1. Technical Field
This invention relates generally to amplifiers and in particular to a system for temperature compensation to improve bias circuit performance that can in turn improve amplifier performance.
2. Related Art
Historically, power amplifiers have had to meet performance specifications based on power output requirements over a temperature range. One such temperature range might be from xe2x88x9230 degrees C. to +85 degrees C. To meet the performance specification, the power amplifier may be required to maintain a specified degree of linear amplification throughout the operating temperature range. Linear amplification of a signal is important to achieve transmission of signals with amplitude information as well as phase information.
These temperature ranges can be problematic for power amplifiers because semiconductor devices are known to behave differently at different temperatures. For a given current bias condition, the turn-on voltage changes due to the natural behavior of a transistor over temperature ranges. Thus, maintaining linear operation over a range of temperatures challenges circuit designers because the amplifier DC bias condition also changes over a range of temperatures. Current mirror circuits are often used to control the bias conditions for a circuit with improved accuracy. During mass production, variations that occur in the current mirror also occur in the radio frequency transistors. A current mirror also provides a low impedance node to meet linearity and current capability requirements. However, at low voltages the current mirror becomes sensitive to the reference voltage.
One proposed solution to the variation in output linearity varying with temperature is to simply use a supply voltage of sufficient magnitude to insure adequate operation throughout the temperature variation. However, the trend in modern communication systems is to reduce the supply voltage. While such reductions improve certain aspects of operation, such as size, weight, and battery life, maintaining linear operation is complicated. For example, at an external regulator voltage of 3.1 volts (Vreference) a small change in the behavior of a power amplifier may change the internal regulator voltage, such as to 2.8 volts. This is sufficient enough percentage of the 3.1 volts reference voltage to adversely affect the biasing conditions in a system.
Another proposed solution to maintain linearity over a temperature range is to increase bias voltage and/or reduce the amplifier""s efficiency. This will increase the linearity margin at normal conditions to meet the specification over the desired temperature range. However, this proposed solution suffers from the disadvantage of greater power usage causing shorter battery life.
Therefore, a need exists for a system to control the linearity of an amplifier""s output over a range of operating temperatures. This need is increased when operating voltages decrease. This invention overcomes the disadvantages of the prior art by providing a system for semiconductor temperature compensation by decreasing the bias current variation over the operating temperature range.
This invention provides a temperature compensation system for use in an amplifier system operating over a range of temperatures. Such an amplifier system may be found in typical wireless communication devices. When these wireless communication devices have circuitry configured to transmit signals their output should operate within a predefined specification over a range of varying temperatures. In particular, at higher operating temperatures, semiconductor devices draw more current. While at lower temperatures, these devices draw less current. In many applications it is desired that the semiconductor devices draw a relatively constant amount of current.
The temperature compensation system of the invention automatically adjusts the current flow into the reference node of the current mirror. This compensates for the changes in current flow in the amplifier that occur over the range of temperatures encountered by the communication device. Thus, at normal temperatures, the compensation scheme is configured to have little affect on the current flow into the amplifier device.
At low temperatures, due to the low control voltage, the current flow in an amplifier device without compensation undesirably decreases as compared to operation at normal operating temperature due to the behavior of the semiconductor devices in the bias circuit. Thus, the compensation module of the invention is configured to increase or not reduce the current flow to the amplifier device at low temperatures.
At high temperatures, due to the low control voltage, the current flow in an amplifier bias circuit without compensation undesirably increases as compared to operation at normal operating temperature. Thus, the compensation module of the invention is configured to decrease the current flow to the amplifier device at high temperatures.
In one embodiment, the temperature compensation system has one or more semiconductor devices in series with one or more resistors. In such a configuration, the number of semiconductor devices in the temperature compensation system may be greater than the number of semiconductor devices in the regulator or amplifier device. Such a ratio insures that the effect, caused by temperature, on the compensation module is greater than the effect on the regulator or amplifier device. This difference in behavior, between the compensation system and the amplifier device, insures that the compensation system adequately compensates the current flowing in the amplifier device. In other embodiments the compensation system may automatically and/or inherently react to the changes in temperature or work in unison with a temperature monitor. Likewise, the invention may be configured for use with a scheme that does not have an amplifier device.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.