Linearity improvement of microwave amplifiers by special harmonic loads

In this paper, the effects of special harmonic loads are presented in the case of microwave amplifiers. The third-order intercept point (IP3) and the 1-dB compression point (P1dB) of the amplifier are improved with the use of special harmonic loads. To demonstrate this effect an amplifier was designed applying a BFG424F NPN type bipolar transistor. The amplifier was designed for a broad band providing gain from 1 GHz to 6 GHz. This way it can be used for testing its performance with the harmonic loads. The effect of loads is demonstrated for a 1.5 GHz sinusoidal signal. Two cases will be shown in this paper. In the first case the harmonic load was designed to reflect the 3 GHz component (the 2^nd harmonic of the input signal) in opposite phase back to the amplifier, while in the second case the 4.5 GHz component (the 3^rd harmonic of the input signal) is reflected. With the use of this load the gain at 1.5 GHz is improved by 2 dB and the third-order input intercept point is also improved by 1 dB. A significant advantage of the present approach is that the delay time of the harmonic reflection is very short which means a large bandwidth.