# Capacitor between emitter and collector

Considering the circuit below.:

What is the purpose of capacitor C5? I know it provides positive feedback to sustain the LC oscillations, but how does this work ?

• I think you need to be more specific about what you don't understand - clearly you understand that it provides positive feedback so what don't you understand - maybe you don't understand that the emitter can be an input? Or maybe you don't recognize that the final transistor is common-base configuration (regarding the carrier)? Or maybe you don't understand how the reverse biased base-collector can modulate the output carrier? Apr 11, 2014 at 18:38
• I understand all the points that you have mentioned. What is dont understand, is does just connecting a capacitor from output to input create a feedback ? Apr 12, 2014 at 11:04
• That's the normal way feedback happens - using a capacitor or resistor. Apr 12, 2014 at 11:18
• Why would only a part of the collector voltage go back to the emitter ? The capacitor is a low impedance for high frequencies, so wouldn't all of the output just go through the capacitor ? Apr 12, 2014 at 11:30
• The emitter, as an input is very low impedance, probably in the order of ten ohms. I estimate this oscillates at about 50 MHz. At that frequency the 5pF has an impedance of over 600 ohms. Apr 12, 2014 at 11:47

For a circuit to oscillate it is necessary to have a LOOP GAIN of unity (Barkhausen criterion). Hence, the amount of output voltage that is fed back to the input of the amplifying unit depends on the gain (magnitude and phase!). In the present case, only a part of the output voltage is created across the impedance seen at the emitter node because of the voltage division rule. This emitter node impedance is $re=r6||(1/gm)$ with $gm=transconductance$.