Variable RF attenuator

A variable attenuator for a source of R.F. signal, said attenuator having an attenuation characteristic that is a function of the level of a source of d.c. control voltage, comprising a pi R.F. attenuator having shunt input and output branches between which is connected a series branch, each of said branches including a voltage controlled variable impedance PIN diode, means responsive to the control voltage for converting the level of the control voltage into a first d.c. current source having a magnitude substantially equal to K.sub.3 +K.sub.2 e.sup.-K 1.sup.V, where e is the base of natural logarithms, K.sub.1, K.sub.2 and K.sub.3 are constants and V is the level of the control voltage, a first d.c. bias circuit for supplying the current of the first source as a d.c. bias for the diode of the series branch while isolating the d.c. current of the first source from the PIN diodes of the shunt branches, means responsive to the control voltage for converting the level of the control voltage into a second d.c. current source having a magnitude substantially equal to K.sub.4 +K.sub. 5 V, where K.sub.4 and K.sub.5 are constants, a second d.c. bias circuit for supplying the current of the second source as d.c. biases for the diodes of both shunt branches while isolating the d.c. current of the second source from the PIN diode of the series branch.