Abstract: A second order all pass network for wide-band signals, and comprising second and third voltage-current converting circuits with their respective positive input terminals supplied with input signals from an input terminal of the network and their negative input terminals supplied with output signals from an output terminal of the network. A first voltage-current converting circuit has its positive input terminal supplied with a positive output of the second voltage-current converting circuit and a negative input terminal supplied with the output signals. A first capacitor is connected between the input and output terminals, and a second capacitor is connected between ground and the positive output of the second voltage-current coverting circuit.

Abstract: A transconductance circuit (10) has its signal input terminals (28,40) at the gates of a pairs of MOSFETS (16,22; 32,38) which are forced to operate in the triode mode. The outputs of the triode mode MOSFET pair are fed to a cascode transistor (18,20; 34,36) for treatment as a differential signal. The differential output (30,42) of the cascode transistors is highly linear with respect to the input signal at the gates of the triode mode transistors. Bias voltages for the gates of the cascode transistors are generated by a bias network (14). The transconductance circuit 12 includes a cross-coupled set of compensation capacitors (62, 64; 66, 68) formed from devices with the same geometries as the triode mode transistors to compensate for high frequency loss due to the Miller effect in the input transistors.

Abstract: A predetermined and variable synthesized capacitance which may be incorporated into the resonant portion of an electronic oscillator for the purpose of tuning the oscillator comprises a programmable operational amplifier circuit. The operational amplifier circuit has its output connected to its inverting input, in a "follower" configuration, by a network which is low impedance at the operational frequency of the circuit. The output of the operational amplifier is also connected to the non-inverting input by a capacitor. The non-inverting input appears as a synthesized capacitance which may be varied with a variation in gain-bandwidth product of the operational amplifier circuit. The gain-bandwidth product may, in turn, be varied with a variation in input set current with a digital to analog converter whose output is varied with a command word. The output impedance of the circuit may also be varied by varying the output set current.

Abstract: An NF (Negative Feedback) tone control circuit in which a circuit gain of unity is maintained for a flat frequency response without the addition of active components. A voltage divider circuit which divides only an a.c. component of the output of an operational circuit is connected between the output of the operational amplifier circuit and a variable impedance circuit, the latter including the variable resistors of the tone control circuit.

Abstract: Space feedback comprising a shield over the tuning elements of a FET radio frequency amplifier provides positive feedback between the gate and drain terminals to raise the gain of the amplifier above what it would be without the space feedback, while simultaneously lowering the noise figure. Not only is the relative noise lowered, but the absolute amount of noise is lowered.

Abstract: An amplifier having a very high gain-bandwidth product includes a first operational amplifier having its positive input connected to the amplifier's input. The output of the first operational amplifier is connected to the base of a transistor, the emitter of which is coupled by a gain-setting resistor to ground and is also directly connected to the negative input of the first operational amplifier. The collector of the transistor is connected by a resistive load to a supply voltage and is also directly connected to the negative input of a second operational amplifier. The output of the second operational amplifier is coupled by a capacitor back to its negative input. The output of the second operational amplifier is coupled by a feedback resistor back to the emitter of the transistor. The output of the second operational amplifier is connected to the output terminal of the amplifier.

Abstract: An amplifier exploits Miller-effect capacitance in a cascaded multi-stage self-limiting amplifier arrangement to achieve automatic adaptive bandwidth behavior (without the use of an active feedback control loop). The Miller-effect capacitance varies in accordance with the amplifier signal-level, with the capacitance in turn varying the amplifier bandwidth. Bandwidth is decreased for low-signal-levels and increased for relatively high-signal-levels. The adaptive amplifier technique is especially useful to reduce broadband noise in an IF stage of a communications device before the usable signal is fed to a detector stage.

Abstract: An amplifier uses non-dissipative elements to provide current and voltage feedback. The base amplifier has two input terminals, and the current and voltage feedback signals are applied to respective input terminals. The non-dissipative elements may be either transformers or coupled-line pairs. These elements allow broad band operation with low noise. The feedback configuration results in higher input impedence.

Abstract: Disclosed is a gain circuit which comprises a transistor for amplifying a high-frequency signal, a variable impedance element connected with an emitter of the transistor and having an impedance variable in accordance with an amount of current flowing therethrough, a control circuit for controlling the amount of current flowing through the variable impedance element, a series circuit composed of a coil and a resistor connected in series with each other, and another resistor connected between the series circuit and a power supply terminal, the variable impedance element and the series circuit being connected in parallel with each other.

