Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2000-02-02
2003-02-11
Nguyen, Patricia T. (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S284000, C330S285000, C375S296000, C375S297000
Reexamination Certificate
active
06518840
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an amplifying circuit for a portable radiotelephone and, more particularly, to a circuit and method for controlling the power transmitted by a portable radiotelephone.
BACKGROUND OF THE INVENTION
A power amplifier (PA) is used in a wireless communication device such as a cellular radiotelephone to amplify radio frequency (RF) signals so that the device can communicate with a fixed site transceiver. Considerable power in a wireless communication device is dissipated in the power amplifier. For example, in a portable cellular radiotelephone, a significant percent of the power dissipation is in the power amplifier. Efficiencies of a power amplifier significantly depend upon the source and load variations the power amplifier experiences over the operating frequency of a cellular radiotelephone. One problem associated with designing a high-efficiency power amplifier is adequately compensating for these source and load variations.
Wireless communication devices typically transmit radio frequency signals at a plurality of power levels. For example, cellular radiotelephones require seven 4 dB steps in output power of the radio transmitter. However, the efficiency of the power amplifier significantly varies over the output power range. Because current drain efficiency of the power amplifier is most affected at a higher output power, the power amplifier is designed to maximize efficiency at higher output power levels. One technique to improve power efficiency requires switching the quiescent current of the power amplifier in response to a power amplifier output step change. At the lowest power step, the power amplifier is normally in class A mode of operation. By changing the bias conditions of the power amplifier at the lower steps, the power amplifier could be kept in class AB mode with a corresponding improvement in efficiency.
Operational efficiency is also particularly dependent on the load impedance variation caused by impedance changes of the duplex filter over a wide bandwidth or environmental factors such as the placement of a user's hand or body near the antenna. The power amplifier is generally designed with the saturated output power higher than the typical operating output power to accommodate power fluctuations caused by load impedance variations. To compensate for the reduced efficiency of the power amplifier designed at such a higher output power, the load at the output of the power amplifier can be varied by switching a diode in or out at the output. For example, a dual mode power amplifier operates in either the linear mode or the saturation mode based upon load switching. This load switching accommodates operation of the device on two different cellular radiotelephone systems. However, such discrete switching of the load at the output provides some improvement in efficiency, but does not maximize efficiency.
Improving the power amplifier efficiency is essential to increasing the operating time for a given battery of the wireless communication device. Accordingly, there is a need for a method and apparatus for amplifying a radio frequency signal with greater efficiency over source and load variations. There is also a need for operating a power amplifier efficiently over a wide range of operating voltages.
A portable (e.g., hand-held) radiotelephone (e.g., cellular phone) which is designed to operate in dual modes generally requires different radiating power levels for each of its modes. As such, each mode requires different battery power to operate. Conventional dual mode portable radiotelephones, however, are designed having power supplies (batteries) that generate power at levels to accommodate the mode requiring the most power to operate. As a result, unnecessary power is dissipated while the portable radiotelephone is operating in the mode, which requires less power, which results in a shortened battery life.
In addition to the different power requirements of dual mode portable radiotelephones, portable radiotelephones may also transmit signals at different power levels depending on the area in which they are operated. Specifically, the dual mode portable radiotelephone can effectively communicate with low power in a region exhibiting a strong electric field, i.e., in the proximity of a base station, but requires high power to communicate in a region exhibiting a weak electric field, i.e., in a remote place from the base station. In any event, dual mode portable radiotelephones are generally used in close proximity to the base station, i.e, a region of strong electric field. Therefore, on average, dual mode portable radiotelephones can operate with lower transmitting power levels.
In the conventional portable radiotelephone, however, the voltage applied to the power amplifier
30
for amplifying the transmission signals is set at 3.3 V in consideration of the situation where the portable radiotelephone is operating in a region of a weak electric field, which requires higher transmission power levels to communicate effectively. Therefore, since the 3.3 V applied to the power amplifier remains constant even when the portable radiotelephone is operating in a region exhibiting a strong electric field (which requires less power), unnecessary power is dissipated resulting in shortening the battery life. Moreover, the temperature of the portable radiotelephone increases, which adversely affects the circuitry.
Referring to
FIG. 1
, a diagram illustrating a power supply circuit of a conventional portable radiotelephone is shown. The power supply circuit of the conventional portable radiotelephone includes a power supply
10
(“battery”), a DC-to-DC Converter
20
connected to the output of battery
10
, an inductor L
1
connected to the output of the DC-to-DC converter, a pair of voltage dividing resistors R
1
and R
2
, which are serially connected between the output of the DC-to-DC converter
20
and a ground, a power amplifier
30
and a linear regulator
31
, which are connected to the output of the DC-to-DC converter
20
, and a capacitor C
1
connected between an input to the power amplifier
30
and ground.
The output voltage of the DC-to-DC converter
20
is determined by the voltage dividing resistors R
1
and R
2
. In the conventional method, the resistance values of resistors R
1
and R
2
are preset so that the required 3.3 V is applied to the power amplifier
30
.
In a dual stage power amplifier having a first pre-amplifying stage coupled with a second amplifying stage coupled with a transmitting antenna of a cellular phone it is desired to lessen the output power provided to the transmitting antenna under certain circumstances. For example, when a transmitting cellular phone is very close to a fixed site transceiver to which it is transmitting, there is no advantage in outputting a substantially high power output signal when less power will suffice. In other instances, it may be advantageous to completely switch off the transmitting amplifier for a short interval of time. This can be accomplished by providing less input power to the input stage of the dual stage amplifier, or, alternatively the output power of the transistors used as the first and second amplifying stages can be controlled so as to lessen the intensity of their output by controlling a biasing voltage thereon.
It is well known to control the output power of a first and second stage amplifying transistor of a two stage system as is shown in prior art
FIG. 2
by controlling the DC collector voltage provided to the collectors
26
a
and
26
b
of the first stage transistor
20
a
and the second stage transistor
20
b
respectively. Thus, by lowering VCC
1
and VCC
2
the output power at the terminal OUT of an input signal coupled into the terminal labeled IN is lessened. In this configuration for example with the antenna in a receive mode, switching off the second power transistor is accomplished by switching off the power from the battery VCC
2
. In order to lessen the power to the terminal OUT of the collector
26
b
a variable att
Julien Marquis Christian
Rahn David George
Freedman & Associates
Nguyen Patricia T.
SiGe Semiconductor Inc.
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