Patent GB-806795-A (GB)
Title: Improvements in or relating to apparatus for testing transistors
IPC: G01R31/26
Filed: 1956-11-21
Abstract: 806,795. Transistor testing. WESTERN ELECTRIC CO. Inc. Nov. 21, 1956 [Nov. 23, 1955], No. 35591/56. Class 37. . Apparatus for measuring the current amplification factor (alpha) of a transistor 11, Fig. 2, having emitter, base and collector electrodes e, b, c, comprises means for supplying D.C. potential to the electrodes, a potentiometer 19 connected between two of the electrodes, for example electrode c and b, an input alternating signal source 17 connected between the third electrode, for example electrode e, and the potentiometer slider, and a null voltage responsive means, for example voltmeter 18, connected between the electrodes c and b. The slider setting necessary for the volt-meter to read null indicates the value of alpha. It is shown mathematically that: where Rp is the total resistance of the potentiometer comprising resistances R1+R2 separated by its slider. In a modification, Fig. 3, two alternative potentiometers 21, 22 in series with resistor 20 replaces potentiometer 19, with a switch 23 selecting the movable contact of either potentiometer for the emitter signal return. By making potentiometer 22 of lower resistance than 21 the precision of determining alpha near unity is increased. The high gain frequency selective amplifier 34 and indicator 35 replace voltmeter 19; the amplifier 34 being tuned to signal frequency and the collector electrode c being shunted to ground by a capacitor 33. A double-shielded transformer 24 in the signal input circuit attenuates stray circulating ground currents at the signal frequency. A D.C. source 28 biases emitter e in the forward direction through a resistor 29 and an inductance coil 30, the collector c being biased in the reverse direction to emitter e by D.C. source 32. In operation, the desired range for alpha is selected by switch 23 and the potentiometer adjusted until indicator 35 reads zero; alpha is then read directly from the potentiometer calibrations. The contact noise in the working potentiometer is balanced out at the detector, and noise may be further reduced by a highly selective null detector; making possible the measurement of junction transistors. In a modification, Fig. 4, a servo-amplifier 36, selective to the input frequency f1, replaces amplifier 34 and feeds the control phase of a two-phase A.C. servomotor 37, whose reference phase is fed from the input signal. An attenuator 38 is connected between signal source 17 and transformer 24 since the motor 37 requires more power than the input signal. The motor is coupled to a balancing potentiometer 19 so that an unbalance signal to amplifier 36 energizes the motor to adjust the potentiometer for null at a setting representing alpha. In Fig. 5, not shown, a commutator is connected to the motor shaft to automatically and selectively operate a sorting device to grade the transistors in accordance with plural preset values of alpha. A drum recorder, Fig. 6, (not shown), may be used to produce a direct reading graph showing alpha as a function of an independent variable (e.g. emitter current) affecting transistor operation.