| 摘要 |
A transconductance amplifier has an input stage and an output stage. The input stage includes a differential input circuit for converting a differential voltage signal applied thereto into a differential current signal, a first pair of regulated cascode circuits for adjusting output voltages of the differential input circuit depending upon a control voltage applied thereto, and an input section of a pair of current mirror circuits for mirroring the differential current signal from the differential input circuit. The differential input circuit, the first pair of regulated cascode circuits, and the input section of the pair of current mirror circuits are connected in series with each other between a voltage source and a ground. The output stage includes an output section of the pair of current mirror circuits, a differential current source circuit, a second pair of regulated cascode circuits for keeping output voltages of the current mirror circuit to a first bias voltage applied thereto, a third pair of regulated cascode circuits for keeping output voltages of the differential current source circuit to a second bias voltage applied thereto, output terminals of the third pair of regulated cascode circuits being connected to output terminals of the second pair of regulated cascode circuits, and a pair of amplifier output terminals connected to the output terminals of the second and third pairs of regulated cascode circuits. The output section of the pair of current mirror circuits, the second and third pairs of regulated cascode circuits and the differential current source circuit are connected in series with each other between the voltage source and the ground. A transconductance amplifier tuning circuitry includes two reference signal input terminals to which reference frequency signals are input, an RC phase-shifter connected to the two reference signal input terminals and provided with two transconductance amplifiers each having the aforementioned configuration and two capacitors, a multiplier with input terminals connected to two output terminals of the RC phase-shifter and to the two reference signal input terminals, and an operational amplifier with two differential input terminals connected respectively to two output terminals of the multiplier and with an output terminal connected to control terminals of the transconductance amplifiers. The tuning circuitry feedback-controls gains of the two transconductance amplifiers so that the RC phase-shifter always shifts a phase of the input reference frequency signals by 90 degrees.
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