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I've been searching all around to find a tutorial on analyzing/designing a positive feedback amplifier, especially a single transistor in common collector mode, but with no luck...

So, lets say I have this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

I was able to design the DC part of the oscillator, the operating point. But I just cant get my head around, how to design the AC part.

How would I calculate the values for the positive feedback capacitive divider network? Also, is there any universal way of analyzing transistor feedback circuits? I would really appreciate a step by step guide.

Thanks in advance!

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"How would I calculate the values for the positive feedback capacitive divider network? Also, is there any universal way of analyzing transistor feedback circuits? I would really appreciate a step by step guide."

What is your intention for positive feedback? For normal amplification purposes we use negative feedback (dc and ac). There is one application only which uses positive signal feedback: Harmonic oscillators.

Regarding an "universal way" for analyzing feedback loops: At first, you must identify the feedback loop. There are some special methods to calculate/simulate the gain within this loop. This "loop gain LG" is used to prove/justify stability properties of the circuit. By the way, this "loop gain" LG also appears in the denominator D of the closed-loop gain function (D=1-LG).

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  • \$\begingroup\$ My intentions are to keep the oscillations going. Isn't this what positive feedback is about? I guess, I forgot to add, I am a hobbyst electrical engineer and am not yet familiar with these symbols. If its possible, I would ask for a bit more in depth calculations. \$\endgroup\$ – Golaž Aug 14 '14 at 15:46
  • \$\begingroup\$ So you intend to design an oscillator, correct? In this case, you must ensure that the oscillation condition (unity loop gain) is fulfilled for one singel frequency only! \$\endgroup\$ – LvW Aug 14 '14 at 15:58
  • \$\begingroup\$ Yes, I intent to design a 27 MHz oscillator using XTAL. How would I ensure that? \$\endgroup\$ – Golaž Aug 14 '14 at 16:02
  • \$\begingroup\$ What is the source of the circuit? Either you have designed it by yourself or you have found it somewhere. In the first case - you know the task of each part, and in the second case you should read the circuit´s description. \$\endgroup\$ – LvW Aug 14 '14 at 16:27
  • \$\begingroup\$ The original circuit was meant for 12V, while I intend to power it from 6V. Which requires different values. I would also like to learn analyzing them, thats why I am asking. \$\endgroup\$ – Golaž Aug 14 '14 at 16:46

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