Mindanao State University-Iligan Institute of
Technology College of Engineering and
Technology
BANDGAP VOLTAGE REFERENCE
A Technical Report Presented to
Prof. Allen D. Lowaton
Of DEET Faculty
________________
In Partial Fulfillment for the course
ECE132.1 – Mixed IC Signal Laboratory
________________
MQ Edson T. Rosete
BSECE-3 Student
ID Number: 2018-0089
INTRODUCTION
In designing voltage or current references, one important property that must be present is
its independence to power supply or temperature variations. This is because the reference circuit
is basically the anchor of the whole circuit; if slight changes were to occur, it would mean that the
circuit would not behave in the expected manner. In the previous lab activity, the constant gm
current reference was designed. One desirable quality of the constant gm current reference is its
immunity to power supply variations. However, it is still susceptible to temperature variations.
Thus, a reference circuit must be made that is immune to both power supply and temperature
variations. That is where the bandgap voltage reference comes into the picture. What makes the
bandgap voltage reference immune to temperature variations is the presence of the BJT’s in the
circuit. The figure below shows a bandgap voltage reference circuit.
Figure 1 – Bandgap Voltage Reference Circuit
OBJECTIVES

To design a bandgap voltage reference using HSPICE software

To compare the performance of a bandgap voltage reference circuit when using an ideal
operational amplifier and a non-ideal operational amplifier

To simulate the bandgap voltage reference across all process corners
MATERIALS NEEDED

