A low distortion transconductance amplifier provides current to a grounded load using a virtual ground input stage, a pair of current mirrors, and a bias current source. The virtual ground input stage may include transistors arranged as a Darlington pair. The low distortion transconductance amplifier can function as a voltage-controlled AC current source that is operable at high frequencies.

A circuit, system, and method for measuring capacitance are described. A current may be received at an input of a conversion circuit. The current may be converted to a voltage signal which may be used to create a negative feedback current to the input of the conversion circuit and which may be demodulated digitally to provide a static digital output representative of a capacitance.

A compensation circuit of a power amplifier includes a varactor, a voltage sensor and a control circuit. The varactor is coupled to an input terminal of the power amplifier. The voltage sensor is arranged for detecting an amplitude of an input signal of the power amplifier to generate a detecting result. The control circuit is coupled to the varactor and the voltage sensor, and is arranged for controlling a bias voltage of the varactor to adjust a capacitance of the varactor according to the detecting result.

A circuit, system, and method for measuring capacitance are described. A current may be received at an input of a conversion circuit. The current may be converted to a voltage signal which may be used to create a negative feedback current to the input of the conversion circuit and which may be demodulated digitally to provide a static digital output representative of a capacitance.