A Wien-bridge oscillator is built with and in the Wien network
PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Feedback, operational amplifiers and analogue signal processing · Feedback, operational amplifiers and analogue signal processing
Electronics for Measurement, Ch Active filters, oscillators and timers
Problem
A Wien-bridge oscillator is built with R=12kΩ and C=6.8nF in the Wien network, and the amplifier gain is trimmed to exactly K=3.00. The builder writes two claims in a lab notebook:
((a)) "Setting K to exactly 3.00 makes the Barkhausen condition satisfied once and for all, so the output amplitude will settle at some fixed value and stay there indefinitely, with no need for anything else in the loop."
((b)) "Just as putting a unity-gain follower between two passive RC stages removes the loading and improves a filter, putting a unity-gain follower between the Wien network and the amplifier's input would remove any loading on the network and let the same K=3.00 amplifier oscillate with a sharper, better-defined frequency." State whether each claim is true or false, and justify each answer from the Barkhausen condition and from the reason an active filter stage differs from a buffered passive cascade.