A gain-controllable magnetoresistive analog amplifier comprises a substrate located in an X-Y plane, an output signal magnetoresistive sensor located on the substrate, and an input signal coil and a gain adjustment coil. The input signal coil and the gain adjustment coil are respectively located on two side surfaces of the output signal magnetoresistive sensor. The gain adjustment coil is used to input a gain signal by the generation of a gain magnetic field, in order to set the gain the magnetic field is applied along a magnetization direction of a free layer of the output signal magnetoresistive sensor, thereby adjusting the slope of the input resistance-magnetic field transfer curve of the output signal magnetoresistive sensor. The input signal coil is used for inputting a current signal to generate an input magnetic field, in order to apply the input magnetic field to a magnetization direction of a pinned layer of the output signal magnetoresistive sensor, thereby controlling the gain signal to adjust a gain factor of an output signal after the current signal passes through the output signal magnetoresistive sensor. This magnetoresistive analog amplifier provides isolation between input signals, output signals, and controllable gain signals.

FETs used in a conventional current-to-voltage converter lack current-to-voltage conversion efficiency and have a narrow operating frequency range when operated at cryogenic temperatures, and it is difficult to sensitively measure current. A desired low-temperature environment cannot be realized either due to power consumption in the current-to-voltage converter. A current-to-voltage converter is provided that sensitively measures small currents even in extremely low-temperature conditions. The current-to-voltage converter of the present disclosure uses elements specifically optimized for low-temperature operation (e.g., HEMTs) as electronic elements for current-to-voltage conversion. This configuration realizes significantly more excellent current-to-voltage conversion characteristics than those of the conventional technique even when the current-to-voltage converter is operated at a low temperature of 150K or less or in cryogenic temperature conditions close to absolute zero.

An amplifier circuit includes an amplifier and a bias circuit. The bias circuit includes a bias transistor having a base terminal and a collector terminal, a transistor having a gate terminal, a source terminal, and a drain terminal, a transistor having a gate terminal, a source terminal, and a drain terminal, resistors, and a current source. The source terminals are connected to a power source. One end portion of the resistor is connected to the base terminal, the other end portion of the resistor is connected to the drain terminal, one end portion of the resistor is connected to the other end portion of the resistor, the other end portion of the resistor is connected to the bias output terminal, and the bias circuit further includes a feedback circuit that controls the electric potential of the base terminal based on the electric potential of the collector terminal.

FETs used in a conventional current-to-voltage converter lack current-to-voltage conversion efficiency when operated at cryogenic temperatures, and it is difficult to sensitively measure current. A desired low-temperature environment cannot be realized either due to the heat inflow into a cooling device from outside. A current-to-voltage converter is provided that sensitively measures small currents even in extremely low-temperature conditions. The current-to-voltage converter of the present disclosure uses elements exclusively optimized for low-temperature operation (e.g., HEMTs) as electronic elements for current-to-voltage conversion. Significantly more excellent current-to-voltage conversion characteristics than those of the conventional technique are realized even when the current-to-voltage converter is operated at a low temperature of 150K or less or in cryogenic temperature conditions close to absolute zero. Power supply to the current-to-voltage conversion circuit and a bias circuit are simplified, and the heat inflow into the cooling device from outside is suppressed, thus reducing the load on the cooling device.

A high-linearity amplifier including a main operational amplifier, a feedback circuit, and a compensation circuit is shown. The feedback circuit couples an output signal of the main operational amplifier to an input port of the main operational amplifier. The compensation circuit couples a former-stage circuit of the amplifier to the input port of the main operational amplifier to compensate for the non-linearity of the feedback circuit. The compensation circuit and the feedback circuit form an inverse paralleling linearization architecture. In the inverse paralleling linearization architecture, a resistor in the feedback circuit corresponds to a resistor in the compensation circuit which is biased in an inversed way in comparison with the corresponding resistor in the feedback circuit.

A high-linearity amplifier including a main operational amplifier, a feedback circuit, and a compensation circuit is shown. The feedback circuit couples an output signal of the main operational amplifier to an input port of the main operational amplifier. The compensation circuit couples a former-stage circuit of the amplifier to the input port of the main operational amplifier to compensate for the non-linearity of the feedback circuit. The compensation circuit and the feedback circuit form an inverse paralleling linearization architecture. In the inverse paralleling linearization architecture, a resistor in the feedback circuit corresponds to a resistor in the compensation circuit which is biased in an inversed way in comparison with the corresponding resistor in the feedback circuit.

Apparatus and methods for power detection with enhanced dynamic range are provided. In certain embodiments, a front end system includes a power amplifier that amplifies a radio frequency (RF) input signal to generate an RF output signal, a directional coupler that generates a sensed RF signal based on sensing the RF output signal from the power amplifier, and a power detector that processes the sensed RF signal to generate a detection signal indicating an output power of the power amplifier. Additionally, the power detector includes two or more detection paths providing different amounts of gain to the sensed RF signal from the directional coupler.

Various examples are directed to a frequency-compensated amplifier circuit comprising a first multi-stage amplifier comprising a first amplifier input node, a first amplifier output node, and a first amplifier intermediate node. A first feedback path between the first amplifier input node and the first amplifier output node comprises a feedback resistance. A second feedback path between the first amplifier output node and the first amplifier intermediate node comprises a first capacitor and a portion of the feedback resistance. A first switch circuit may be electrically coupled to the first capacitor and to the feedback resistance. The first switch circuit may have a first state in which the first capacitor is coupled to a first tap point of the feedback resistance and the portion of the feedback resistance has a first value. The first switch circuit may also have a second state in which the first capacitor is coupled to a second tap point of the feedback resistance and the portion of the feedback resistance has a second value different than the first value.

When a current-to-voltage converter is used at low temperatures, the frequency band of measurable small currents is limited. Stray capacitance of a coaxial cable that takes out an output voltage of the current-to-voltage converter from inside to outside a cooling device narrows the operating frequency band of the current-to-voltage converter. The current-to-voltage converter of the present disclosure uses elements exclusively optimized for low-temperature operation (e.g., HEMTs) as electronic elements for current-to-voltage conversion. This configuration realizes current-to-voltage conversion characteristics with significantly more excellent sensitivity than that of the conventional technique even when the current-to-voltage converter is operated at a low temperature of 150 K or less or in cryogenic temperature conditions close to absolute zero. Further, a source follower circuit is added to an output stage of the current-to-voltage conversion circuit to isolate the effect of stray capacitance added to the output side of the current-to-voltage conversion circuit and achieve a wider bandwidth.

In a limiting circuit that limits an output voltage of an operational amplifier, the signal quality of the output voltage is improved. The limiting circuit includes a short-circuit transistor and a gate voltage supply unit. In the limiting circuit, the short-circuit transistor short-circuits a path between an input terminal and an output terminal of the operational amplifier in a case where a voltage between the input terminal of the operational amplifier and the gate is higher than a predetermined threshold voltage. Furthermore, in the limiting circuit, the gate voltage supply unit supplies a voltage to the gate, the voltage depending on the threshold voltage and the output voltage of the output terminal.