Patent ID: 7894727
Filing Date: 2011-02-22
Classification: H03F

Abstract:
1. A bandpass filter circuit, comprising: a first transconductance amplifier circuit for converting a differential input voltage into a differential output current; a second transconductance amplifier circuit for converting a differential input voltage into a differential output current; a third transconductance amplifier circuit for converting a differential input voltage into a differential output current; a first common-mode feedback circuit to which a differential output voltage of the first transconductance amplifier circuit and a reference voltage are input, and which outputs a first control signal to the first transconductance amplifier circuit so as to make a D.C. voltage level of a differential output voltage of the first transconductance amplifier circuit equal to the reference voltage; a second common-mode feedback circuit to which a differential output voltage of the second transconductance amplifier circuit and the reference voltage are input, and which outputs a second control signal to the second transconductance amplifier circuit so as to make a D.C. voltage level of a differential output voltage of the second transconductance amplifier circuit equal to the reference voltage; a voltage supply circuit for supplying, as the reference voltage, a voltage which is hardly affected by the power source voltage to the first and second common-mode feedback circuits; and a first capacitor; a second capacitor; and a third capacitor, wherein: a noninverting input terminal is connected, via the first capacitor, to (i) a noninverting output section of the first transconductance amplifier circuit and (ii) a noninverting input section of the second transconductance amplifier circuit; an inverting input terminal is connected, via the second capacitor, to (i) an inverting output section of the first transconductance amplifier circuit and (ii) an inverting input section of the second transconductance amplifier circuit; a noninverting output section of the second transconductance amplifier circuit is connected to (i) an inverting input section of the first transconductance amplifier circuit, (ii) a noninverting input section and an inverting output section of the third transconductance amplifier circuit, and (iii) one end of the third capacitor; an inverting output section of the second transconductance amplifier circuit is connected to (i) a noninverting input section of the first transconductance amplifier circuit, (ii) an inverting input section and a noninverting output section of the third transconductance amplifier circuit, and (iii) another end of the third capacitor; the noninverting output section of the third transconductance amplifier circuit serves as an inverting output terminal, the inverting output section of the third transconductance amplifier circuit serves as a noninverting output terminal; the noninverting output section and the inverting output section of the first transconductance amplifier circuit serve as input terminals of the first common-mode feedback circuit; the noninverting output section and the inverting output section of the second transconductance amplifier circuit serve as input terminals of the second common-mode feedback circuit; and the power source voltage is supplied from a power source terminal to each of the circuits.