PURPOSE: To set optionally an impedance viewed from a signal terminal with less current value by connectig respectively plural diodes in forward connection between the signal input terminal and a power terminal and a constant current source between the signal input terminal and earth respectively.

CONSTITUTION: The titled circuit consists of n-set of diodes 5 in p-n forward direction connected in cascade between the signal input terminal 3 connected to a high impedance circuit 4 and a power terminal 1 and the constant current source 6 connected between the terminal 3 and earth 2. The impedance viewing the circuit from the terminal 3 is decided by an impedance Z1 of the diodes 5 and expressed as Z1=nkT/qI, where (k) is Boltzmann's constant, T is a temper ature (q) is an electric charge of an electron, and I is a current value of the current source 6. Further, the operating bias point VOP of the terminal 3 is expressed as VOP=V-n×(kT/q)ln(I-IS), where (n) since is an integer. In giving three conditions of Z1, VOP, (n), a constant current value is obtained and since a value kT/q is 26mV at 27°C, the impedance per diode is given as 26Ω at 1mA.