A circuit structure for microwave frequency doublers which features effective suppression of fundamental and odd harmonics (>50 dB), conversion gain (>3 dB) and simplicity of the circuit is presented. Either a balanced or unbalanced configuration can be selected at the output without requiring baluns or transformers, thereby reducing the loss and simplifying the circuit. Nonlinear computer simulation was carried out during the design of a monolithic-microwave-integrated-circuit (MMIC) doubler using the present circuit configuration. Very good performance was predicted and verified by experimental results.<<ETX>>