Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-05-23
2003-03-04
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S540000, C323S315000
Reexamination Certificate
active
06529065
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a circuit configuration for controlling operating points of a power amplifier, in particular such as those used for mobile telephones.
In many applications, linear power amplifiers are operated with a very low quiescent current in order to achieve high efficiency levels, in particular with reduced output power levels. The electrical characteristics of such an amplifier used in the Class AB mode are very heavily dependent on the value of the quiescent current. Within the overall temperature range in which the power amplifier is operated, a constant quiescent current is a precondition for constant and reproducible electrical characteristic data.
If the supply voltages are low, quiescent current control is additionally complicated by the fact that the available voltage range of the amplifier is tightly limited. Exact quiescent current adjustment is problematic, especially in the case of power amplifiers using heterobipolar transistors with high base-emitter forward voltages of, typically, 1.3 V and a supply voltage of 3 V.
Circuit configurations known in the prior art for adjusting the operating point for a power amplifier have the problem that the maximum available voltage range is not sufficient to ensure stable adjustment of the operating point.
If the operating point of the power amplifier is configured for a low quiescent current, then, the efficiency of the power amplifier rises and thus the operating duration of an appliance with a limited energy reservoir increases. On the other hand, the maximum amplification power is limited if the quiescent current is low. In many applications, in particular including mobile radio technology, the amplification power requirements vary, however, so that it is difficult to find an optimum operating point for the amplifier if it is also necessary for the power amplifier to have as high an efficiency level as possible at the same time.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a circuit configuration for controlling the operating point of a power amplifier that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the quiescent current is as constant as possible irrespective of the temperature, thus ensuring an operating point which is as stable as possible even when the quiescent current is low. A further object of the invention is to control the operating point such that the efficiency of the power amplifier is as high as possible even if the amplification power levels vary.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for active control of an operating point of a bipolar transistor in a power amplifier circuit. The circuit configuration contains a current mirror circuit having a first bipolar transistor and a second bipolar transistor. The first bipolar transistor has a control input, a collector and an emitter, and the second bipolar transistor has a control input, a collector and an emitter. A third transistor is connected as a common emitter amplifier circuit and has a control input, a collector and an emitter. The control input of the third transistor is connected to the collector of the second bipolar transistor. A fourth transistor is connected as a common emitter amplifier circuit and has a control input, a collector and an emitter. The control input of the fourth transistor is connected to the collector of the third transistor. A fifth transistor is connected as a common collector circuit and has a control input, a collector and an emitter. The control input of the fifth transistor is connected to the collector of the fourth transistor and the emitter of the fifth transistor is connected to the control input of the second bipolar transistor. A resistor is connected to the collector of the second bipolar transistor, and a voltage source is connected to the resistor and through the resistor the voltage source is connected to the collector of the second bipolar transistor.
The term current mirror circuit should be understood in a wide sense here. The current mirror circuit is formed by two transistors whose control inputs are connected to one another, so that a collector current in one transistor produces a mirrored collector current in the other transistor. Mirrored in this context results in that both the currents behave linearly with respect to one another in the area of interest, in an ideal situation. In this case, with regard to process technology, the two transistors are preferably physically identical, so that the linearity of the two currents with respect to one another is maintained even, for example, in the event of temperature fluctuations. If the two transistors also have the same emitter-base area and if the emitters of the two transistors are at the same potential, then the ratio of the two currents is essentially unity.
The circuit configuration according to the invention allows the current through the collector of the second bipolar transistor to be regulated at a fixed value even in the event of temperature fluctuations. The quiescent current thus also remains constant in the first bipolar transistor, so that the operating point of the bipolar transistor connected in the amplifier circuit is more stable. Control of the current through the collector of the second bipolar transistor is ensured by the active feedback circuit from the collector of the second bipolar transistor to the control input of the second bipolar transistor. The common collector circuit produced with the fifth transistor results in that the output of the feedback circuit also has a low output impedance, so that high output power levels can also be accepted on the first bipolar transistor. Furthermore, the circuit according to the invention has the advantage that the possible voltage range on the first bipolar transistor is wide, since only one diode voltage need be subtracted from the total available voltage.
In one preferred embodiment, the control input of the first bipolar transistor and the control input of the second bipolar transistor are connected to one another via a resistor. The resistor allows the amplifier linearity of the first bipolar transistor to be optimized.
In a further preferred embodiment, the collector of the fifth transistor, and the collector of the fourth transistor are connected to the voltage source via a resistor, and/or the collector of the third transistor is connected to the voltage source via a resistor. This embodiment allows the circuit to be configured with a minimum number of voltage sources. Alternatively, it is possible to provide a separate voltage source with a very high current rating for the collector of the fifth transistor, in order that the fifth transistor can supply a current which is as high as possible to the control input of the first bipolar transistor when the amplifier power level is high.
The transistors which have been mentioned are preferably npn bipolar transistors, and are preferably heterobipolar transistors owing to their superiority in terms of noise response and linearity when the signals to be amplified are analog signals at high frequencies.
In a further preferred embodiment, the control input of the third transistor is connected to the reference ground potential via a resistor, and the control input of the third transistor is connected via a resistor to the collector of the second bipolar transistor, and, furthermore, the control input of the third transistor is preferably connected to the collector of the third transistor via a resistor. The choice of these three resistors allows the operating points of the common collector amplifiers to be set, and thus allows the gain and linearity to be adjusted. Further resistors can also be fit.
The first and second bipolar transistors are advantageously constructed and disposed with respect to one another in such a manner that they have the same thermal characteristics and are largely subject to the same tem
Forstner Johann-Peter
Sinnesbichler Franz-Xaver
Cunningham Terry D.
Mayback Gregory L.
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