Voltage Reference is a circuit which generated a stable reference voltage across Process, Voltage and Temperature. Generally, Analog circuits such as DC-DC converters, power management circuits, Analog to Digital Converters require a stable voltage reference circuit with a tight specification for voltage variation in the range of 4ppm/C. Most of the voltage references used in analog circuits are Bandgap voltage reference circuits. Bandgap Voltage reference circuits generate a stable reference voltage of 1.2V. Since Silicon Bandgap energy is 1.12eV, so the circuit is called Bandgap reference.
Bandgap Reference
Bandgap reference can be generated by adding positive temperature coefficient voltage with the negative temperature coefficient voltage to cancel the effect of temperature. In CMOS technology, negative tempco voltage can be generated with a PNP/NPN transistor VBE voltage and positive tempco voltage can be generated using the difference of two VBE voltages(VBE1-VBE2)
VREF= VBE2 + (VBE1-VBE2)
VBE1 = VT*ln(Jc/Jc0),
where VT is the Volt equivalent of temperature,
Jc is the collector current density,
Jc0 is the reverse saturation current density
Basic bandgap reference voltage circuit is shown in the figure below:
Figure 1: Bandgap Voltage Reference
Q1 is PNP transistor and Q2 are N such PNP transistors
In the above circuit, I1 = (VEB1-VEB2)/R1 = VT * ln(N)
So, VREF = VEB2 + (VEB1-VEB2)/R1* R2
Reference Voltage, VREF = VEB2 + VT * ln(N) * R2/R1
The above Bandgap voltage reference circuit requires an opamp which requires and additional current, so for low power voltage reference circuit(current less than 100nA), the circuit given below can be used. The reference voltage can be designed with negative tempco of Vbe of PNP transistor and positive tempco of Vt(thermal voltage). The transistors in this circuit should be in subthreshold region. The challenge in this design is to get a reference voltage less than 1V with low current across process
Figure 2: Low power Voltage Reference