Abstract: A solid state electrical instrument amplifier includes an instrument signal input, a preamplifier, and circuitry in association with the preamplifier and preferably in a feedback loop of the preamplifier, for adjusting the frequency compensation of the preamplifier. The amplifier may also include distortion circuitry, again preferably located in a feedback loop of the preamplifier, which compresses the peaks of the input signal thereby giving a distorted output of the preamplifier which maintains the relative even and odd harmonic content of the input. The amplifier may also include a variable band reject filter operated by a sweep rate generator which varies the light output of a light emitting diode which in turn varies the resistance of photoresistors in the variable band reject filter. If signal delay circuitry is included to create a reverberation effect, the amplifier also includes a high pass filter in parallel with the delay circuitry to increase the high frequency content of the output signal.

Abstract: A signal voltage from a signal processing path is coupled via an interfacing terminal to an input port of a filter, the output port of which is connected to the base input of a current source transistor to supply thereto a signal voltage which exhibits the filter transfer function. The collector output of the current source transistor is also coupled to the interfacing terminal and conducts a current exhibiting the filter transfer function to the signal path via the interfacing terminal. The base input of the current source transistor is driven with filter output sigals via a low output impedance voltage source (e.g., an emitter follower stage). The impedance presented to the base of the transistor is primarily determined by the low output impedance of the voltage source.

Abstract: This invention is directed to a narrow band filter for eliminating out-of-band noise in the oscillator signal of a doppler radar system. It provides a very high Q filter derived from a Bulk Acoustic Resonator's (BAR) high Q, L-Band response and uses thermal feedback to lock the BAR's response band onto the input frequency. The BAR is a very narrow band filter with a Q near 40,000 and a comblike frequency response having passbands occurring every 5 MHz, approximately. The comblike response of the BAR shifts down 60 KHz for every 1.degree. C. rise in temperature. In this invention a frequency sensitive heater is used to move the response comb of the BAR up or down in accordance with changes in the frequency of the incoming signal. The input frequency can be allowed to drift and the heat source will produce more heat or less heat, keeping the response right at the input frequency.

Abstract: A dual-stage band pass-band reject filter with feedback. Two state variable band pass filters with identical topology are cascaded as first and second stages. Each stage employs two inverting adders and two inverting integrators in a state variable topology, and each provides a band pass output and a band reject output. Both stages are selectively tuned to the same fundamental frequency. The filter output may be switched from a band pass to a band reject mode. In the band reject mode decreased attenuation of harmonics of the fundamental frequency is achieved by providing a predetermined amount of feedback from either the band reject output or band pass output of the second stage to the input of a respective one of the two adders of the first stage.

Abstract: An electrical signal processing network includes a high order, two port, three terminal filter, and means coupled to the filter for synthesizing, at a single terminal, an impedance exhibiting the transfer function of the filter. The synthesized impedance can be coupled to a single point in a signal path for filtering signals in accordance with the transfer function of the filter.

Abstract: An amplifier comprising a pair of differential input MISFETs, a current mirror circuit connected between the drains of the differential input MISFETs and a power source terminal, a phase compensation circuit connected to the drain of one of the differential input MISFETs, an output stage amplification circuit amplifying the signal produced at the drain of one of the differential input MISFETs, a phase regulation circuit such as a capacitor connected to the other of the differential input transistors, and a feedback circuit feeding back the output signal produced from the output stage amplification circuit to the other of the differential input MISFETs in order to apply negative feedback to the amplifier. Since the capacitor is provided, the phase of the power source noise can be made substantially equal to the phase of noise occurring at the drain of one of the differential input MISFETs due to the power source noise. Hence, hardly any noise is produced from the output stage amplification circuit.

Abstract: An RC active filter device which is implemented with integrated circuit technology. Each RC filter element of the device has a distributed series resistance and a distributed shunt capacitance. The series resistance is formed by a strip of resistive material which overlies a resistive plate (or substrate) with an intervening insulating layer to form the shunt capacitance. The filter device can be designed to have a pass-band ripple response which is not affected by variation in nominal absolute resistance and capacitance values resulting from process spreads. These variations only stretch or compress the filter response along the frequency axis. The filter device comprises three RC filter elements and an operational amplifier. The RC filter elements nearest the operational amplifier determine a low pass band, while the input filter element prevents high frequency components outside the low pass band being leaked directly to the filter device output.

