Patent GB-1119708-A (GB)
Title: Improvements in or relating to tuned circuits employing transistors
IPC: H03B5/24
Filed: 1965-10-15
Abstract: 1,119,708. Transistor amplifying and oscillating circuits; remote control of radio receivers. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 15 Oct., 1965 [15 Oct., 1964; 13 Feb., 1965], No. 43799/65. Headings H3Q and H3T. An R.C. coupled transistor circuit which can be adjusted to act as either a high Q circuit or alternatively as an oscillator comprises a first transistor T 1  with its collector connected to the base of a second transistor T 2 . A phase-shifting feedback loop is provided between a further electrode of the second transistor and one of the first transistor, e.g. the network C, R 1 , R 2  connected between the emitters; the arrangement is characterized in that the collector circuit of the first transistor T 1  includes at least two semi-conductor diodes D 1 , D 2  connected in series and arranged in the same direction of conduction as transistor T 1 . The collector circuit also comprises phase-shifting elements, e.g. capacitor C 5  such as to cause a phase-shift equal to but of opposite sign to that of the feedback network at the resonance frequency of the circuit, which is also arranged so that at that frequency the loop gain is substantially equal to unity. The Specification gives mathematical expressions for the necessary conditions, which comprise the inclusion in the collector circuit of transistor T 1  of series resistor R 5  of value double the emitter input resistance of transistor T 2 : each of the diodes D 1 , D 2  should have the same value of differential resistance as the emitter input resistance of transistor T 1 , and this can be varied so as to change the resonance frequency of the circuit by adjustment of the emitter-collector current, by means of resistor R 1 . The value of capacitor C 5  determines the effective Q of the circuit; it is approximately half C, lesser values making Q negative, i.e. causing sustained oscillations to occur. The Specification describes numerous modifications to the circuit, including means for maintaining the resonance frequency constant despite temperature variations (Figs. 3-5, not shown) and the use of transistors to replace resistors R 1 , R 2  (Figs. 6 et seq., not shown); transistor T 2  may also be replaced by a Darling- ton pair (Fig. 6, not shown) and resistor R 5  by diodes (Fig. 7, not shown). Two such resonant circuits may be tuned simultaneously by means of a single variable resistor (Fig. 8, not shown) and may be adjusted so that their resonant frequencies differ by a constant frequency, thus providing gang-tuned input and oscillator circuits for a superheterodyne receiver. In alternative circuits the feedback loop is located between the collector of the second transistor and the base of the first (Figs. 9-13, not shown) and in these the phase-shift may be shared between two stages (Fig. 11, not shown). Transistors of different conductivity type may be employed (Fig. 13, not shown). The Specification also discloses the manner in which two such resonant circuits may be coupled together (Figs. 14, 15, not shown) and the use of a shunt diode in the coupling network to act as a variable impedance for the purpose of gain control (Fig. 17, not shown). The arrangement may be modified by the inclusion of resistance in series with the capacitance C 5  to compensate at high frequencies for the effect of the capacitance between emitter and base (Fig. 16, not shown). When the arrangement is used as local oscillator in a superheterodyne receiver, the frequency may be automatically controlled by a voltage depending on the detected signal; again, the resistor R 1  may be of light-dependent type and controlled, e.g., from a remote light source. In another, e.g. remote control circuit, a single variable resistor may be used to tune over a number of separate desired wavebands, intervening frequencies being excluded by providing gaps in the resistor track which are closed by means of fixed resistors (Fig. 18, not shown). The arrangement may also be used as an amplifier of the modulation type; e.g. a capacitance pick-up may be connected in shunt with capacitor C 5  to vary the circuit Q, i.e. provide amplitude modulation, or resistor R 1  may be varied as a function of the signal, to provide oscillations modulated in frequency and amplitude. In an alternative embodiment, a small part of the current flowing through resistor R 1  is diverted from transistor T 1 , through a signalcontrolled transistor T 5  (Fig. 19, not shown) and flows direct into the diodes D 1 , D 2 . The Specification also discloses modifications of the circuit in which the resistance R 5  is considerably increased in value, to reduce the effect of non- linearity in diodes D 1 , D 2  with appreciable signal amplitudes, resistance being connected in series with capacitor C in compensation (Figs. 20-22, not shown). In an arrangement for providing a high Q circuit of improved stability, a second similar circuit, adjusted just to oscillate, is also provided its output being detected and supplied as a Q control to both circuits.