Question

Find the value of Ra and Rb from the circuit shown?

(a) Ra =4.6kΩ ; Rb= 9kΩ

(b) Ra =7.3kΩ ; Rb= 3.4kΩ

(c) Ra =2.5kΩ ; Rb= 5.1kΩ

(d) Ra =4kΩ ; Rb= 10kΩ

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Enquiry is from Input Offset Voltage in section Operational Amplifier Fundamentals of Linear Integrated Circuits

Answer 1

Correct answer is (d) Ra =4kΩ ; Rb= 10kΩ

Simply put: We know that input offset voltage, Vio =(Rc*Vmax)/ Rb

=> Rb = Vmax*(Rc / Rb ) = (10v/10mv)*10Ω (∵ Vio specified on the datasheet is 10mv for LM307 op-amp).

=> Rb =10000 = 10kΩ.

Since Rb > Rmax let us choose Rb = 10*Rmax. (Where Rmax = Ra/4).

∴ Rb = (10*Rb)/4  and Ra = Rb/2.5 = 10kΩ/2.5=4kΩ.