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S/24/244 | N 12%S42/72/7 N S/6%Z N Aug. 20, 1963 D. R. HOLCOMB ETAL 3,101,452 WOLTAGE-WARIABLE CAPACITOR BRIDGE AMPLIFIER Filed June 30, 1959 ------------------- -------- 24 44242.42 17a A12A12 62A62624 nazza s/24/244 | N 12%S42/72/7 N s/6%Z N 42.vaaza4%. 4242. 444 Zaas2-w, s/62-(214. A/ 42//7. 3,101,452 United States Patent Office Patiented Aug. 20, 1963 2 tapped impedance element which completes the bridge 3,101,452 circuit of the balanced modulator also forms a part of VOLTAGE-VARABLE CAPACEOR BREDGE the phase detector. That is, the carrier wave which is AMPLEFER used as a phase reference voltage is taken from across the Don R. Holcomb and David B. Leeson, Los AEageles, two halves of the tapped impedance element. This aim Calif., assignors to Hughes Aircraft Company, Calver plifier is referred to as a variable parameter amplifier. City, Calif., a corporation of Delaware The following specification and the accompanying Fied June 30, 1959, Ser. No. 823,879 drawing describe and illustrate exemplifications of the Caifa. (C. 330-0) present invention. Consideration of the specification and The present invention relates to voltage-variable capaci. O the drawing will lead to an understanding of the inven tor circuits and, more particularly, to a bridge circuit tion including the novel features and objects thereof. utilizing the capacitance variation of semiconductor ele Like reference characters are used to designate like parts ments with applied potential as a balanced modulator cir throughout the figures of the drawing. cuit or as a variable parameter amplifier. FIG. 1 is a diagram of an embodiment of a balanced Heretofore, electronic circuits have often utilized active modulator circuit in accordance with the invention; elements such as vacuum tubes or transistors to provide FiG. 2 is a diagram of an embodiment of a variable modulation or amplification. Vacuum tubes and tran parameter amplifier in accordance with the invention; and sistors have several disadvantages, for example, they are FIG. 3 illustrates waveforms of signals appearing in relatively unstable, introduce noise into the circuit and are the circuits of FIGS. and 2. inefficient. Direct-coupled vacuum tube amplifiers are 20 Semiconductor devices have, in addition to their unilat very unstable and are inefficient in that power is required eral conduction properties, capacitance or the ability to to heat the vacuum tube cathode. A transistor amplifier store an electrical charge, when they are biased to be cannot be connected to a signal source having a high out nonconductive. At a P-N junction the density of charge put impedance without loading the source because tran carriers (electrons in the N region and holes in the P sistors are low impedance devices. The input level of a 25 region) is reduced virtually to zero when a voltage is transistor amplifier drifts over a period of time which is applied across the junction in the opposite direction from equivalent to increasing the noise introduced into the cir that causing easy current flow. As the voltage increases, cuit. the region of zero carrier density known as the depletion Accordingly, it is an object of the present invention to region, becomes wider. In effect, this increases the separa provide a modulator or amplifier circuit which utilizes 30 tion between the two charge-carrying areas and thereby passive elements. decreases the capacitance of the semiconductor device, as Another object of the invention is to provide a modula though there were two metal plates separated by a die tor or amplifier circuit which introduces substantially no electric whose thickness was variable. The area of the OS. plates remains the same; the dielectric constant is un Yet another object of the invention is to provide a 35 changed; but the thickness of the dielectric varies accord modulator or amplifier circuit having a high input im ing to the applied voltage. pedance. For capacitor action, it may appear that the semicon An even further object of the invention is the provision ductor device must always be biased so that the net volt of a modulator or amplifier circuit which introduces sub age applied to the junction never falls to zero. It has been stantially no amplitude, phase, and frequency distortion. 