PURPOSE: To reduce number of components, to increase the loop gain with a simple circuit and to obtain the effect of stable level control by adopting the constitution such that a slope of a switching characteristic of transistors(TRs) for a DC amplifier in an automatic level control circuit of a microwave amplifier fed back to an attenuator.

CONSTITUTION: Let a monitor output be XmV with a high frequency output power Px at an output terminal RFPOUT in a transistor switch circuit, then when a power RFPIN is applied and the RFPOUT, does not reach Px, a potential difference between a monitor output voltage and a reference voltage is nearly 0.6V or over, the 1st TR 16 is turned on and the 2nd TR 17 is turned off. with the RFPIN increasing, the RFPOUT reaches the Px. Then the monitor output voltage is XmV and the difference between the monitor output voltage and the reference voltage is nearly 0.6V or below, the TR 16 is turned off and the TR 17 is turned on. Thus, the high frequency output power RFPOUT with respect to the high frequency input power RFPIN is constant in a range of a prescribed level control.