Abstract: A multi-stage amplifier (21, 22, 23, or 24) has three or more amplifier stages (A1, A2, and A3) arranged in a capacitatively nested configuration for frequency compensation. The technique consists of nesting two of the stages together with a pole-splitting capacitor (C1) to form a stable device (21 or 22) and then nesting this device and a third of the stages together with another pole-splitting capacitor (C2) to form the amplifier.

Abstract: An amplifier arrangement includes a first section and a second section. The second section has an output transistor which is protected by a protection circuit. The protection circuit acts on the input of the second section in order to obtain a higher gain in the protection loop. In order to preclude instabilities, the protection circuit has a falling frequency response with a flat portion in view of the frequency compensation of the second section.

Abstract: A predistortion circuit for a microwave power amplifier which exhibits phase and amplitude nonlinearities and has a small signal gain equal to a. The phase distortion circuit includes a 180.degree. coupler, a first coupler for coupling a given fraction of the input power to the 180.degree. coupler and a second coupler for coupling the given fraction of the power output from the amplifier, reduced by a, to the 180.degree. coupler. The 180.degree. coupler has a difference output port at which a signal appears which corresponds substantially to the difference in phase between the two signals input to the 180.degree. coupler and has a sum port at which a signal appears which corresponds substantially to the sum in amplitude between the two signals input thereto.A phase change circuit and attenuator are coupled between the first coupler and input to the power amplifier. Signals related to the signals produced at the difference port and sum port of the 180.degree.

Abstract: An operational amplifier includes an input stage, an output stage including first, second and third NPN output transistors, and an intermediate stage. The output stage is driven by a thermal current having a positive temperature coefficient so as to keep the gain of the output stage substantially constant over temperature. Thus, the amplifiers Miller loop stability will also remain substantially constant over temperature.

Abstract: An operational amplifier includes an input stage, an output stage including first, second and third NPN output transistors, and an intermediate stage. The first NPN output transistor sources load current to an output terminal while the second and third output transistors sink load current therefrom. The output of the intermediate stage is coupled to the base of the third NPN transistor and, via a resistor, to the base of the second NPN transistor. A first Miller capacitor is coupled between the input of the intermediate stage and the amplifier's output terminal, and a second Miller capacitor is coupled between the base of the second NPN transistor and the output terminal.

Abstract: An RF MESFET amplifier having a feedback resistor further incorporates two sets of quarter wavelength low impedance pads and quarter wavelength high impedance lines in the resistor feedback path. These RF impedance elements prevent RF feedback as well as thermal noise generated by the feedback resistor from being coupled to the input. Moreover, they effectively isolate RF interaction between the drain and gate of the MESFET; as a result, the amplifier has excellent stability. The low impedance quarter wavelength pads serve as an RF bypass (to ground), while the high impedance quarter wavelength lines provide high attenuation near frequencies at the given wavelength. Consequently, thermal noise generated by the feedback resistor will be bypassed to ground and will not leak into the gate. The value of the feedback resistor can be selected and optimized to obtain the desired VSWR, bandwidth and gain flatness.

Abstract: An operational amplifier which comprises a first amplifying circuit; a second amplifying circuit which is connected to the first amplifying circuit and carries out inverting amplification; a third amplifying circuit which is connected to the second amplifying circuit, carries out noninverting amplification and has a gain of substantially 1; a first feedback circuit which is connected between the output and input terminals of the second amplifying circuit and carries out phase-compensation; and a second feedback circuit which is connected between the output terminal of the third amplifying circuit and the input terminal of the second amplifying circuit, and carries out phase-compensation.

Abstract: A notch filter is provided having a low-pass section and a summing section which are capable of being integrated on a single semiconductor chip. The summing section provides an output signal which is the sum of a first component signal proportional to an output signal of the low-pass section and a second component signal proportional to current flowing in a capacitor in the low-pass section such that the high-frequency response of the low pass section is compensated to provide an output signal characteristic of a notch filter.

