Patent GB-804012-A (GB)
Title: Improvements in and relating to amplifiers
IPC: H03D11/02
Filed: 1956-03-26
Abstract: 804,012. Parametrically-excited resonators. KOKUSAI DENSHIN DENWA CO., Ltd. March 26, 1956 [March 30, 1955; Sept. 16, 1955], No. 9396/56. Class 40 (9). A parametrically-excited resonator in which a ferro-magnetic or ferro-electric reactance tuned to a pilot frequency F is varied by an excitation frequency 2f is arranged to amplify a signal modulating the pilot frequency. The basic arrangement is shown in Fig. 1A and comprises ferrite cores K1, K2 having windings L1, L1<SP>1</SP> connected to a D.C. bias source E and to an excitation source of frequency 2f through a gate G. The tuned circuit comprises opposed core windings L2, L2<SP>1</SP>, a condenser C and a damping resistor R which are associated with a signal voltage e1. of frequency f and produce an output voltage e 1<SP>1</SP>. The variable reactance may alternatively consist of ferro-electric capacitors C1, C2, Fig. 1B, which are tuned by linear reactors L and C. The principle governing operation as an amplifier is explained with reference to Figs. 2A and 2B which respectively show the excitation current I and the output voltage from the resonator to a base of time t. The output voltage e1<SP>1</SP> is equal to the input voltage e1 when gate G is closed, but when the gate is opened at time t = 0 the output voltage rises in accordance with the function el#γ<SP>t</SP> to a maximum value determined by the circuit damping. Subsequently, when the excitation is terminated by closing the gate, the output voltage falls to the initial value of e1. Figs. 4A-4D explain the manner in which this principle is applied to amplify a comparatively lowfrequency signal modulating the pilot frequency, Fig. 4A. As shown, the excitation current I, Fig. 4C, is periodically applied to the resonator by opening the gate G for duration #, Fig. 4B, in successive and equal time periods T, the duration # being shorter than that required for the output voltage to reach its possible maximum value. As a result this output voltage has the exponentially-modulated waveform shown in Fig. 4D. Since the growth of the output modulation in each of the time periods T is determined by the coefficient e1 of the expression el#γ<SP>t</SP>, it follows that the growth of this modulation is determined by the instantaneous modulation depth of e1 each time gate G opens, and that the envelope of the output modulation comprises an amplified form of the input modulation. Similar results may be obtained by frequency modulating the exciting voltage, or by periodically varying the bias E, instead of using a gate G. The amplified signal is segregated from the output voltage by the use of a bandpass filter and a demodulating circuit. An amplifier of higher gain may be formed by connecting several resonators PI-PIV in cascade as shown in Fig. 5B, the excitation circuits being omitted from the Figure. In this arrangement the individual resonators are successively excited in the order of the stages A1-A6, Fig. 9A, by currents i1, i2, i3, Fig. 9B, and are coupled by impedances Z. As the arrangement is liable to self-oscillation due to back-coupling of the amplified signal, the cascade stages may each be modified as shown in Fig. 7A. As shown, the resonator comprises four cores L1-L4 which provide a neutralizing transformer coupling between the output and input circuits T2 and T1. Separate windings Ld, L2f are provided for the biasing and excitation currents. An alternative bridge connection of ferro-magnetic cores is shown in Fig. 7B. The modification may be applied to amplifiers utilizing ferro-electric capacitors C1-C4, Fig. 7D, which illustrates resonators I-III connected in cascade. Where the value of C necessary for resonance is unduly large, a transformer coupling TS1, TS2, Fig. 8A, between cascaded stages may be used, or the condenser may be coupled to the resonator separately by a transformer TS, Fig. 8B. It is stated that the resonators act as balanced modulators, since with excitation frequency 2f and input frequency f 1 there is produced output frequencies 2f + f1 = f2 and 2f - f1. When f1 and f2 have quite different values, the input and output signals can be separated by filters to eliminate backcoupling and heat. For cascade connections an even number of stages gives an amplified output corresponding in frequency to the input, while with an odd number of stages the amplifier functions as a frequency converter.