An LTspice BANDGAP example

The bandgap has become an essential part of all integrated circuits not only for references but also making stable bias circuits over temperature, supply, and process variations. Understanding it's basic operation and tradeoffs is essential to mixed signal design. 

A classic way to create a stable bandgap voltage reference is to use two matched bipolar transistors that are operated at different current densities. Even though the devices are physically identical, biasing them differently causes their base-emitter voltages (V_BE) to diverge slightly.

Here’s the key idea:

• V_BE decreases with temperature (about –2 mV/°C), which is a CTAT (Complementary to Absolute Temperature) behavior.
• The difference in V_BE between the two transistors, when run at different current densities, is proportional to absolute temperature (PTAT).
• By scaling and summing these two voltages — the CTAT V_BE and the PTAT ΔV_BE — you can cancel out the temperature dependence, producing a nearly constant voltage close to the silicon bandgap (~1.2 V).

Download our example circuit and begin simulating in LTSPICE. Modern references employ all manner of curvature correction methods, but this basic circuit still remains at the heart of any good reference.