FM transmitter and capacitor

[mstechca]

Take a look at this site: **broken link removed**

and now take a look at this site: **broken link removed**

Ignore the audio amplification sections, and concentrate on the carrier oscillator sections only.

Why is it that in one circuit the capacitor is connected from the base of the right-most transistor to VCC, and in the other circuit, the capacitor is connected from the base of the right-most transistor to ground?

and which way is better in terms of stability and range?

 

[Nigel Goodwin]

Why is it that in one circuit the capacitor is connected from the base of the right-most transistor to VCC, and in the other circuit, the capacitor is connected from the base of the right-most transistor to ground?

and which way is better in terms of stability and range?

There's no difference, as far as signal is concerned Vcc and ground are the same point. However BOTH circuits should have a decoupling capacitor from Vcc to ground (shown as optional on the first circuit) - in fact ALL electronic circuits should have one!. Without the capacitor it relies on the internal impedance of the battery to connect Vcc and ground - it's not really a good idea.

 

[audioguru]

Without a supply bypass capacitor in the 2nd circuit, the positive supply voltage will fluctuate at the RF frequency more and more as the battery runs down because its internal resistance rises.
The cap from the positive supply to the base will feed the supply fluctuations to the base and since it is in-phase with the collector voltage fluctuations then the oscillator's output will be reduced.