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Bandgap Reference Circuit


We designed a voltage reference circuit with minimal temperature variation from -40 ℃ to +120 ℃ with a  = +40 ℃. The challenges are to get two opposite temperature coefficients then add them with proper weighting, making a zero temperature coefficient. Another challenge is to minimize the VoutVout to have good stability and noise reduction. Lastly, we have to modify the circuit so T0 equals +40 ℃. 

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Device Parameters

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Design Process

Our overall design is seen above. M0, M1 and M2 work as a current mirror, and they are the same size. The current of M2 is PTAT current, and M3 and M4 work together. If all of the transistors carry zero current, they may remain off. To avoid this situation, we have implemented M5 as a start-up circuit to ensure the circuit turns on.


As seen from the images below, our temperature curve when plotted against the voltage is a down bow. We achieved a maximum Vout of 1.308 V at 40 ℃ and a minimum Vout of 1.3001 V at -40 ℃. The downward bowing effect can be attributed to the cancellation of the negative temperature coefficients. As the temperature increases, the decrease in forward voltage drop causes Vout to decrease, creating the downward bow which is seen in our graphs. 

To prove our circuit met the appropriate startup conditions, as shown in Figure 6, we can see that our Vout starts at 0, increases to 2.5 V (Vdd), and then stays at that level signifying that it has turned on successfully. Figure 7 provides an accurate derivation of the Vout vs. Temperature as it is a negative linear line which is the correct derivation of a negative parabolic function. 

After we completed our layout, we performed the post-simulation analysis which was extremely similar to the pre-simulation analysis. However, as seen from the placement of the bow, the maximum voltage was achieved at a lower temperature in comparison to the pre-simulation analysis. In the pre-simulation, we achieved a T0 of 40℃, whereas in the post-simulation we achieved a poorer T0 of 17.6℃.

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Temperature vs. Voltage Curve

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Layout Part 1

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Layout Part 2


In this design, we used a PTAT circuit, BJTs, and resistors to design a temperature independent voltage circuit and create a fully-functional layout. If we had more time, we would solve the problem of asymmetry of the two currents. Instead, we could implement a cascode structure to remove this issue.

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