mirror circuit 14 is connected between the v cc rail and the current flow path of the npn transistor 24 of the translinear circuit 11 additionally the second pnp transistor 49 is connected in series with the third pnp transistor 50 which has its base connected to the collector of the npn transistor 24 of the translinear circuit 11 thus the current that flows in the pnp transistor 50 mirrors the current that flows through the pnp transistor 48 and the npn transistor 24 when the circuit 10 is selected in dependence upon the state of the bias voltage appearing on line 17 the third output mirror circuit 15 is a cross coupled collector current source type with the current supplied to each leg via the pnp transistors 47 and 50 of the current mirrors 13 and 14 the current mirror 15 includes three npn transistors 52 53 and 54 the npn transistors 52 and 53 are connected from the collectors of the pnp transistors 47 and 50 respectively to the reference potential rail 22 an n channel mos transistor 60 which may be a linear mos device is connected between the input stage 61 of the video preamplifier and the reference potential rail 22 with its gate connected to the collector of the mirror transistor 52 one of the advantages derived from the use of a mos device of the type described is that such mos device has essentially zero current leakage from its base therefore the charge on the capacitor 64 used to establish the control voltage.