Class E/F switching power amplifiers

Details Class E/F switching power amplifiers Class E/F switching power amplifiers

2001-10-09

2004-04-20

Nguyen, Patricia (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including class d amplifier

C330S20700P, C330S292000

Reexamination Certificate

active

06724255

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to high efficiency power amplifiers and more particularly to a new class of switching power amplifiers that is a hybrid of class E and inverse class F (class F
−1
) power amplifiers.
2. Description of the Related Art
Power amplifiers are classified in several different categories such as A, AB, B, C, D, E, F, S, etc. based on their fundamental characteristics, which relate to circuit topology and principle of operation. Each class presents relative advantages and disadvantages in their operating characteristics, such as linearity, power efficiency, bandwidth, frequency response, etc., and is chosen according to the application requirements.
More particularly, RF power amplification can be realized using active devices (i.e. transistors, vacuum tubes), that function as linear amplifiers, switching amplifiers or as a combination of both. Since linear amplifiers (e.g. classes A and B) are relatively inefficient at producing radio frequency (RF) output from an applied signal and direct current (DC) supply power, operating an active device as a linear amplifier is not an ideal solution for power amplifier applications requiring high efficiencies. Rather, designing the active device to operate as a switch is preferred because this mode of operation causes the device to be in a saturated or cut-off condition most of the time and therefore dissipates relatively little power by keeping the device out of the much lossier active region. In many applications, such as portable communication devices (e.g. cell phones) and high-power industrial generators (e.g. plasma drivers and broadcast transmitters), where low power consumption and low dissipation are crucial, high efficiency switching amplifiers are an attractive solution due to the performance and cost advantages they allow.
FIG. 1
simplified block diagram of a generic switching power amplifier
6
designed into a conventional RF transmission system
1
. The system includes a driver
4
, the power amplifier
6
, comprising a switch
5
and load network
7
, and a load
8
. The input signal
2
to be amplified is input to the driver stage
4
, which controls the active device
5
in the amplifier. The active device acts substantially as a switch when appropriately driven by the driver and thus is represented as a single-pole, single throw switch. The active device is powered by a dc power supply
3
, and has an output connected to the input of the load network
7
. The output of the load network
7
is connected to the load
8
, such as an antenna. As the switch
5
is cyclically operated at the desired output frequency, or fundamental frequency, f
o
, the dc energy is converted into ac energy at this switching frequency and its harmonics. The load network
7
may employ one or more filters to control the power dissipation caused by switching action (i.e. the efficiency of the device), reduce the level of the harmonic overtones at the load, and/or provide impedance transformation. The design of the load network determines the behavior of the voltage and currents in the switching amplifier
6
, and thus the class of operation by which the amplifier is denoted.
Realizing highly efficient switching operation at high frequencies, however, has been challenging due to finite switching times in the active device and package parasitic impedances. Nonetheless, among the known types of power amplifiers, when an application requires highly power-efficient amplification at high operating frequencies, ostensibly the most appropriate known types are class E and F amplifiers.
Class E Amplifiers
The class E amplifier achieves high efficiency at high frequencies by essentially eliminating the dominant cause of the switching power dissipation that occurs in other types of switching amplifiers, namely the loss associated with capacitive discharge. In virtually every switching-mode power amplifier, a capacitance, CS, shunts the power switch. At a minimum, this capacitance is the inherent parasitic capacitance, C
out
, of the circuit components (transistor) and wiring; the circuit designer might intentionally wish to add additional capacitance. In other types of switching amplifiers (other than the Class E amplifier), this shunt capacitance is typically undesirable. The reason is that if the switch is turned on when the voltage across the switch and its shunt capacitance is nonzero, the energy stored in the charged capacitance will be dissipated as heat; the energy is C
s
V
2
/2, where C
s
is the capacitance shunting the switch and V is the voltage across the switch (and hence across the capacitance) when the switch is turned on. If the switching frequency is f
o
, the power dissipation is C
s
V
2
f
o
/2. Note that the power dissipation is directly proportional to the switching frequency. Thus, for a high-frequency power amplifier, this power dissipation can become a severe drawback, often becoming the dominant power loss mechanism. Moreover, while the switch is discharging this capacitor, the switch is subjected to both the capacitor voltage and the discharge current, simultaneously. If the simultaneous voltage and current are large enough, they can cause destructive failure and/or performance degradation of the power transistor.
These difficulties can be avoided by ensuring Zero-Voltage-Switching (ZVS) operation, i.e. demanding that the voltage across the switch be substantially zero when the switch is turned on. This feature of the class-E amplifier allows this class to readily accommodate the switching device output capacitance without seriously degrading performance by using this capacitance in the load network and designing the load network so that the capacitor voltage is zero at just before the device turn-on.
In addition to the problems with turning on the switch, switching off (opening) a power switch inherently subjects it to simultaneous high voltage and high current (hence further power dissipation and device stress). Fortunately, unlike the turn-on loss, this loss mechanism can be made arbitrarily small by choosing a faster device or increasing the device drive level sufficiently so as to reduce the device turn-off time. Although it is possible to design a switching amplifier to achieve ZCS (zero-current-switched) operation, wherein the device current is zero just before the transistor switches off thereby eliminating turn-off loss, it is believed to be impossible to achieve ZVS and ZCS conditions simultaneously. While the turn-off loss can be reduced by other means, the turn-on loss is dependent only on the switching voltage and the capacitance, C
S
, which cannot be reduced arbitrarily as it is an inherent property of the active device. Therefore, ZVS switching has been found to be the most appropriate for high-efficiency operation using modern high-speed devices. By properly choosing the relative values of the circuit components (including the switch capacitance C
S
, the load resistance R
L
, and load inductance L
L
), class E therefore allows for ZVS switching to reduce switching loss using a very simple circuit.
Thus, with relatively simple circuit topology, class E operation achieves low power dissipation and low device stress by (a) incorporating the switch shunt capacitance as part of a network, allowing its detrimental effects to be accounted for and minimized and (b) using a resonant load network whose transient response after the switch turn-off brings the switch voltage back to zero (or nearly zero) at the time the switch will next be turned on. A schematic of a typical class E amplifier circuit is shown in the simplified diagram of FIG.
2
. The power amplifier
10
includes a switching device
12
and a load network
20
. DC power is supplied to the device
12
via a choke
14
. The network includes a simple filter
24
which is connected in series to an RL load, represented by L
L
26
, and R
L
28
, respectively. As a class E device, the filter acts as a short circuit at the fundamental frequency, and an open circuit at all harmonics.

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