Some Solutions to Problems of Operating Germanium Transistor Servo Amplifiers at High Ambient Temperatures

Abstract

Some solutions are outlined to the problem of using readily available types of germanium power transistor at ambient temperatures as high as 85°C, such as might be found in aircraft. Germanium transistors can be used at ambients within 5°C of their maximum storage temperatures if the power dissipated in the transistor is kept low and there is provision to handle the high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_{c0}</tex> . Both the mean power dissipation and the peak instantaneous dissipation must be considered, and the thermal and electrical circuits designed to insure that the junctions never, even instantaneously, exceed the maximum temperatures set by the transistor manufacturers. A typical design for high-temperature operation is described, consisting of a three-stage direct-coupled amplifier to operate a standard 3-watt 400 cps servo motor. This amplifier, in which sound engineering practices are stressed, sacrifices some gain for stability, reliability, and efficiency. It is designed to tolerate wide variations in transistors and most other components with a minimum change in gain or available output. The maximum mean power dissipation is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\frac{1}{2}</tex> watt, and the maximum peak instantaneous dissipation is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1\frac{1}{2}</tex> watts. The first of these amplifiers was built two years ago, and although several have been in operation, there have been no failures. The techniques are applicable to other circuits where transistors must operate close to their maximum storage temperature.