The circuit exploits the fact that when fed from a constant current source, the forward voltage of a silicon diode varies with temperature in a reasonably linear way. Diode Dl and resistor R2 form a potential divider fed from the constant current source. As the temperature rises, the forward voltage of Dl falls tending to turn Ql off. The output voltage from Ql will thus rise, and this is used as the control voltage for the CMOS VCO
With the values shown, the device gave an increase of just under 3 Hz/Â°C (between 0 Â°C and 60 Â°C) giving a frequency of 470 Hz at 0 Â°C.