PURPOSE: To prevent recoupling from the output to the input by providing a shield metallic case shielding a waveguide fundamental transmission mode with respect to a signal band to an output circuit of at least one amplifier element in microwave amplifiers.
CONSTITUTION: Matching circuits 3, 4 having amplifier elements 1, 2 are formed to the metallic case of W1 wide. The metallic case 6 in the output direction of the amplifier element 2 is reduced for the width W2 and an output matching circuit 5 is formed in the inside. Moreover, a high impedance line 12a in λ/4 long in the power supply circuit 12 is provided and the line is folded to make the width W2 narrower. Further, the width W2 of the metallic case 6 is selected to be the size in giving the cut-off frequency of the waveguide fundamental transmission mode in the signal frequency. Thus, most power radiated from the output matching circuit 5 of the next stage is cut off and the recoupling with a first stage input matching circuit 3 due to space propagation is prevented. There is no size limit in the matching circuits 3, 4 and all circuit components are contained and the deterioration in the characteristic is prevented.
OKABE KAORU
MIYAZAKI MASAO
OTA TOMOZO
