Hi I am trying to find out more about this MW radio transmitter,
Miniature MW Transmitter
This is one I built on bread board, I would like to know why it has no resistor from Base to ground, and what type of amp are they using common emitter or common collector
First, we should say that the NPN BJT (Bipolar Junction Transistor) is in an _oscillator_ circuit configuration as opposed to an _amplifier_ circuit configuration.
Notice the feedback from Collector to Emitter (resulting in "in-phase" feedback) using a .01 uF cap.
And in this configuration (an osc) it isn't often necessary to use a resistor from base to ground; if this were being used as an
amplifier, instead of an oscillator, it might be desirable to use a resistor from base to ground to stabilize the base-bias over temperature extremes. ALso notice that the resistor in the base circuit is 100 K Ohm, a fairly high value for a BJT transistor circuit. In reality, that resistor plays a part in getting the transistor started oscillating, after which it plays much less of a part although a DC return to +V or ground is still necessary since BJTs are current, not voltage operated devices.
it does look like a common collector but I don't know how this LC circuit behaves when DC is pulsing through it. Does the LC circuit become treated as a normal resistor and would this resistance be the total impedance of the LC circuit at resonant frequency.
why does a 0.01uf capacitor get used I have found it's Xc 15 ohm, is it to match to the small antenna or just block DC and let RF through and if so how would I find out that with maths.
The circuit configuration is more analogous to a "common base" than anything else, I think.
The actual Xc of the .01 is not important, and there is probably wide latitude in choosing this value. The frequency of oscillation is set by the Inductor and Variable cap shown adjacent to each other.
Note that the modulating audio is applied to the base, and this affects the "amplitude" of the oscillating signal produced by the transistor, thereby modulating the RF 'wave' generated by the transistor.
Also note the tuned circuit is 'tapped down' (lower impedance point) impedance-wise where the feedback point is tapped off through the .01 uF cap to the Emittter.
Did I miss anything? I kind of bounced around on subject points in the response above ...
Jim