| 摘要 |
A thermal power sensor for providing an output signal that is linearly related to the amount of power derived from a power source over a first range and that is square law related to the amount of power derived from a power source over a second range which may overlap the first range. The apparatus comprises a pair of matched transistors connected in a common emitter configuration. Each of the matched transistors is thermally coupled to a matched resistor so that heat generated in the resistor is transmitted to its respective transistor to cause temperature variations therein. The current through each transistor is a function of the temperature of the transistor. The matched transistors are connected in series with a pair of cascode transistors. The cascode transistors are connected to a first differential amplifier which senses and amplifies the differential voltage which results from any difference in current flow through the matched transistors. The output of the first differential amplifier is connected to the bases of the cascode transistors to vary the bias voltage applied to the matched transistors. A second differential amplifier is connected to the differential output of the first differential amplifier and has its output connected to the second matched resistor. The effect of the feedback loop which includes the first differential amplifier, the cascode transistor and the matched transistors is to maintain the current flow through the matched transistors constant, whereby their collector-emitter voltage is linearly proportional to the power being sensed. The effect of the second loop, including the second differential amplifier and the second matched resistor, is to maintain thermal equilibrium and equality between the matched transistors and their associated resistors to provide a square law indication of the power being sensed. A combined indicator is provided for providing an indication of sensed power over the first and second ranges.
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