Question 19.2: Consider the bandgap circuit shown in Fig. 19.27. Choose the...

Design of Analog CMOS Integrated Circuits

Consider the bandgap circuit shown in Fig. 19.27. Choose the values of n, $R_1 \text {, and } R_2$ such that $V_{\text {out }}$ exhibits a zero temperature coefficient and the layout can be designed for high precision.

19.27

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Since $V_{\text {out }}=V_{B E 3}+V_T\left(R_2 / R_1\right) \ln n$ , we must find convenient values of n, $R_1 , and R_2$ such that $\left(R_2 / R_1\right) \ln n \approx 17.2$ (Chapter 12). If n=31, then $R_2 / R_1 \approx 5$ , yielding the layout of Fig. 19.28(a). Note that $R_1$ is placed in the middle to partially cancel the effect of gradients.

Now suppose we choose n=25, obtaining $R_2 / R_1=5.34$ . Such a value cannot be accurately established by simply adjusting the dimensions of $R_2 and R_1$ . Rather, we write $R_2 / R_1=16 / 3$ and construct the resistors as shown in Fig. 19.28(b).

19.28

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