low frequency noise fig4 and fig5 are used to explain in specifics the characteristics wherein the impedance is high being sharp at the transition point between the low frequencies wherein the impedance is low and the high frequencies in the variable impedance circuit za fig4 is a diagram to which is added the flow of the electric current to assist in explaining the operation of the conventional variable impedance circuit z 1 illustrated in fig9 fig5 is a diagram to which is added the flow of the electric current to assist in explaining the operation of the variable impedance circuit za of the example illustrated in fig2 first the operation will be explained based on the conventional variable impedance circuit z 1 the input electric current iin_ac is branched into iac_r 2 iac_c 1 and iac_q 1 b depending on the frequency characteristics of the lpf 1 which are determined primarily by the time constant of the resistor r 2 the capacitor c 1 and the resistor r 5 the frequency characteristics are such that the iac_c 1 is large and iac_q 1 b is small from the lpf cutoff frequency fc 1 point with the result that with an input electric current of a frequency that is higher than fc 1 the collector current of the transistor q 1 the ac component can be small and the base current of the transistor.