Emitter follower design for VCO

emitter follower design

Hi all,

I have a question about Emitter follower design for VCO. Right now I have several choices: 1. incorporate emitter follower into VCO feedback path. 2. Use DC coupling between emitter follower and VCO. 3. Use AC coupling between emitter follower and VCO. Does anyone know the advantage/disadvantage of these configurations???

BTW, in papers, I have ever seen people using DC coupling between emitter follower and VCO. But since the base of the emitter follower is DC biased to its collector as VCC through VCO inductor, why doesn’t the transistor go into saturation region during every positive VCO cycle???

Thank you very much

emitter follower

Hey, can somebody gives me a hint?? Thanks

rf emitter follower

You can use an emitter follower as an oscillator but, due to its inherent instability, in this configuration the right topology to be used is the one that is known as a negative resistance topology. In this topology the collector is grounded for RF and DC, being the VCC applied to the emitter. If you have a problem with negative power supply, you can choose a PNP transistor.
RF wise, the resonator is connected between base and ground and the signal is taken from the emitter through a small coupling capacitor.
The linear analysis of a negative resistance oscillator consist to check if the phase of S11 is crossing 0deg in a point where you have the magnitude of S11 negative.
There is another way to analyze this oscillator using a linear approach through virtual grounds. This technique is well described in the literature.

NandoPG