28 February 2013 99 9K Report

I have extracted these questions from the discussions (leaded by Lutz von Wangenheim) about the Barkhausen criterion, RC oscillators and phase shift in RC circuits where we tried to answer the question, "How do sine oscillations arise in RC oscillators?":

https://www.researchgate.net/post/Is_the_Barkhausen_criterion_about_the_loop_gain_right_in_the_case_of_the_Wien_bridge_oscillator1

https://www.researchgate.net/post/How_do_sinusoidal_oscillations_arise_in_RC_oscillators#share

https://www.researchgate.net/post/Why_is_there_a_phase_shift_in_RC_circuits_How_do_we_make_it_exactly_90_degrees

I have stated several times that we can find the answer of this question in the time domain by following the sine "movement" of the output voltage between the supply rails. I suggest to do it here by investigating the structure and the operation of the ubiquitous Wien bridge oscillator. Let's begin with the structure; here are my speculations:

To realize this exotic circuit solution, we have "to see the forest for the trees":), i.e. to group the particular elements in well-known functional blocks. Thus, we may first group the two resistors Rf, Rb and the op-amp U1 (see the attached picture below) into a low-gain (≈ 3) single-ended nonlinear amplifier (the classic non-inverting op-amp amplifier) with a Wien network (R1 = R2, C1 = C2) connected in the positive feedback loop. Rb (a bulb) self heats and reduces the amplifier gain until the point is reached that there is just enough (maybe, 3?) gain to sustain the sine oscillations without reaching the saturation point of the amplifier. So, from this viewpoint, the Wien bridge oscillator is considered as two connected in a loop devices - a non-inverting amplifier and a Wien network (a non-inverting amplifier with a Wien network positive feedback).

Then, we may group (in a little more exotic way) the non-inverting amplifier above with the upper part (R2, C2) of the Wien network into a current-driven negative impedance circuit (INIC). Its impedance is roughly equal to the "positive" impedance of the lower part (R1, C1) so that the two opposite impedances roughly neutralize each other at the equilibrium point.

The next powerful idea is to see the whole Wien bridge circuit (Rf, Rb, R1, C1, R2, C2) and to consider the Wien bridge oscillator as a combination of an op-amp and a Wien bridge connected in the positive feedback loop between the op-amp output and its differential input. The loop gain is a product of the very high op-amp gain and the very low bridge ratio. At the oscillating frequency, the bridge is slightly unbalanced and has a very small transfer ratio; so, the loop gain is about unity.

The final, and maybe the most popular viewpoint, is to break down the Wien bridge into two half bridges, and to consider the overall feedback as composed of two partial feedbacks - a nonlinear negative feedback (the voltage divider Rb-Rf connected to the inverting op-amp input) and a frequency-dependent positive feedback (the Wien network connected to the non-inverting input). Thus the feedback voltage applied to the op-amp differential input is the difference between the two partial voltages.

Now about the operation...

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