Question 12.9: What is V out for a transistor log amplifier with Vin = 3 V ...
What is V_{\text {out }} for a transistor log amplifier with V_{i n}= 3 V and R_1 = 68 kΩ? Assume I_{\mathrm{EBO}}= 40 nA.
V_{\text {out }}=-(0.025 \mathrm{~V}) \ln \left(\frac{V_{\text {in }}}{I_{\mathrm{EBO}} R_1}\right)=-(0.025 \mathrm{~V}) \ln \left(\frac{3 \mathrm{~V}}{(40 \mathrm{nA})(68 \mathrm{k} \Omega)}\right)
=-(0.025 \mathrm{~V}) \ln (1103)=-\mathbf{0 . 1 7 5} \mathrm{V}
P R A C T I C E EXERCISE
Calculate V_{\text {out }} \text { if } R_1 is changed to 33 kΩ.
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