|Object Name:||Transistor Amplifier Module|
|Year of Manufacture:||Unknown|
|Location:||Archive Object Store|
Type No. 3C5161.A2
Ser. No 2066
In the mid-1950s germanium junction transistors were becoming available from several manufacturers and a detailed investigation was carried out to assess their suitability for avionic applications. One of the outcomes was a standard general-purpose transistor amplifier 3C5 161 which was used in a number of Elliott avionics programmes. At that time there were several large active avionic projects in the pipeline, notably the Blue Steel inertial navigation system for the V-bomber force, the automatic flight control systems for the Lightning and Buccaneer aircraft, 0R946 (OR946 was a de facto standard for a set of Navigation Display instruments (e.g. attitude indicator, altimeter, flight director, airspeed indicator and standby versions of them, etc.); amongst others. At that stage, silicon transistors were not available and the voltage, power handling and temperature limitations meant that transistors were restricted in application to low-level applications and thermionic valves and magnetic amplifiers were used where large voltage or power handling was required.
The amplifier had 2 stages of Class-A amplification using Mullard 0C71 germanium transistors and a Class-B push-pull output stage with two OC72 germanium power transistors. The actual power available from the amplifier was severely limited by the high ambient temperature in which it was to work which imposed limits on the allowable internal dissipation. The design gain was in the order of 2000-5000 and the design load was 400 ohms which represented the input impedance of the following ring demodulator and magnetic amplifier. Tantalum capacitors produced by Plessey were used for coupling and decoupling because of their high capacity, small size, and ability to withstand high temperatures. (The low impedances in transistor circuitry necessitated the high capacitances.)
These transistor amplifiers were used in some numbers in the inertial navigator for Blue Steel where they functioned as pre-amplifiers for a 5 watt magnetic amplifiers Type 3C516 driving Sperry motor-tacho servos, and similarly for a 9-watt magnetic amplifier. Similar arrangements were used in the 0R946 barometric air data computer and flight control system. As the servo was a 400Hz carrier system, the amplifier did not need a wide bandwidth and thus small transformers could be used.
Germanium transistors were severely temperature-limited and would fail completely at 75°C; the amplifier circuit therefore incorporated a temperature compensating circuit using a germanium diode sensor and its power levels were heavily restricted. A temperature of 70°C was expected to be encountered. In the Lightning aircraft a temperature of 120°C was expected and germanium transistors would not have survived; that flight control system used magnetic amplifiers. The transistor amplifier may have been used on the Buccaneer flight control system but it is also possible that this used a later amplifier using silicon transistors which became available and which had a maximum temperature rating of 150°C.
The first application was in the 0R946 the transistor amplifier and the magnetic amplifier were combined on a common chassis known as the "cheese-shaped amplifier" a number of them were mounted radially in a cylindrical container. At a later stage, the transistor amplifier was re-packaged in a rectangular form to be used without the magnetic amplifier or in a different packaging arrangement.
In the Blue Steel inertial navigator the transistor amplifier was used as a pre-amplifier for 5 watt and 9 watt magnetic amplifiers in the gyro and computing servo arrangements. This gave a measure of standardisation, although most of the electronic circuitry in the navigator was specially designed using a mixture of thermionic tubes, magnetic amplifiers, germanium transistors and, in the power supplies, silicon diodes and power transistors.
The amplifier was produced in quantity; more than 20,000 are believed to have been made.
(Thanks to R. Bristow for permission to use this text)