Temperature setpoint circuit with hysteresis

bebebeA temperature setpoint circuit comprises bipolar transistors Q and Q which receive currents I and I at their respective collectors and are operated at unequal current densities, with a resistance R connected between their bases such that the difference in their base-emitter voltages (ΔV) appears across R. An additional PTAT current I is maintained in a constant ratio to I and I and provided to the collector of Q while Q is off, and is not provided while Q is on. The circuit is arranged such that Q is turned on and conducts a current equal to Ia when:ΔV=(kT/q)ln(NI/Ia), where Ia=I+I, the temperature T at which ΔV=(kT/q)ln(NI/Ia) being the circuit's setpoint temperature, such that the switching of current I provides hysteresis for the setpoint temperature which is approximately constant over temperature.