A bi-polar modulator that can perform quadrature modulation using amplitude modulators is described. In one design, the bi-polar modulator includes first and second amplitude modulators and a summer. The first amplitude modulator amplitude modulates a first carrier signal with a first input signal and provides a first amplitude modulated signal. The second amplitude modulator amplitude modulates a second carrier signal with a second input signal and provides a second amplitude modulated signal. The summer sums the first and second amplitude modulated signals and provides a quadrature modulated signal that is both amplitude and phase modulated. The first and second input signals may be obtained based on absolute values of first and second modulating signals, respectively. The first and second carrier signals have phases determined based on the sign of the first and second modulating signals, respectively. Each amplitude modulator may be implemented with a class-E amplifier.