GB763443A - Improvements in or relating to transistor circuits for amplifying electrical signals - Google Patents

Abstract

763,443. Semi-conductor amplifying circuits. PHILIPS ELECTRICAL INDUSTRIES, Ltd. June 22, 1953 [June 25, 1952], No. 17221/53. Class 40(6) A circuit for amplifying an intelligence signal shown in the Figure as derived from a source Vi comprises a transistor 1 to the input circuit of which the intelligence signal is applied together with an auxiliary oscillation Vo, having a higher frequency fo than the band of frequencies indicated generally as fi occupied by the intelligence signal Vi, so that an output signal of a suitable combination frequency, e.g. (fo + fi) is developed across the resonant circuit 9 tuned to the selected combination frequency. By suitable demodulation of the signal developed across the resonant circuit 9, an amplified version of the input signal Vi may be derived, the object of the invention being to derive such an amplified signal having a low signal-to-noise ratio. This is achieved by suitably biasing the transistor 1 and by selecting appropriate values for the elements in the output circuit of the transistor. It is stated that the total impedance in the output circuit must be high at the combination frequency (fo + fi); this is achieved by the tuning of the resonant circuit 9 to that frequency. Further a low impedance is desired at the frequency fo of the auxiliary oscillation Vo, while a high impedance is desired at the frequency of the intelligence signal fi, a suitably designed resistance capacitance circuit 10 being provided for this latter purpose. A comparatively low resistance 25 may further be introduced into the output circuit of the transistor to provide negative feed-back at the frequency fo and maintains the level of noise generated at the collector contact at that frequency constant. A modified form of circuit is illustrated in Fig. 4 wherein the effective base resistance is reduced to zero by means of a current negative feed-back provided by the phasereversing transformer 12 and resistor 13. It is stated that assuming the transformer 12 has a one-to-one ratio the resistor 13 should have a valve equal to the natural value of the base impedance. Fig 5 shows a further modification wherein the local oscillation of frequency fo is generated in the circuit itself by means of the tuned circuit 15 connected in the base circuit of the transistor 1. The diode 21 and by-pass resistor 23 are provided to limit the amplitude of the oscillation developed across the circuit 15. In a further modification Fig. 6 (not shown) the circuit of Fig. 5 is modified by connecting a tapped-off portion only of the inductor of the circuit 15 in the emitter-base circuit of the transistor.