class D & class E Amplifiers

[umair khan]

class e switching

can anybody tell me about class d and class e amplifiers.
and also tell me in cell phones , which amplifier is used and why??

 

[ArjunSC]

class e switching amplifiers

A class D amplifier is a power amplifier where all power devices are operated in on/off mode. Output stages such as those used in pulse generators are examples of class D amplifiers. Mostly though, the term applies to devices intended to reproduce signals with a bandwidth well below the switching frequency. These amplifiers use pulse width modulation, pulse density modulation (sometimes referred to as pulse frequency modulation) or more advanced form of modulation such as Sigma delta modulation (see for example Analog Devices AD1990 Class-D audio power amplifier). The input signal is converted to a sequence of pulses whose averaged value is directly proportional to the amplitude of the signal at that time.

The class E/F amplifier is a highly efficient switching power amplifier, typically used at radio frequencies. The main concept used in this amplifier is to model the active switching device, such as a transistor or MOSFET, as a linear combination of two parts: (1) a (theoretical) "perfect" switching element, and (2) a complex network of parasitic elements attached to it (capacitors, inductors and resistors).

 

[neils_arm_strong]

I think cell-phones use class B amplifier.Because when there is no signal the ampp will be turned off and power is saved.

 

[electronics_kumar]

Radio frequency amplifiers having high efficiency..deeply cuttoff biased... class C will also comes under this RF power amplifiers

 

[jjohn]

A switching amplifier or class-D amplifier is a power amplifier where the active devices (especially in the output stage) are operated in on/off mode (i.e., as switches). Output stages such as those used in pulse generators are examples of class D amplifiers. However, the term mostly applies to devices intended to reproduce signals with a bandwidth well below the switching frequency. These amplifiers use pulse width modulation (PWM), pulse density modulation (sometimes referred to as pulse frequency modulation) or more advanced forms of modulation such as Sigma delta modulation (see for example Analog Devices AD1990 Class-D audio power amplifier). The input signal is converted to a sequence of pulses whose average value is directly proportional to the amplitude of the signal at that time. The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal. The final switching output consists of a train of pulses whose width is a function of the amplitude & frequency of the signal being amplified, and hence these amplifiers are also called PWM amplifiers. The output contains, in addition to the required amplified signal, unwanted spectral components (i.e. the pulse frequency and its harmonics) that must be removed by a passive filter. The filter is made using (theoretically) lossless components like inductors and capacitors in order to maintain efficiency.

A PWM amplifier operates similiar to a switching power supply (SMPS), except that the reference voltage of a PWM amplifier is the varying audio signal instead of a fixed voltage as in a switching power supply. A switching amplifier must not be confused with an amplifier that uses a SMPS. A switching amplifier may use any type of power supply but the amplifier itself uses switching of output devices though to achieve amplification.




The class E/F amplifier is a highly efficient switching power amplifier, typically used at radio frequencies. The main concept used in this amplifier is to model the active switching device, such as a transistor or MOSFET, as a linear combination of two parts: (1) a (theoretical) "perfect" switching element, and (2) a complex network of parasitic elements attached to it (capacitors, inductors and resistors). After the decomposition, it becomes trivial to eliminate the losses of each part:

(1) The "perfect" switching element should be switched ON during zero-voltage crossing, and should be switched OFF during zero-current crossing. Thus the switching element either conducts current, or has some non-zero voltage on it, but never both at the same time. Because the dissipated power is equal to current x voltage, it becomes zero. This can be arranged by adjusting the phase (capacitor) and DC bias (resistor) of the signal going into the transistor input.

(2) the Imaginary part of the impedance of the parasitic elements can be tuned, one by one, by matching them to another passive element with the complex conjugate impedance, thus leaving only the real part of the complex impedance.

In theory, the only remaining loss is the real part of the impedance of the parasitic elements in the system, which cannot be avoided. This class of amplifier is unique to radio frequency ranges, where the amplifier analysis is usually done in the frequency domain and not in the voltage/current domain. This class is further divided to subclasses depending on which harmonics of the signal are taken into account during zero-voltage switching (ZVS) and zero-current switching (ZCS), with names such as Class E/F2,odd; Class F^-1; and so on. It is still an active area of research and new variants appear from time to time, usually with the letters "E" and "F" somewhere in class name.