I was reading up on bandgap references. I noticed that a CTAT voltage can be simply produced by taking the Vbe voltage of a BJT.
However, when generating the PTAT voltage, most books seem to mention that you need two bipolar transistors operating at unequal current densities, then the difference between their base-emitter voltages is PTAT.
My question is, if we have an easy way of generating a CTAT, why do we need a PTAT? Can we simply not just generate another CTAT and invert it by plugging it into the inverting input of an differential amplifier? Why isn't that good enough?