+ regions, as shown in the following figure.
Working of PMOS
When the gate terminal is given a negative potential at VGG than the drain source voltage VDD, then due to the P+ regions present, the hole current is increased through the diffused P channel and the PMOS works in Enhancement Mode.
When the gate terminal is given a positive potential at VGG than the drain source voltage VDD, then due to the repulsion, the depletion occurs due to which the flow of current reduces. Thus PMOS works in Depletion Mode. Though the construction differs, the working is similar in both the type of MOSFETs. Hence with the change in voltage polarity both of the types can be used in both the modes.
This can be better understood by having an idea on the drain characteristics curve.
The drain characteristics of a MOSFET are drawn between the drain current ID and the drain source voltage VDS. The characteristic curve is as shown below for different values of inputs.
Actually when VDS is increased, the drain current ID should increase, but due to the applied VGS, the drain current is controlled at certain level. Hence the gate current controls the output drain current.
Transfer characteristics define the change in the value of VDS with the change in ID and VGS in both depletion and enhancement modes. The below transfer characteristic curve is drawn for drain current versus gate to source voltage.
Comparison between BJT, FET and MOSFET
Mosfet Drain Current
Now that we have discussed all the above three, let us try to compare some of their properties.
|Device type||Current controlled||Voltage controlled||Voltage Controlled|
|Operational modes||No modes||Depletion mode only||Both Enhancement and Depletion modes|
|Input impedance||Low||High||Very high|
So far, we have discussed various electronic components and their types along with their construction and working. All of these components have various uses in the electronics field. To have a practical knowledge on how these components are used in practical circuits, please refer to the ELECTRONIC CIRCUITS tutorial.