due to the current density ratios of the current mirror the mirrored current i out has a positive temperature coefficient and is converted to a voltage with similar temperature dependence by resistor r c2 the voltage dropped across r a2 is then added to the base to emitter voltage v be of transistor tr 3 which has a negative temperature coefficient to result in the voltage vref in this particular circuit vref is fed to a feedback amplifier a 1 and is also used to generate the input current for the current mirror fig2 b shows another exemplary bandgap voltage reference circuit a ptat current source is provided by generating a fixed ratio of current densities through two sets of transistors q 1 q 2 and q 4 q 5 the voltage difference due to the different current densities is applied across resistor r b1 the value of the ptat current is nominally given by where v t 0 0267 volts at 37 degrees c and is proportional to absolute temperature a voltage reference vref is generated by summing a base to emitter voltage v be with the voltage across resistor r b2 through which the ptat current is flowing many different bandgap voltage reference circuits have been designed but virtually all work basically.