40 found, however, that when using silicon semiconductor In accordance with these and other objects of the in devices, the bias can be as low as zero volts or even up vention, a circuit is provided which utilizes variations in to 0.4 volt in the conducting direction. Silicon devices the capacitance of semiconductor elements with variations also have extremely small leakage currents when biased in applied potential. A pair of voltage-variable capaci to be nonconductive and have a sufficiently high Q, or tors having a capacitance which varies with applied poten 5 figure of merit, to fulfill the requirements of nearly all tial are connected in series with the same polarity. That capacitor applications. is, the cathode of one is connected to the anode of the Although any semiconductor devices, such as, for ex other so that a potential applied from a point between ample, germanium diodes, may be used in circuits ar the two voltage-variable capacitors and the outer ends will 50 ranged according to the present invention, it has been cause the capacitance of one to increase and the capaci found that silicon voltage-variable capacitors manufac tance of the other to decrease. A center-tapped imped tured by Hughes Aircraft Company and bearing the type ance element is connected in parallel with the series-con number HC7001 through HC7008 are particularly satis nected voltage-variable capacitors to form a bridge cir factory. Silicon voltage-variable capacitors are silicon cuit. An input signal source is connected in series with 55 diodes which have been developed and selected for their a load from the junction between the two voltage-variable capacitance characteristics. capacitors to the center tap of the impedance element. A Referring now to FIG. 1 of the drawing, wherein there source of a carrier wave, or pump, is coupled to the im is shown a balanced modulator circuit in accordance with pedance element to develop a carrier wave thereacross. the invention, first and second voltage-variable capacitors By virtue of the bridge arrangement, any noise developed 60 10 and 1 are provided. The voltage-variable capacitors by the pump is canceled. The variations of the input 19 and 11 are connected in series, and with the same signal cause a change in the capacitance of the voltage polarity; that is, the cathode of the first voltage-variable variable capacitors, one increasing in capacitance and the capacitor 10 is connected to the anode of the second volt other decreasing, causing the carrier wave to be modulated age-variable capacitor 11 at a junction point 12. An im by the input signal waveform. If the circuit is balanced, 65 pedance element, such as inductor 13 having a tap 14 the carrier wave will be suppressed at the output terminals which may be centrally disposed, is coupled in parallel and only the modulation sidebands will appear. with the voltage-variable capacitors 10 and 11 by means In accordance with another embodiment of the inven of a pair of series blocking capacitors 15 and 16. The tion, a phase detector is coupled between the bridge cir center-tapped inductor i3 in conjunction with the voltage cuit and the load to provide a reproduction of the input variable capacitors 18 and 1 form a bridge circuit. signal at a much lower impedance level, thus providing To balance the capacitance of the voltage-variable ca power amplification. The circuit is arranged so that the pacitors ié) and , first and second variable trimming 3,01,453 3 4. capacitors 17 and 18 are provided, each being individual bands, the carrier wave being suppressed or eliminated. ly connected in parallel with one of the voltage-variable This may be seen from the waveforms shown in FIG. 3. capacitors 10 and E. More precisely, the first variable The carrier wave applied to the coupling coil 21 is in capacitor 7 is connected in parallel with the first volt duced into the inductor 13 by means of the mutual in age-variable capacitor 10 and the second variable capaci 5 ductance therebetween, indicated on the drawing by the tor 18 is connected in parallel with the second voltage brace symbol designated M joining the coupling coil 21 variable capacitor 1. and the inductor 13. The carrier wave voltage developed A carrier wave source, or pump 29, is coupled to the across the inductor 13 is applied across the series-con inductor 13 by means of a coupling coil 21, the carrier nected voltage-variable capacitors 50 and 1 by means of wave having a frequency which may be, for example, two O the coupling capacitors 55 and 16. megacycles per second. The carrier wave source, or As indicated by the second waveform of FIG. 3, the pump 20, may be a conventional oscillator. When the carrier wave is a sine wave whose instantaneous ampli bridge is balanced, no carrier wave voltage will appear tude varies at a high frequency. The capacitance of the between the junction point 12 intermediate the voltage voltage-variable capacitors 6 and if varies correspond variable capacitors G and 1 and the center tap 14 of 5 ingly, both increasing or both decreasing, but when the the inductor 13. Also, the balanced bridge arrangement bridge is balanced and when no input signal is applied, cancels any noise developed by the carrier wave source 28. no difference in potential appears between the junction A load or utilization circuit 22 has one side connected point 2 intermediate the voltage-variable capacitors 10 to the center tap 4 of the inductor i3 and the other and fi and the center tap 14 of the inductor.
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