The predistortion circuit with concave characteristic is developed from a novel functional model for class-AB solid-state bipolar amplifiers. The model, based on a Fourier sine-series is simple and can provide closed-form expressions for the intermodulation products resulting from exciting the power amplifier by a multi-sinusoidal signal. The special case of a two-tone equal-amplitude signal is considered in detail. The present model can be used to build a database for the intermodulation performance of the class-AB bipolar SSPAs excited by a multi-sinusoidal signal. Using this database, the diode-based predistortion circuit with concave characteristic for linearizing the class-AB solid-state power amplifier is designed.