Abstract: A filter circuit which operates as a substantially optimum integrating circuit comprises input and output terminals and a differential amplifier, the amplifier having an inverting input connected to the output via a series circuit including a first capacitor and a first resistor. The amplifier output is coupled via a series circuit of a second resistor and a second capacitor to a terminal carrying a reference voltage. The non-inverting input to the amplifier has a DC connection to the reference voltage terminal. One input to the filter circuit connects to the reference voltage terminal, the other filter circuit input being in circuit with the inverting input to the amplifier. The junction between the second resistor and the second capacitor connects to the filter circuit output terminal. The RC time constant of the first capacitor and the first resistor is substantially equal to the RC time constant of the second capacitor and the second resistor.

Abstract: An adjustable filter arrangement is disclosed which is implemented in integrated circuit technologies. The filter configuration corresponds to a SALLEN-KEY filter in which the amplifier function is obtained by a transistor device, the R1 and R2 resistor functions are obtained by a first pair of suitably connected diodes, and the C1 and C2 functions are also integrated into the semiconductor chip. Adjustment of the natural frequency of the filter is obtained by the value of an externally supplied control current through the pair of diodes corresponding to R1 and R2. Adjustment of the damping characteristic of the filter is obtained by supplying a second control current to cause a relative change between the values of R1 and R2. No external components need be attached to the integrated circuit filter.

Abstract: An operational amplifier includes an input stage, an output stage including first, second and third NPN output transistors, and an intermediate stage. The first NPN output transistor sources load current to an output terminal while the second and third output transistors sink load current therefrom. A first Miller capacitor is coupled between the input of the intermediate stage and the amplifier's output terminal. A second Miller capacitor is coupled between the base and collector terminals of the third NPN output transistor.

Abstract: An operational amplifier with frequency compensation is described. The amplifier includes a first amplifier with a low-impedance output followed by a transconductance amplifier with a capacitive feed-forward. The transconductance amplifier is followed by a Miller integrator.

Abstract: An improved output circuit for an operational amplifier which is controlled to operate in one of two modes. In the first mode, the output of the operational amplifier tracks a reference signal or ground. In the second mode, the output of the operational amplifier tracks the level of a time varying second voltage signal. The improvement consists of replacing the stabilization capacitor of prior art output circuits with a pair of stabilization capacitors connected in parallel. Each of the pair of capacitors has an associated series connected switch for switching the capacitor into and out of the circuit. The switches are operated by a respective one of a pair of external non-overlapping clock pulse trains so as to not be closed simultaneously.

Abstract: An amplifier topology suitable for amplification of the frequency dependent output of a transducer to provide a frequency equalized amplifier output includes a first transconductive stage for converting a voltage input signal to the current domain through a signal node and a second transreactive stage for converting the signal current into an output voltage. An active equalization network feeds back a representation of the output signal to the signal node in such a way that the fed back signal voltage error is not affected by the high impedance thereat to provide error correction feedback that is achromatic with frequency and, accordingly, does not vary across the range of frequencies amplified, to provide the second amplifier with a frequency independent closure ratio, while not reducing second amplifier voltage gain.

Abstract: Headphones have a small cavity between the diaphragm and the ear canal with a microphone in the cavity closely adjacent to the diaphragm providing a feedback signal that is combined with the input electrical signal to be reproduced by the headphones to provide a combined signal that is power amplified for driving the diaphragm.

Abstract: An infra-red receiver front-end includes a trans-impedance amplifier connected to receive an input current and provided with frequency selective transadmittance feedback between its output and input, the transadmittance feedback having a DC path to ground for low frequency noise components in the input current, caused by background illumination.

Abstract: Improved radio transmitter modulation control circuitry is described which significantly increases the average power of the modulating signal, resulting in enhanced audio signal quality since the signal-to-noise ratio is likewise increased. The inventive modulation control circuitry includes a microphone 110, audio amplifier 120, splatter filter 130, modulator 150, potentiometer 140, oscillator and transmitter 180, and antenna 190. The audio amplifier 120 further includes an operational amplifier 160 having an input resistor 162 coupled via capacitor 168 to microphone 110, a phase-lag filter 170 coupled to the output of the operational amplifier 160, and a feedback resistor 161 coupled between the filter 170 and the input resistor 162. When driven into amplitude limiting, the phase and amplitude of harmonic signals from the operational amplifier 160 are altered by phase-lag filter 170 to produce a waveform similar to that shown in FIG. 2.

