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This circuit is a small signal sine wave generator.

R7 and R8 are used to control the amplitude of the output as desired.

After a few simulations, it looks like the gain of the Q3 stage is constant with the collector feedback bias resistance. Also, those R7 and R8 actually end up controlling the amplitude of the oscillation in Q1's base. I've got no idea why.

Please explain this to me. At first I thought those resistances were there to create a collector feedback bias, and such high values would limit the gain significantly, but it remains constant.

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2 Answers 2

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The resistive chain R7-R8-R9 provides negative feedback for Q3 - for DC (bias point stabilization) as well as for ac. This is because of the finite output resistance of the Q2 stage. Hence, any modification of this feedback chain will influence the gain of the final stage.

(I think, the "virtual ground" principle does not apply here. It is valid only for opamps with a (nearly) infinite gain).

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  • \$\begingroup\$ That was right, thank you! After some calculations, the negative feedback composed of R10 and R7-R8-R9 provides an output resistance of R10 || R7-R8-R9 ~ R10, that can be finely tuned with R7-R8. Also, the transfer function of the negative feedback is independent of R10. \$\endgroup\$
    – user2210558
    Oct 21, 2017 at 15:11
  • \$\begingroup\$ I cannot fully agree. (1) Feedback is composed of R7....R10 as well as R5||rin of Q3. (2) The feeedback transfer function is not indepencent on R10, however, the influence of R10 is rather small. \$\endgroup\$
    – LvW
    Oct 21, 2017 at 15:58
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Q3 operates like an opamp. The base of Q3 is the virtual ground of the opamp.

The charge arriving through C5, arriving at the base of Q3, is almost totally extracted from the base node through series resistor chain R7/R8/R9.

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