This circuit provides square-law response to better than +-1% over the range of 0.1<V<8.5 volts for a varicap diode as the square of the angle of rotation of a potentiometer control. Therefore, it results in linear voltage across the varicap diode, the dial of a radio used this schematic also will be linear. The circuit uses two 9 volts power sources, two 2N5172 bipolar NPN transistors, 10k potentiometer and 10k resistor.
The original circuit of the square-law potentiometer by F. N. H. Robinson, Clarendon Laboratory, Oxford.
Circuit diagram of the square-law potentiometer in LTSpice program.
Output voltage for different potentiometer angles of the circuit simulated in LTSpice program.
Angle–voltage characteristic of the circuit simulated in LTSpice program.
Transistors T1 and T2 can be replaced with common NPN 2N3904 transistors or similar.
«Wireless World» (UK), 1973, December.