A resistor (9-1) and a resistor (11-1) are connected in parallel with each other between a source of an input transistor (7-1) and the ground. A switch (12-1) is provided between the resistor (11-1) and the source. A variable resistor circuit may be constituted by the resistor (9-1), the resistor (11-1) and the switch (12-1). Further, a capacitor (10-1) and a variable capacitor (13-1) are connected in series with each other between the source and the ground. A control terminal (14-1) to which a voltage is applied when capacitance of the variable capacitor (13-1) is controlled is provided between the capacitor (10-1) and the variable capacitor (13-1). A variable capacitor circuit may be constituted by the capacitor (10-1) and the variable capacitor (13-1). An input capacitance change circuit may be constituted by the variable resistor circuit and the variable capacitor circuit.