OPTIMIZED TEMPERATURE COMPENSATION CIRCUIT APPLIED IN THE HIGH POWER AMPLIFIER
Publication Date: 2014-May-20
The IP.com Prior Art Database
The invention proposes a technique that provides optimized temperature compensation circuit applied in a high power amplifier. Three more temperature sensors are added to the amplifier with two sensors already in it. Sensor 3 is used to monitor water temperature from sensor 1 and sensor 5 locations. Cold plate temperature is close to linearity from sensor 2 to sensor 4. The cold plate temperature is also close to linearity. Four sensors are used instead of two in a linear configuration as there exists a temperature step change from sensor 5 to sensor 2.
FIELD OF INVENTION
The invention generally relates to high power amplifier and more particularly to optimized temperature compensation circuit applied in the high power amplifier.
BACKGROUND OF THE INVENTION
In general, radio frequency (RF) coil in magnetic resonance (MR) system outputs RF excitation which generates magnetic (B1) field. Due to nonlinearity of RF amplifier, the actual RF amplifier output have some distortion compared with the input, which results in poor slice profile.
To improve linearity, digital pre-distortion, analog pre-distortion, feedforward, and backward, technologies are developed very quickly in recent years, especially, in mobile communication systems. However, between MR system and communication systems, MR has higher power than communication or mobile systems. High dissipated power results in heat, which increases junction temperature and cold plate temperature. Further, quiescent operating point floats away and the heat results in gain decrease during period time of one pulse. If the quiescent operating point does not resolve correctly, it poses a big challenge to all linearity technologies.
Junction temperature and cold plate compensation arise in high power amplifiers. FIG. 1 depicts conventional compensation circuit.
As depicted from the above figure, the output voltage is applied on all the modules of the final stages.
FIG. 2 depicts temperature on final stages.
As illustrated in the above figure, two temperature sensors are mounted on TC1 and TC2 location, and the designer uses TC1 and TC2 average voltage to control gating-source voltage. TC1 and TC2 are lower than temperature tested when the charge sensitive amplifier (CSA) is in working state.
FIG. 3 depicts cold plate temperature compensation.
As illustrated in the above figure, static current is maintained flat when the temperature goes high in figure 3B while bias current goes up in figure 3A.
A conventional technique uses a pair of thermistors which are connected in parallel to a resistor of a value chosen with respect to the resistance of its associated thermistor over a given temperature range. The technique controls the base current over a range of ambient temperature variation. However, the bias circuit is used for bias current and not for bias voltage.
Another conventional technique uses a transistor package with the temperature coefficient of the diode similar to the base to emitter junction temperature coefficient of the associated transistor. The diode is closely spaced to its associated transistor to provide a fast thermal response to maintain a stable quiescent point and therefore provide thermal compensation. However, the technique is used for integrated circuits (IC) but not for popular metal oxide semiconductor field-effect transistor (MOSFET) or metal–semiconductor field effect transistor (MESFET) and laterally diffused metal oxide sem...