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Publication Date: 2015-Aug-07
Document File: 7 page(s) / 289K

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A technique for compensating a gain or phase in a radio frequency amplifiers is disclosed. The technique proposes a temperature sensor and an adjustment module in an existing circuit. The temperature sensor is a temperature circuit which outputs temperature signals. The adjustment module is a temperature processing circuit that matches control signal requirements of a virtual valve amplifier (VVA) or variable gain amplifier (VGA) and a phase shifter. The adjustment module determines one or more signals to be used based on gain or phase temperature variance of a pre-amplifier, an amplifier and others. The adjustment module provides appropriate compensation amplitude that may be either positive or negative.

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The present invention relates generally to power amplifier systems, and more particularly to a technique for compensating gain or phase in a radio frequency amplifier.

High Power Amplifiers (HPAs) are crucial component in transmit chain of a magnetic resonance (MR) system.  The high power amplifiers are required to possess good operating characteristics, such as good linearity, wide dynamic range, stability and high efficiency.  The linearity is improved by several techniques, such as a forward technology, a feedback technology, and a digital/analog pre-distortion.  Further, there is a shift in gain or phase of an amplifier with respect to temperature even if various compensation techniques are used.  For example, a cold plate temperature bias compensation, a junction temperature bias compensation, and closed loop method are used as compensation techniques.

Figure 1

Figure 1 depicts a topology of a linear radio frequency amplifier.  Referring to figure 1, a shift in phase or gain in the amplifier arises from a pre-driver section and a main amplifier.  The pre-driver section includes a variable voltage attenuator (VVA), a variable gain amplifier (VGA), a phase shifter, a power splitter, radio frequency (RF) detector.  It is observed that the gain or phase of such devices varies with temperature.  If the pre-driver section is out of close loop (feedback), then the gain or phase variable of the pre-driver section contributes to the gain or phase of the main amplifier.

If the pre-driver section is included in a close loop (i.e. a feedback), the gain or phase variable is improved by the close loop.  The high gain is needed to pull the gain or phase variable to a desired target.  Unfortunately, the high gain of the close loop worsens stability due to decreased phase margin of the loop.  Therefore, an envelope bandwidth of the loop also deteriorates.

Figure 2

Figure 2 depicts a circuit diagram of an amplifier. Referring to figure 2, the circuit includes a radio frequency amplifier (RFA), a power splitter or combiner, and a driver amplifier.  In some embodiments, the RFA is a radio frequency metal–oxide–semiconductor field-effect transistor (RF MOSFET) and forward trans-conductance of the MOSFET is shown in Figure3.  There is a need to keep static quiescent bias point stable and the forward trans-conductance constant.  However, both the static quiescent bias point and the forward trans-conductance decrease with the temperature. 

Figure 3

One conventional technique proposes an electronic circuit that contains a controller integrated circuit and a power amplifier MMIC (Monolithic Microwave ICs) connected to a controller IC.  The controller IC has an operational amplifier that supplies a DC bias to a base of an RF power amplifier through a ballast resistor.  The operational amplifier has an output slope that compensates either partially or entirely for the volta...