Patent GB-703685-A (GB)
Title: Electric circuits utilizing semiconductor amplifiers
IPC: H03K3/281
Filed: 1950-02-28
Abstract: 703,685. Transitor generating circuits. WESTERN ELECTRIC CO., Inc. Feb. 28, 1950 [March 5, 1949], No. 5071/50. Class 40 (6) An electric circuit comprises a transistor and a network which provides a positive feedback and includes a biasing source such that over a desired range the current amplification α exceeds unity and at least one part of the circuit has a dynamic characteristic in which a negative resistance portion is bounded on each side by a positive resistance portion, the network having an impedance element which determines the extent of the range. Arrangements comprising one transistor adapted to produce saw-tooth oscillations, pulses, and trigger circuits with one or two stable states are described. In Fig. 5, a transistor having base electrode 2, emitter 3 and collector 4 is grounded emitter connected, an inductance L and resistance Rb being connected in series with the base electrode. With suitable bias potentials the dynamic characteristic of base voltage (Eb) against base current 1b is an S- shaped curve as shown, in Fig. 6, and a method of deriving this characteristic is described. The curve possesses a centre portion of negative resistance, bordered by two portions of positive resistance. Fig. 6 also includes the load line corresponding to resistance Rb and bias source Vb, which cuts the Eb Ib characteristic at a, a stable operating point. When the base circuit is first closed, inductance L prevents any sudden change of current and the operating point therefore proceeds from b along the S-curve, tending to approach point a. At point d, the inductance causes the operating point to snap to point e on the curve. The operating point then continues to f where a similar action takes place, the operating point snapping over to g on the lower portion of the curve. The stable operating point a is thus never reached, and the system oscillates continuously. The waveforms produced at the electrodes are described, saw-tooth oscillations appearing across the base resistance Rb, and rectangular pulses being produced in the collector circuit. If the bias source Vb and the resistance Rb are such that the load line cuts at only one positive resistance portion of the curve (e.g. at point a<SP>1</SP> or a<SP>11</SP>) the arrangement is normally stable, but may be triggered by pulses to the base or emitter electrodes, for example from a source 15, so that one complete oscillation takes place, the circuit then returning to its stable position. Alternatively, the inductance may be omitted, and resistance Rb increased so that the load line cuts the S curve in both the positive resistance and the negative resistance portions. This arrangement has two stable states, the change-over being initiated by pulses applied either to the emitter or base electrodes, and may be used as a slicer. Fig. 12 shows a transistor connected in grounded base configuration, but with a resistance Rb in the base circuit which is large enough to dynamically affect both the emitter-base, and collector-base potentials. The Specification describes how the emitter voltage (Ee), emitter current (Ie) dynamic chatacteristic may be obtained, which with suitable values of collector bias voltage and resistance Rb, may be an N-shaped curve having a negative resistance portion bordered by two positive resistance portions as shown in Fig. 13. In this case a resistance Re in parallel with a condenser Ce is connected in the emitter circuit in series with the emitter bias source Ve. The condenser resists sudden changes of voltage in the emitter circuit, and in this way the system may be made to operate in a fashion corresponding in principle to that of Figs. 5 and 6, so as to produce sawtooth oscillations, rectangular pulses, or to provide trigger circuits with one or two stable states. The effect on the characteristics, of varying the value of resistances in series with the electrodes is described. Differentiating or integrating circuits may be added to modify the waveforms produced. Specifications 694,021 and 700,232 are referred to: