Telecommunications – Transmitter – Power control – power supply – or bias voltage supply
Reexamination Certificate
2000-08-28
2003-07-15
Chin, Vivian (Department: 2682)
Telecommunications
Transmitter
Power control, power supply, or bias voltage supply
C455S550100, C330S096000
Reexamination Certificate
active
06594474
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electronics, and, more particularly, to a controlled-gain power amplifier device which is also known as a controlled-gain power driver. The present invention applies advantageously, but is not limited to radio-frequency circuits, particularly circuits used in cellular mobile telephones.
BACKGROUND OF THE INVENTION
Radio-frequency circuits used in cellular mobile telephones do not incorporate the power amplifier. The power amplifier is generally an external circuit formed using gallium arsenide (GaAs) technology. On the transmission side, the last stage incorporated into the radio-frequency circuit is a power amplifier device. The power amplifier device has a controlled gain, and is also known as a power driver. A power driver typically delivers a maximum level of only a few dBm (0 to 6 dBm) at 1 or 2 GHz depending on the frequency band. The power level delivered by the specific power amplification circuit that is formed within the radio-frequency circuits provides about 27 to 33 dBm. The power amplification circuit is formed from gallium arsenide, for example.
Moreover, for new generation telephones using the Code Division Multiple Access (CDMA) mode which includes embedding several tens of communications within the same frequency band (e.g., 1.25 MHz), the power amplification end stage must be furnished with variable-gain control whose dynamic range may reach 20 dB. It is necessary to control the power transmitted in view of a particular average power level requirement in transmissions using the CDMA mode.
Finally, the power driver must exhibit linearity constraints, i.e., a linearity of the transfer function linking the output power to the level of the input signal. Stated otherwise, if the input signal level increases linearly, the power output must also increase linearly. The requirements for this last stage of the radio-frequency circuit depend on a compromise between current consumption, linearity, supply voltage, circuit area (influencing the cost of the silicon), and noise floor.
Such a device or controlled-gain power amplifier stage receives a signal, which usually originates from an external filter whose output impedance requires a 50 ohm matching for template compliance constraints. One therefore generally finds in series from the input to the output of a power amplifier device an input impedance matching network, a voltage/current transconductor block, gain control carried out by shunting a variable proportion of the current from the transconductor block to the output load, and a network for matching power to the input of the specific power amplification circuit (which follows the integrated radio-frequency circuit).
The design most frequently encountered uses an input transconductor block which is a common emitter transistor (single-input version) or two transistors with linked common emitters (differential-input version). These are known for their high power gain. After the input transconductor block is a pair of transistors with linked emitters, which shunt part of the current originating from the transconductor block. The transistors of the transconductor block may be conventionally biased by a decoupled current source.
In this type of structure, the linked emitters of the transistors of the current shunting means or circuit are linked to the collector of the transistors of the transconductor block, and are consequently current-driven. Moreover, the input of the device is linked to the bases of the transistors of the transconductor block. Finally, the input matching of this type of stage involves using an inductance connected between the emitters of the transistors of the transconductor block and ground (differential-input version) to form an impedance having a real part with respect to the input.
This type of controlled-gain power amplification stage exhibits numerous drawbacks and limitations. First, the greater the linearity requirement, the greater the consumption of the stage. Moreover, such a prior art structure leads to a limitation in the maximum power output. This is because the limitation in the maximum voltage swing can be applied to the output without saturating the output transistor, i.e., the transistors of the current shunting circuit.
Also, the conventional bias circuits merely intensify the above limitation by adding the breakdown voltage of the transistor forming the bias current source. Furthermore, whereas the use of an inductance connected between the emitter of the input transistor and ground (single-input version) makes it possible to form an input impedance whose real part is significant, the natural input impedance of a bipolar transistor degenerated by such an inductance depends on numerous parameters.
Hence, the input matching network reflects towards the input of the arrangement the drifting of the nominal input impedance of the transconductor block related to the process and temperature variations. This often leads to unstable optimizations which may impair the efficiency of production. It may also result in different characteristics of the various batches produced.
Moreover, the order of magnitude of the natural input impedance of a common emitter arrangement is greater than 200 ohms. A 50 ohm matching therefore causes, on crossing the input impedance matching network, an over voltage whose coefficient is in the ratio of the square root of the impedances. The matching is frequently rendered impossible if this coefficient is too large. The matching is then done by degrading its quality coefficient, i.e., by introducing losses which degrade the noise factor.
Moreover, the search for high linearity is frustrated by the fact that the rise in the degeneracy inductance results in an increase in the input impedance of the transconductor block, and hence in the voltage at the input of the device. This goes against the sought-after effect since the input transconductor is voltage-controlled. Furthermore, the use of a degeneracy inductance across the terminals of the transistors of the transconductor block is expensive in terms of silicon area.
SUMMARY OF THE INVENTION
In view of the foregoing background, it is therefore an object of the present invention to provide a controlled-gain power amplifier device offering a better linearity/current consumption compromise, as well as better control of the input impedance of the device. This is applicable to a single input structure and a differential input structure.
Another object of the present invention is to provide a saving in terms of silicon area relative to the prior art devices.
Yet another object of the present invention is to provide a stable input matching that is easy to achieve.
A further object of the present invention is also to provide an arrangement allowing separate optimization of gain, noise and linearity.
Yet a further object of the present invention is to provide a biasing that allows accurate control of the quiescent currents, and which is not sensitive to the offset voltage of the biased transistors at high current, and is not sensitive to differences of the coefficients &bgr; of these transistors. There are different values for the coefficients &bgr; of the transistors formed in the silicon, although in theory they are identical.
These and other objects, advantages, and features in accordance with the present invention are provided by a controlled-gain power amplifier device comprising voltage/current transconductor means or a voltage/current transconductor circuit, and gain control means or a gain control circuit comprising shunting means or a shunting circuit able to shunt to the output of the device, in response to a control signal, all or some of the current delivered by the transconductor circuit. A control circuit delivers the control signal.
According to a general characteristic of the invention, the device comprises at least one pair of transistors having linked emitters (bipolar transistors) or sources (field effect transistors), controlled at their base or gate by the control circuit. This pair of transistor
Grasset Jean-Charles
Pinatel Christophe
Afshar Kamran
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Chin Vivian
Jorgenson Lisa K.
STMicroelectronics SA
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