A bypass circuit is disclosed for use with lower power supply voltage PC cards. The bypass circuit controls the power supply voltage fed to a power amplifier when switching between a lower power 8-PSK modulation mode and a higher power GMSK modulation mode. A step-up DC/DC converter provides a higher voltage to the power amplifier than can be supplied by an original power supply. Switch control logic controls a step-up switch and a battery switch. The step-up switch is turned on when operating in an 8-PSK modulation mode to provide a higher voltage to the power amplifier than the original power supply voltage. The battery switch is turned on when operating in the GMSK modulation mode to provide the original power supply voltage to the power amplifier.