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 2nd harmonic of the input signal) in opposite phase back to the amplifier, while in the second case the 4.5 GHz component (the 3rd 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.

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