TRANSMISSION CIRCUIT

PURPOSE:To prevent power efficiency from being degraded, current consumption from being increased or output distortion from being enlarged even when a carrier wave signal level is changed by changing a power supply voltage and a bias voltage to be impressed to a power amplifier each time the carrier wave signal level is changed. CONSTITUTION:First of all, a control circuit 16 outputs a voltage source control signal (Vcon) in proportion to a reference voltage (Vref). A constant DC voltage (Vdc) is applied to a variable voltage power source 18, converted into any suitable voltages (Vdd and Vgg) by the control voltage (Vcon) and supplied to a power amplifier 15. There are seven stages in the level of the output voltage (Pout) of the transmission circuit to be used for a portable telephone set and corresponding to the respective levels, the power supply voltage (Vdd) and the bias voltage (Vgg) of the power amplifier 15 are controlled so as not to degrade the power efficiency. This is to decide the control voltage (Vcon) corresponding to the reference voltage (Vref) for deciding the respective levels.