Consider the bandgap circuit shown in Fig. 19.27. Choose the values of n, R1, and R2 such that Vout exhibits a zero temperature coefficient and the layout can be designed for high precision.

Question

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

Accepted Answer

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

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

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

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