Abstract: A system suitable for the amplification of wide-band signals, with frequencies from zero to the gigahertz order, for oscilloscopic observation or recording. Included are first and second amplifier circuits. The first amplifier circuit, typically of multistage configuration, has two inputs, one for the reception of a wide-band signal to be amplified and the other for the reception of a low-band signal, and puts out an amplified wide-band signal in response to the wide- and low-band input signals. Comprising an operational amplifier, the second amplifier circuit also has two inputs for the reception of the low-frequency components of the wide-band input and output signals of the first amplifier circuit and, in response to these inputs, produces the low-band signal for delivery to the mentioned other input of the first amplifier circuit.

Abstract: A translating circuit couples an electrical signal filter to a signal path conveying signals to be filtered. The translating circuit exhibits unity current gain and a voltage gain other than unity between the signal path and the filter, and applies signals from the signal path to the filter. By setting the scale factor (e.g., attenuation) of the translating circuit, the impedance of the filter can be set to any desired level without affecting the operating parameters of the signal path. In the case of an inductive filter, the translating circuit permits the filter to be designed with a lower impedance, and a smaller value inductor less susceptible to picking up interference signals likely to adversely affect signals in the signal path, without requiring a corresponding adjustment in the operating parameters of the signal path.

Abstract: A CMOS linear amplifier is disclosed with a frequency compensation circuit that employs a Miller integrater construction in which the feedback capacitor is coupled by way of a noninverting amplifier operating at constant current and therefore does not load the inverting amplifier input or bypass the integrator amplifier.

Abstract: A differential amplifier which incorporates at its preceding stage resistances and condensers combined with each other and serving as a low-pass filter, thereby omitting a conventional low-pass filter separate from a differential amplifier provided at a weight detection circuit for a load cell weighing scale or other detection circuit.

Abstract: A feedback amplifier is provided with a voltage-controlled compensation circuit which varies the effective feedback capacitance by controlling a proportional amount of feedback current returned via the capacitor to the amplifier summing node. The proportionality and current steering of the feedback current is effected by a multiplier circuit, the bias of which is controlled by an adjustable DC voltage. Several feedback amplifier configurations are shown herein, exemplifying the versatility of the compensation circuit of the present invention.

Abstract: This application discloses a virtual ground pre-amplifier for application with magnetic phono cartridges such as moving coil, moving magnet, moving iron, or similar types. The preamplifier according to the present invention comprises a forward path (an amplifier), and a reverse path (shunt feedback). By presenting a relatively low load impedance to the phono cartridge, the preamplifier effectively damps mechanical resonances of the stylus-cartridge system, and thus serves as a "stylus stabilizer."Also disclosed are techniques for equalizing the overall response of the stylus-cartridge-preamp system so as to provide a predetermined (e.g., flat) frequency response. This predetermined frequency response can be achieved by utilizing any of several circuit arrangements which serve to compensate for the intrinsic inductance and dc resistance of the magnetic phono cartridge.

Abstract: An equalizer circuit is provided which has a high D.C. stability without degrading the equalization characteristic. The equalizer circuit comprises an equalizer amplifier, an equalizer element, a direct current component detecting circuit, and a negative feedback circuit. The equalizer amplifier amplifies an input signal of the equalizer circuit in accordance with RIAA equalization characteristic. The output of the equalizer amplifier is applied to the equalizer element which has an impedance corresponding to an inversed RIAA equalization characteristic. The output from the equalizer element, which is not subject to the RIAA equalization characteristic due to the provision of the equalizer element, is supplied to the direct current component detecting circuit in order to derive a direct current component from the output. The derived direct current component is negative fed-back to the equalizer amplifier as a negative feedback signal thereof.

Abstract: A circuit for producing a compensated output signal includes a first and second stage of amplification (18) and (20) and a compensation circuit (16). The first stage of amplification (18) has a signal input, a signal output and a control input. The second stage of amplification (20) also has a signal input, a signal output and a control input with the signal input of the second stage (20) connected to the signal output of the first stage (18). A passive feedback network comprised of a series connected resistor (R.sub.F) and capacitor (C.sub.F) is connected between the signal input and the signal output of the second stage (20). The combination of the passive feedback network, the first stage (18) and the second stage (20) provide a gain and frequency response that is defined by three poles and one zero. The zero overlaps one of the poles thereby providing an extended frequency response. The passive feedback network varies the frequency response in response to manufacturing process variations.

