Voltage References Basics

Voltage reference is a reference voltage Vref stable against process, voltage and temperature. Voltage reference is a basic block used in analog/mixed signal Integrated Circuit(IC) and radio frequency ICs. Voltage reference is used in DC-DC converter, power management circuits, ADC and DAC. So there are more than one voltage reference designed in an analog/mixed signal IC. The design of voltage reference is very important because the accuracy of the voltage reference decides the accuracy and performance of the whole system.

For example, in a voltage regulator the line regulation is one of the important specifications. Line regulation refers to variation in output voltage with the variation in input supply voltages. In a voltage regulator, a voltage reference is generated and is multiplied by a specific ratio to get the regulated output voltage. So if the voltage reference doesn’t vary with the input supply voltage, then the regulated output voltage doesn’t vary with supply voltage. So the accuracy of the output voltage is decided by the accuracy of the reference voltage.

For the figure shown below, 

If VREF changes, then VOUT changes by  1+ R1/R2, so there is a gain from VREF to VOUT. If VOUT specification is 2.5V ± 1%. Then ∆VOUT = ±25mV. If VREF=1.2V and changes by 100ppm/C i.e., For change in temperature of 100C, Vref=1.2±1.2×100×1e-6×100=1.2±0.012V. Let  1+ R1/R2=2.1, then VOUT changes by 2.5±0.0252. From the above example, the accuracy of the VREF decides the accuracy of the regulator output voltage VOUT.

So Voltage reference accuracy determines the accuracy and performance of the system. Generally in a mixed signal system and radio frequency integrated circuits, more than one voltage references are used. So voltage reference should be designed properly and the error sources in the reference generation circuit should be reduced.

Let us study few voltage reference circuits:

A resistive divider can be used a voltage reference. 
VREF=VDD ×  R2/(R1+R2), 
But this type of voltage reference has poor supply and temperature sensitivity.

To generate a stable voltage reference, a feedback mechanism should be used to keep it stable against supply variations and to reduce the temperature sensitivity a positive tempco voltage should be added with the negative tempco voltage.

A BJT base emitter voltage has negative temperature coefficient. Threshold voltage of MOSFET also has negative tempco.Let us consider a PNP transistor as shown in figure below:

where, VBE is base emitter voltage of pnp transistor
VT is volt equivalent of temperature given by kT/q
Ic is collector current
If a positive tempco voltage is added with the negative tempco voltage, a stable reference across temperature can be generated. Widlar was the first one who used this concept to get the stable Bandgap reference. A positive tempco voltage is generated using two BJTs and generating a current which is a function of difference of two VBE voltages.

VBE  is inverse PTAT at roughly -2.2mV/C at room temperature

VT is PTAT that has a temperature coefficient of +0.085mV/C at room temperature.

where γ=3       

         Differentiate with respect to temperature T, assuming JC is proportional to temperature as Tα .

Therefore, 

For zero temperature coefficient at T=T0 , VBE and ΔVBE with respect to temperature T should add up to zero.

Solving for K yields,        

Substituting above equation In VREF = VBE + KVT0

VREF |T=T0 = VG0 + VT0(Υ-α)
For typical values γ=3.2 and α=1, VREF = 1.262V at 300K, which is approximately equal to the bandgap energy of the silicon i.e., 1.12eV, so the circuit generating this voltage is called Bandgap Voltage reference generator.
Typical Example:

where AE1 and AE2 are emitter areas of the BJT transistors

Then, VREF  = VBE2 + K * VT