1. 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.
2. Description of the Related Art
Mobile cellular communications, like many other electronic applications, requires a bias circuit to bias an amplifier which 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 filed. 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 D1 to the base of the bipolar transistor Q1 via the coil L2. The base voltage of transistor Q1 is adjusted by varying the resistance of variable resistor VR1 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 VR1, 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 Q1 via coil L1 and to the bias circuit series variable resistor VR1 and diode D1 are provided by separate paths and generally operate independently such that changes in the RF transistor Q1 operating characteristics due to, for example, changes in the RF transistor Q1 operating temperature, does not track the changes in the bias voltage provided by the series variable resistor VR1 and diode D1. Thus, the bias voltage provided to the base of the RF transistor does not track the electrical characteristic changes of the RF transistor Q1 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 Q1. Second, the conventional circuit has some inherent temperature stabilization because D1 and Q1 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 VR1. This causes the total resistance from Vcc to D1 to increase when the temperature rises, thus decreasing the base voltage of Q1 and collector current of Q1. 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 Q1 at various times during circuit operation given that the RF transistor Q1 and the temperature compensation circuit are in different physical locations. Third, the thermal matching of the transistor Q1 and diode D1 pair has unit to unit variation so even though the transistor Q1 and diode D1 pair are matched as best as possible at their nominal values, the use of a particular transistor for transistor Q1 and a particular diode for diode D1 does not generally result in perfect thermal matching. Fourth, in high power conditions the RF transistor Q1 is at higher temperature than the diode D1 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 Q1 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 Q1 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.
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 active 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. In the case of an amplitude varying input signal, the auto-bias system of the present invention provides a sample and hold function so that the amplifier bias is the same during high power periods and low power periods, even though during high power periods the amplifier may be operating in a non-linear region. Further, the present amplifier auto-bias system 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 active bias feedback loop is provide which includes a bias measurement device connected to an amplifier transistor so as to measure a physical quantity, for example, a voltage, current, temperature etc. The measured physical quantity is provided to a control circuit which monitors the physical quantity and continuously adjusts its output according to variations in the physical quantity. The output of the control circuit (alternatively the output of the measurement device) is input to a sample and hold device, e.g., a capacitor, and to the bias circuit so as to vary the amplifier bias in a manner that will provide a relatively stable amplifier operating point in response to variation in the physical quantity being measured. Thus, the bias method of the present invention continuously adjusts the amplifier bias so as to improve the amplifier characteristics by maintaining a desired amplifier operating level.
According to another variation of the invention, a bias measurement is provided by measuring a current provided to an input terminal (e.g., collector) of an amplifier RF transistor. Measurement and control circuits includes a parallel set of series resistors, which act as voltage dividers, and a sample amplifier, which in conjunction operate to provide a varying voltage to a sample and hold capacitor based on a measured transistor input voltage. The sample and hold capacitor is connected to the input of an amplifier (e.g., a unity amplifier) which provides the desired bias voltage to the control terminal of the RF transistor. In operation, the sample and hold capacitor is charged to the desired RF transistor bias voltage by the output of the measurement and control circuits during the class-A (low power) operation as a function of, for example, the input voltage to the transistor terminal biased by VCC, and maintains that bias voltage during the class-AB (high power) operation of the amplifier.
According to an even further variation of the invention, the pair of series resistors and the sample amplifier are replaced with a sampling transistor biased with a reference voltage, which acts as the measurement and control circuit. In operation, the sample and hold capacitor is charged to the desired RF transistor bias voltage by the output of the sampling transistor during the class-A (low power) operation as a function of, for example, the input current to the transistor terminal biased by VCC, and maintains that bias voltage during the class-AB (high power) operation of the amplifier.
The invention is particularly useful for setting an amplifier bias in general because the bias control is automated and generally more accurate over time and temperature. Generally, the bias method of the present invention improves the amplifier characteristics, for example, current, temperature compensation, frequency response, and power. The invention provides various particular advantages which includes: (1) enabling the use of any type of transistor in the amplifier and eliminating the need for separate bias circuit design for various particular transistor types or transistors from different manufacturers; (2) eliminating the need to tune the amplifier in production because it is automatically tuned based on the design of the auto-bias system; (3) eliminating the need for providing an automatic temperature compensation feature because such compensation is inherent in the auto-biasing system of the present invention; and (4) eliminating the long term drift effects of bias parameters as well as a lag in the bias tracking that may result due to rapid amplifier loading.
The invention is also particularly useful for RF amplifiers and for biasing amplifiers that must operate both in linear and non-linear regions. As a result of using the auto-bias invention, the same desired transistor bias may be provided during both class-A operation and class-AB operation of the amplifier when using an AM or amplitude varying input signal.