Amplifiers – With semiconductor amplifying device – Including particular biasing arrangement
Reexamination Certificate
2000-07-31
2002-02-26
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including particular biasing arrangement
C455S126000
Reexamination Certificate
active
06351189
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to electrical amplification, and more particularly to an auto-bias system and method for an amplifier.
BACKGROUND OF THE INVENTION
Mobile cellular communications, like many other electronic applications, requires a bias circuit to bias an amplifier that is used, for example, in the transmitter of a cellular base station or mobile telephone. In one conventional RF amplifier using a bipolar transistor, the general bias method for the RF amplifier has been to set a fixed DC-voltage to the base of the transistor. The collector current of the RF transistor is controlled by way of adjusting the DC-voltage during the production process in manufacturing the device using, for example, a variable resistor and diode. Once the bias is adjusted and set in production the bias of the amplifier remains substantially the same unless manually altered in the field. One such circuit is shown in FIG.
1
.
In the conventional amplifier shown in
FIG. 1
, the base voltage of an RF transistor is set by transferring the knee voltage of the diode D
1
to the base of the bipolar transistor Q
1
via the coil L
2
. The base voltage of transistor Q
1
is adjusted by varying the resistance of variable resistor VR
1
so as to control the bias current of the diode. Once the bias voltage is set during production of the circuit by adjusting the variable resistor VR
1
, it generally is not changed again even though the component characteristics will change over time. In the conventional bias method the power provided by Vcc to the RF transistor Q
1
via coil L
1
and to the bias circuit series variable resistor VR
1
and diode D
1
are provided by separate paths and generally operate independently such that changes in the RF transistor Q
1
operating characteristics due to, for example, changes in the RF transistor Q
1
operating temperature, does not track the changes in the bias voltage provided by the series variable resistor VR
1
and diode D
1
. Thus, the bias voltage provided to the base of the RF transistor does not track the electrical characteristic changes of the RF transistor Q
1
and the bias voltage does not provide a sufficiently stable operating point for the RF transistor in all circumstances.
Further, the conventional bias method illustrated in
FIG. 1
has the following problems and/or disadvantages. First, the bias circuit needs tuning during production which takes time and increases the risk of error in setting the correct bias voltage supplied to the base of RF transistor Q
1
. Second, the conventional circuit has some inherent temperature stabilization because D
1
and Q
1
have almost the same, but not exactly the same temperature characteristics. Thus the conventional circuit often needs an extra temperature compensation circuit added to provide the necessary variation in the bias voltage so as to stabilize the amplifier operation as required by some applications. For example, one such temperature compensation circuit is provided by adding a positive temperature coefficient (PTC) resistor connected in series with VR
1
. This causes the total resistance from Vcc to D
1
to increase when the temperature rises, thus decreasing the base voltage of Q
1
and collector current of Q
1
. However, even with the addition of such a temperature compensation circuit the conventional method of biasing results in a bias condition that tends to drift as a function of temperature, because the temperature compensation circuit is not exactly at the same temperature as the RF transistor Q
1
at various times during circuit operation given that the RF transistor Q
1
and the temperature compensation circuit are in different physical locations. Third, the thermal matching of the transistor Q
1
and diode D
1
pair has unit to unit variation so even though the transistor Q
1
and diode D
1
pair are matched as best as possible at their nominal values, the use of a particular transistor for transistor Q
1
and a particular diode for diode D
1
does not generally result in perfect thermal matching. Fourth, in high power conditions the RF transistor Q
1
is at higher temperature than the diode D
1
and causes more inaccuracy to the thermal compensation (i.e., power related temperature transients). Finally, the conventional bias method requires that during the design phase every different transistor type (e.g., transistors having different electrical and temperature characteristics) that is to be used as the transistor Q
1
in the amplifier requires a different individual thermal compensation design so as to provide a design that is properly temperature compensated. Different transistor types occur, for example, when the RF transistor Q
1
will be provided by more than one manufacturer and there is manufacturer-to-manufacturer variation or when a different style of transistor is going to be used, e.g., bipolar, MOSFET, LDMOS, or GASFET.
SUMMARY OF THE INVENTION
The present invention overcomes many of the foregoing problems and/or disadvantages by providing a method and apparatus for auto-biasing an amplifier. The invention is particularly useful in biasing non-linear amplifiers and amplifiers whose input signal are amplitude modulated (AM) or includes sufficient amplitude variations. The auto-bias system of the present invention has an auto-bias feedback loop that continuously adjusts the bias condition of an amplifier to a wanted state during amplifier operation by monitoring the operating state of the amplifier and controlling the amplifier bias so as to control the amplifier operating point sufficiently to compensate for variations in amplifier electrical characteristics, amplifier load, amplifier temperature, and input signals. The system further adjusts the amplifier operating point based on the modulation scheme used to modulate information included in an input signal provided to the amplifier, thereby allowing the amplifier to operate in any one of multiple signal modulation systems. Further, the invention eliminates the need for manually adjusting the amplifier bias during production and enables use of any transistor type in the amplifier.
According to one variation of the invention, an auto-bias feedback loop is provided which includes a bias control feedback circuit connected to an amplifier. The bias control feedback circuit measures an operating parameter of the amplifier and adjusts a bias level of the amplifier based on the measured operating parameter when the bias control feedback circuit is closed. When the bias control feedback circuit is open, the bias control feedback circuit holds the bias level of the amplifier that was set when the bias control feedback circuit was closed. According to another variation of the invention, the auto-bias feedback loop is included in a transmitter of a communication device, which communication device further includes a processor coupled to the transmitter. The processor controls the opening and closing of the bias control feedback circuit.
According to a further variation of the invention, an auto-bias feedback loop is provided which includes a bias control feedback circuit connected to an amplifier. The bias control feedback circuit receives an information signal from an information signal source, which information signal is based on a signal modulation scheme. The bias control feedback circuit measures an operating parameter of the amplifier and adjusts a bias level of the amplifier based on the measured operating parameter and the signal modulation scheme. According to yet another variation of the invention, the auto-bias feedback loop is included in a transmitter in a communication device. The communication device further includes an information source and a processor that are each coupled to each other and to the transmitter. The information source sources the information input signal to the bias control feedback circuit. The processor produces a control signal that is based on the signal modulation scheme and is conveyed to the bias control feedback circuit. The bias control feedback c
Banner & Witcoff , Ltd.
Nguyen Khanh Van
Nokia Networks Oy
Pascal Robert
LandOfFree
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