An upconverter and a downconverter having good performance are described. In one design, the upconverter includes first, second, and third sets of transistors. The first set of transistors receives baseband signals and provides an upconverted signal. The second set of transistors switches the transconductance of the transistors in the first set based on transmit (TX) local oscillator (LO) signals. The third set of transistors enables and disables the transistors in the second set based on a TX VCO signal. In one design, the downconverter includes first, second, and third sets of transistors. The first set of transistors receives a modulated signal and provides baseband signals. The second set of transistors switches the transconductance of the transistors in the first set based on receive (RX) LO signals. The third set of transistors enables and disables the transistors in the second set based on an RX VCO signal.