detection circuit includes a current source first to fourth transistors whose emitters are connected in common to the current source a reference voltage source for supplying the bases of the first and the second transistors with a reference voltage input terminal means connected to the bases of the third and the fourth transistors for receiving an input signal a power supply source for supplying the first to fourth transistors with a power supply voltage first collector load means connected between the power supply source and a node connecting the collectors of the first and second transistors in common second collector load means connected between the power supply source and another node connecting the collectors of the third and fourth transistors in common and an output terminal means connected between the nodes for taking out an automatic gain control detection output application_number US 84460886 A description the present invention will be described in detail with reference to the accompanying drawings namely fig2 to 8 throughout the drawings like reference numerals and letters are used to designate like or equivalent elements for the sake of simplicity of explanation referring now to fig3 there is shown an embodiment of an agc detection circuit according to the present invention in the agc detection circuit a power supply source e3 is connected to the collectors of transistors q21 and q22 through a resistor r21 and also to the collectors of transistors q23 and q24 through a resistor r22 for.