Abstract: A treble-control circuit which exhibits a particularly low noise level and which can be controlled digitally in a simple manner. The treble control circuit comprises an amplifier having an output connected to the arrangement of a capacitor in series with a resistor chain. The tappings of said resistor chain are connected to an inverting input of the amplifier via a first switch and to the output of the treble control circuit via a second switch.

Abstract: A low-noise bass-control device, which is digitally controllable in a simple manner, comprises an amplifier having an output connected to the series arrangement of a capacitor and a tapped resistor chain. The taps on said resistor chain are connected to an inverting input of the amplifier via a first switch and to the output of the control device via a second switch.

Abstract: A bandpass filter suitable for use in modem circuitry and other applications which require the filter's center frequency to be switched is disclosed. The filter circuit has the characteristic that although the center frequency is changed the overall gain of the filter remains constant. This characteristic results from using a plurality of filter sections which each have low individual gain and providing compensating gain at the input of the filter. In addition, compensating gain elements are used in each filter section so that when the center frequency of the filter is changed, the gain of each filter stage remains constant.

Abstract: A bandpass amplifier employing a field effect transistor amplifier first stage (10) with a resistive load (R.sub.2) either a.c. or directly coupled to the non-inverting input of an operational amplifier second stage (20) which is loaded in a Wien Bridge configuration. The bandpass amplifier may be operated with a signal injected into the gate terminal (G) of the field effect transistor and the signal output taken from the output terminal of the operational amplifier. The operational amplifier stage appears as an inductive reactance, capacitive reactance and negative resistance at the non-inverting input of the operational amplifier, all of which appear in parallel with the resistive load (R.sub.2) of the field effect transistor.

Abstract: Disclosed herein is a push-pull amplifier circuit employing field-effect transistors in which the field-effect transistors opposite in electrical conductivity to each other are activated by a common input signal to perform a push-pull operation. Secondary distortion components due to the square-law characteristics of the field-effect transistors are cancelled out by impedance elements each being provided between the source of each field-effect transistor and a common reference potential point.

Abstract: One aspect of the invention includes an automatic gain control system (AGC) having a frequency controlled variable gain amplifier stage whose stage gain is a function of a frequency signal applied to a gain control input of the amplifier stage. An input signal to be processed is applied to an input of the amplifier stage. The output of the amplifier is sensed and compared to a reference potential to produce a frequency signal which is applied to the gain control input of the amplifier. Another aspect of the invention includes a frequency controlled variable gain amplifier embodying the invention comprised of an amplifier means having input and feedback networks and whose voltage gain is a function of the impedance of the feedback network with an impedance of the input network.

Abstract: A signal to be delayed is applied to a frequency selective path including an inverting active bandpass filter and to a non-frequency selective path including a voltage-to-current converter. The paths are coupled to a signal combiner which produces an output current having a first component proportional to the output current of the converter and a second component directly proportional to the filter output voltage and inversely proportional to the value of a gain control resistor. The resistor is selected to provide a predetermined ratio of the output current components to provide an overall allpass characteristic for the delay circuit and biased to conduct no current under quiescent signal conditions thereby reducing power dissipation. Several circuits may be cascaded for such purposes as peaking and delay equalization in television receivers, video disc players or the like.

Abstract: An operational amplifier includes an input stage, an output stage including first and second NPN output transistors and an intermediate stage including first and second emitter follower transistors. In this manner, a .beta..sup.3 current gain is achieved from the amplifier output to the output of the input stage to reduce output loading effects on the input stage. A first capacitor network is coupled between the amplifier output and the output of the input stage to frequency stabilize the amplifier. A second capacitor is coupled between the base terminals of the first and second emitter follower transistors, and a resistor (or diode) is coupled between the emitter of the first emitter follower transistor and the base of the second emitter follower transistor. This network frequency stabilizes the parasitic feedback loop associated with the intermediate stages, the NPN output transistor and the first capacitor network.

Abstract: In a video signal play-back circuit connected so that a play-back signal from a magnetic head may be applied to an amplifier circuit through a resonance circuit which consists of an inductance element and a capacitance element, a resistor for adjusting the quality factor Q of the resonance circuit is connected between one terminal of the resonance circuit and one output terminal of the amplifier circuit, thereby to reduce thermal noises which develop from the Q adjusting resistor.