Amplifiers – With semiconductor amplifying device – Including temperature compensation means
Reexamination Certificate
1999-09-16
2001-04-10
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including temperature compensation means
C330S296000
Reexamination Certificate
active
06215358
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to wireless communications networks and, more specifically, to a system for biasing an RF amplifier in a wireless network base station.
BACKGROUND OF THE INVENTION
Wireless networks, including cellular telephone networks, have become ubiquitous in society. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is increased by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. To maximize usage of the available bandwidth in each cell, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
Every cellular base station has an RF transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. It is important that the RF power amplifier in a base station transmitter operate in a highly linear manner, especially when amplifying a signal whose envelope changes in time over a wide range, as in CDMA and multi-carrier systems. It also is important that the RF amplifier have good linearity characteristics across a wide range of operating conditions, because wireless systems cannot tolerate large amounts of signal distortion and may not violate adjacent channel power specifications, such as the IS 95 bandwidth requirements, regarding spectral spreading effects.
The output stage of an RF amplifier typically contains a high-power transistor, such as a class AB laterally diffused metal-oxide-silicon field-effect transistor (LDMOS FET), a gallium-arsenide (GaAs) FET, or, perhaps, a bipolar junction transistor (BJT). In order to maintain linear operation in the RF amplifier, the bias voltage of the output stage high-power transistor must be adjusted so that the bias current of the high-power transistor remains constant over a range of temperature.
For example, in an LDMOS FET, the gate-to-source bias voltage (V
gs
) must vary such that the quiescent current (I
dq
) remains constant as temperature rises. To maintain constant I
dq
over a temperature range, the gate voltage must decrease as temperature increases. The desired slope (mV/C) of the gate voltage varies from one device to the next due to process variation. If I
dq
is not constant over temperature, the device linearity or adjacent channel power ratio (ACPR) degrades. If the ACPR degrades, the RF amplifier output power must be reduced to the point at which it again complies with the J-STD-019 spectral mask. This reduction in output power decreases the overall range and capacity of cellular and PCS base stations.
One technique for biasing the output power transistor is to use a fixed-bias voltage. The fixed-bias approach is generally implemented with a simple voltage divider or adjustable reference voltage. Unfortunately, this technique is not capable of compensating the bias voltage over temperature, nor is it capable of compensating for lot-to-lot device variations. Furthermore, the fixed-bias technique is subject to thermal runaway. If the bias voltage is not temperature compensated, the bias current becomes very large with increased temperature. Under full RF drive conditions, the increase in bias current may become so large that the device overheats to the point of failure. Regardless of failure, the device mean-time-to-failure (MTTF) degrades with increased current and temperature.
Another technique for biasing the output power transistor involves the use of microprocessors and/or electronically programmable resistor arrays. This approach is much more complex and costly and requires input and output data from a master controller card. Furthermore, in order to measure and adjust the quiescent current, the RF input signal to the output power transistor must be temporarily shut off. Obviously, when the RF input signal is removed, the base station no longer transmits and all calls must be dropped. Thus, the base station must go out-of-service just prior to and during adjustment of the bias current.
There is therefore a need in the art for improved systems and methods of biasing the output power transistor of an RF amplifier to compensate for temperature variations. In particular, there is a need for temperature-compensated biasing networks for the output power transistor of an RF amplifier that are simple and inexpensive and that do not require that the base station be temporarily put out of service.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in an RF amplifier containing an output power transistor having a quiescent current set at a selected bias current level, a biasing circuit for maintaining the quiescent current at the selected bias level. The bias circuit comprises 1) a temperature sensor circuit capable of generating a temperature-sensitive control voltage that varies according to changes in temperature of the output power transistor; and 2) a bias voltage generating circuit capable of detecting a variation in the temperature-sensitive control voltage and, in response thereto, adjusting a bias voltage applied to the output power transistor by an amount suitable to offset a change in the selected bias current level caused by a temperature change related to the variation in the temperature-sensitive voltage.
According to one embodiment of the present invention, the bias voltage generating circuit comprises amplification means for scaling a voltage change in the temperature-sensitive control voltage to match a required voltage change in the bias voltage.
According to another embodiment of the present invention, the bias voltage generating circuit further comprises an adjustable voltage divider circuit for further scaling the voltage change in the temperature-sensitive control voltage to match the required voltage change in the bias voltage.
According to still another embodiment of the present invention, the bias voltage generating circuit comprises an operational amplifier having a first input coupled to an output of the temperature sensor circuit.
According to yet another embodiment of the present invention, the operational amplifier has a second input coupled to an output of a precision voltage reference circuit.
According to a further embodiment of the present invention, an output of the operational amplifier is proportional to a difference between the precision voltage reference circuit output and the temperature sensor circuit output.
According to another embodiment of the present invention, the precision voltage reference circuit output provides a DC offset voltage in the operational amplifier output.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize t
Appel Mark J.
Bednekoff George A.
Hon Terry
Choe Henry
Han John C.
Pascal Robert
Samsung Electronics Co,. Ltd.
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