Patent GB-959550-A (GB)
Title: Improvements in or relating to resonant electric circuits
Inventor: BAXANDALL PETER JAMES
IPC: H03B5/12
Filed: 1959-03-13
Abstract: 959,550. Transistor oscillator and amplifier circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 8, 1960 [March 13, 1959], No. 8865/59. Heading H3T. [Also in Division H2] An A.C. generator comprises a parallel resonant circuit connected alternately to opposite poles of a D.C. source to cause the circuit to oscillate and an inductance in a connection between the circuit and the source which forms with capacitance a series circuit resonant to the same frequency as the parallel resonant circuit whereby the changeover of connections is effected when the current is small. The invention also relates to a similar circuit in which the changeover is effected when the voltage is small. The Complete and Provisional Specification describe a number of oscillators but state that the switching devices thereof could alternatively be controlled by an external signal. Fig. 3 shows a basic arrangement described only in the Provisional Specification and not in accordance with the invention, comprising a resonant circuit L1, C1 coupled to a load R1 and connected to a D.C. source Vht through two transistors J1 and J2 which by feedback winding CT2 are rendered alternately conducting to cause the circuit to oscillate. Included in the connection to the source is a choke L2 having a high impedance at the oscillation frequency so that a substantially constant current flows. As a result the collector voltage remains low during the conducting half-cycle and while each transistor is switching. The circuit is made self-starting by providing a small bias voltage through a resistor Rb or through a potentiometer feeding a choke which maintains the bias current constant. Preferably the choke L2 has at an inductance at least ten times the collector - to - collector tuned-circuit inductance L1 so that the energy stored in the choke is approximately a quarter of that stored in the inductance. During the period when one transistor is being switched off and the other being switched on a large reverse base current may flow into the transistor being switched off from the transistor being switched on so that the stored charge in the base is quickly removed and the other transistor is quickly switched on. In a modification of the circuit (Fig. 5, not shown), a tertiary winding is tuned instead of the primary winding and a further winding feeds the load. In the oscillator in accordance with the invention, shown in Fig. 6, the tuned circuit L1, C1 is connected alternately to one side and the other of the D.C. source through transistors J1 and J2 which are rendered alternately fully conducting by feed-back winding L6 and L7. A circuit C2, L4 is series tuned to approximately the oscillation frequency so that although the voltage across the tuned circuit is of sinusoidal form, the voltage at the common connection between the collector and emitter is of square wave form and the change-over of the conduction of the two transistors accordingly takes place when the current through the transistor is at a minimum. If the series resonant circuit C2, L4 is tuned slightly below the oscillation frequency current is still flowing in the circuit when each transistor is being switched off. Accordingly, current may flow through the emitter circuit in the reverse direction for a short time, the emitter operating as a collector. If the tuning capacitance is reduced the current through the circuit is too small when the change-over of conduction takes place and a slight ring in the voltage waveform occurs. Preferably the effective loaded Q values of the coils L4 and L1 are equal and the unloaded values also equal. In an alternative arrangement of this circuit (Fig. 7, not shown), the tuning capacitor C1 is formed by two capacitors in series and the series tuned circuit C2L4 is formed by an inductor connected to the junction of the capacitors and by the two capacitors themselves. Starting of the circuit is ensured by biasing the bases of the transistor in a balanced manner by potentiometers comprising resistors and rectifiers in series, the rectifiers being poled to provide a low resistance for base current when it flows and being by-passed by capacitors. In a further arrangement a parallel resonant circuit is earthed at one end and connected at its opposite end by a capacitor and the moving arm of a switch alternately via respective choke windings on a common core to the opposite poles of a battery. The Complete Specification also describes basic circuits in which the transistors are replaced by switches and states that the circuits described will operate as tuned amplifiers if the input is obtained from an external circuit instead of from a feed-back winding.