Personal Computer/Laptop

HSPICE Software
PROCEDURES
The procedures will be the same for both the bandgap voltage reference with an ideal op-amp and
the bandgap voltage reference with a non-ideal op-amp
Step 1: Simulate the bandgap voltage reference circuit shown in figure 1 in HSPICE. Set the values
of R1, R2, and R3 to 7 kΩ.
Step 2: Observe and record the changes in the base-emitter voltage (VBE1) with respect to room
temperature.
Step 3: Observe the changes in the voltage across R3 (VR3) with respect to room temperature.
Step 4: Observe the changes in VREF with respect to the changing temperature.
Step 5: Calculate the temperature coefficient.
Step 6: Calculate the proper values of R1, R2, and R3.
Step 7: Re-simulate using the new values of R1, R2, and R3.
Step 8: Observe the changes in VREF with respect to the changing temperature across the three
different process corners.
Step 9: Calculate the temperature coefficient.
Step 10: Calculate the sensitivity of VREF to VDD.
Step 11: Compare the results of the simulations with a bandgap voltage reference using an ideal
op-amp to one using a non-ideal op-amp.
RESULTS AND DISCUSSION
First, the circuit was coded in HSPICE software based on the diagram shown in Figure 1.
Initially, the values of R1, R2, and R3 are set to 7 kΩ. The figures below show the HPSICE Code
for the bandgap voltage reference using an ideal op-amp and an actual op-amp.
Figure 2 – HSPICE Code for Bandgap Voltage Reference using Ideal Op-Amp
Figure 3 – HSPICE Code for Bandgap Voltage Reference using Actual Op-Amp
Once the HSPICE Code is finished, simulations are then done. Using AVANWAVES, the
changes in the base-emitter voltage (VBE1), the voltage across R3 (VR3), and VREF are then observed
with respect to room temperature (for VBE1 AND VR3) and to the changing temperature (for VREF).
SIMULATION RESULTS FOR IDEAL OP-AMP:
Figure 4 – Change of VBE1 with respect to room temp.
Figure 5 – Change of VR3 with respect to room temp.
Figure 6 – Change of VREF with respect to temp.
SIMULATION RESULTS FOR ACTUAL OP-AMP:
Figure 7 - Change of VBE1 with respect to room temp.
Figure 8 - Change of VR3 with respect to room temp.
Figure 8 - Change of VREF with respect to temp.
After simulation of the circuits, their respective temperature coefficients were then
calculated using the following equation:
𝑇. 𝐶. =
Ideal Op-Amp
𝑉𝑟𝑒𝑓𝑚𝑎𝑥 − 𝑉𝑟𝑒𝑓𝑚𝑖𝑛
𝑇𝑒𝑚𝑝. 𝑆𝑤𝑒𝑒𝑝 × 𝑉𝑟𝑒𝑓 @ 27°𝐶
Actual Op-Amp
Figure 9 – Temperature Coefficient for Ideal and Actual Op-Amp
From the calculations, it can be seen that the bandgap voltage reference with the actual opamp has the higher temperature coefficient. This is because the op-amp used already contains
actual MOS devices, each having their own non-idealities that they contribute to the circuit. The
former has a lower temperature coefficient since it uses an ideal op-amp. However, both
temperature coefficients are still quite high and need to be lowered to a more acceptable level.
In order to accomplish this, the value of R1, R2, and R3 have to be adjusted accordingly.
The relation below is used to find the appropriate values of these resistors.
𝑉𝑟𝑒𝑓 = 𝑉𝐵𝐸1 + 𝑉𝑅3 (
𝑅1
)
𝑅3
This relation can be manipulated to give the proper equation for finding the appropriate
value of R1, R2, and R3. The figures below show the calculations for both the ideal op-amp and
the actual op-amp.
Ideal Op-Amp
Actual Op-Amp
Figure 10 – Adjusting Resistor Values for Ideal and Actual Op-Amp
Using the new resistor values that were calculated, the circuit is re-simulated. Additionally,
the behavior of VREF with respect to the changing temperature is examined across all three process
corners: SS, TT, and FF.
Figure 11 – HSPICE Code for Bandgap Voltage Reference using Ideal Op-Amp
Figure 12 – HSPICE Code for Bandgap Voltage Reference using Actual Op-Amp
SIMULATION RESULTS FOR IDEAL OP-AMP:
TT:
Max VREF = 1.2397 V
Min VREF = 1.2374 V
VREF @ 27°C = 1.2397 V
Min VREF = 1.2636 V
VREF @ 27°C = 1.2669 V
SS:
Max VREF = 1.2676 V
FF:
Max VREF = 1.2154 V
Min VREF = 1.2105 V
VREF @ 27°C = 1.2148 V
SIMULATION RESULTS FOR ACTUAL OP-AMP:
TT:
Max VREF = 1.2539 V
Min VREF = 1.2524 V
VREF @ 27°C = 1.2527 V
Min VREF = 1.2653 V
VREF @ 27°C = 1.2682 V
SS:
Max VREF = 1.2730 V
FF:
Min VREF = 1.2406V
VREF @ 27°C = 1.2411 V
PROCESS CORNER
IDEAL
ACTUAL
TT
-3.529 ppm/°C
7.484 ppm/°C
SS
1.973 ppm/°C
37.947 ppm/°C
FF
25. 210 ppm/°C
8.561 ppm/°C
Max VREF = 1.2423 V
Table 1 – Temperature Coefficients of the Different Process Corners
From the results above, it can be seen that the bandgap voltage reference using the ideal
op-amp has a more desirable temperature coefficient than the actual op-amp, except for the FF
process corner. The basic principle of this is again, the non-idealities that is introduced by the
actual op-amp that was used in the bandgap voltage reference circuit. Nonetheless, all of the
calculated temperature coefficients are significantly better than the initial temperature coefficient.
Next, the change of VREF with respect to the change in VDD through all process corners is
observed.
SIMULATION RESULTS FOR IDEAL OP-AMP:
TT:
SS:
FF:
SIMULATION RESULTS FOR ACTUAL OP-AMP:
TT:
SS:
FF:
Using the formula below, the sensitivity of VREF to changes in VDD can be calculated for
both the ideal and actual op-amp.
𝑆(
𝑉𝑟𝑒𝑓
𝑉𝐷𝐷 𝑑𝑉𝑟𝑒𝑓
)=
×
× 100
𝑉𝐷𝐷
𝑉𝑟𝑒𝑓 𝑑𝑉𝐷𝐷
PROCESS CORNER
IDEAL
ACTUAL
TT
0.18 %
0.1071 %
SS
0.1757 %
0.1046 %
FF
0.1845 %
0.1902 %
As can be seen from the results above, the bandgap voltage reference circuit has low
sensitivity in both the ideal and actual op-amp implementation. Both are less than 1%, which is
usually the limit when it comes to power supply sensitivity. Therefore, the desired quality of
independence from power supply and temperature variations was achieved.
CONCLUSION
In this laboratory experiment, a bandgap voltage reference was designed, encoded, and
simulated in HSPICE software. Both an ideal and actual op-amp were used in the implementation
of the circuit. Various behaviors of the voltages were examined, like VBE1, VR3, and VREF, as well
as how they changed with respect to temperature. It was seen that when the resistor values were
all equal to each other, the bandgap voltage reference circuit had a high temperature coefficient,
both in the ideal and actual op-amp implementation. Therefore, using the relation between the
resistors and VREF as well as VBE1, the appropriate values for the resistors were calculated. These
new values caused the temperature coefficient of the bandgap voltage reference circuit to improve
drastically. Furthermore, the change in VREF with respect to temperature was examined across the
three process corners – TT, SS, and FF. Lastly, it was seen that the bandgap voltage reference is
quite independent of power supply variations by examining the changes in VREF with respect to
the power supply voltage VDD. The main reason for the difference of values between the ideal and
actual op-amp implementations are the non-idealities introduced by the actual op-amp circuit.
Since it is composed of active MOS devices, some peculiar behavior is to be expected from the
circuit.