The present disclosure relates generally to electronic circuits, and more particularly to bandgap reference circuits. Still more particularly, the present disclosure relates to bandgap reference circuits that can operate at a low voltage.
Reference circuitries generate reference voltages and currents that are used in a variety of semiconductor applications, including flash memories, Dynamic Random Access Memories (DRAMs) and analog devices. These circuitries are required to be stabilized despite process and temperature variations, and must be implemented without modification of its fabrication process. A reference voltage that exhibits little dependence on temperature is essential in many analog circuits. If a voltage reference is temperature independent, it is usually process independent as well, since variations in most process parameters affect voltage reference through variations in temperature.
A conventional bandgap reference generator is one of the more popular reference voltage generators that can stabilize reference voltage despite process and temperature variations. Bandgap is the energy gap in a semiconductor that separates the valence band, where electrons cannot conduct, and the conduction band, where electrons can conduct. A bandgap reference generator typically operates by creating a device that has a nominally zero temperature coefficient. One method of achieving the nominally zero temperature coefficient is to use a positive temperature coefficient of one part of the device to cancel out a negative temperature coefficient of the other part of the device.
Bipolar transistors may be used for forming the bandgap reference circuits. The base-emitter voltage of a bipolar transistor typically exhibits a negative temperature coefficient. The difference between the base-emitter voltages of two bipolar transistors with unequal current densities operating together exhibit a positive temperature coefficient. Therefore, a bandgap voltage generator may be designed by connecting two bipolar transistors in parallel with unequal current densities and ensuring that the positive and negative temperature coefficients cancel each other out.
Typically, the minimum operating voltage to drive a reference voltage generator must exceed 1.25 volts, or the bandgap voltage of silicon, because the common-collector structure of a bipolar transistor and the input common-mode voltage of an amplifier require at least that much voltage to drive any bandgap reference voltage generator.
However, with the spread of battery-operated, portable applications such as cellular phones and wearable computing devices, device designs increasingly demand low-power and low-voltage circuitries due to power supply limitations. In addition, advanced deep sub-micron Complementary Metal-Oxide-Semiconductor (CMOS) technologies require low power supply voltage. Therefore, it is understood that in the near future, the operating voltage of most devices will be below 1 volt.
Desirable in the art of bandgap reference voltage generator designs are additional designs and methods with which bandgap reference circuitries can successfully operate with a low operating voltage such as one below one volt.