miniature mw transmitter

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 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 transistor biasing is only with base resotor to the supply line. The 0.01 uF is a feeedback to enable the carrier oscillator.

Hello my friend. it is kind of bjt biasing that has no R2 ( from base to ground ) . and also it is the worst biasing circuit of transistor . and instead of RF signal it is common base and instead of audio signal it is common emitter. and about the lc system , it is a tank circuit that has impedance about infinite per RF signal . and the 0.01 uf capacitor is feed back capacitor to provide oscillations.
Best Wishes
Goldsmith

samuelmpuk said:

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

Click to expand...

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.

Click to expand...

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

Dear RF_Jim
Hi
The value of feed back capacitor is very important , because it is your beta coefficient and you should pay more attention to select that .
Respectfully
Goldsmith

Thanks for your help that has cleared things up.I was thinking it was just a amp powering a LC circuit as load in place of a speaker as I thought this would cause RF to be generated.

Would understanding common base circuit and oscillators be a good start to understanding this circuit, Could a AMP ever be used as MW AM transmission or you would always have to generate the frequency with a oscillator.

goldsmith said:

Dear RF_Jim
Hi
The value of feed back capacitor is very important , because it is your beta coefficient and you should pay more attention to select that .
Respectfully
Goldsmith

Click to expand...

goldsmith,

I beg to disagree. In an oscillator as simple as this there is a lot of leeway in component value selection such as the 'feedback' capacitor. The important parts are the tuning inductor and (variable) capacitor

Two points offered in contrast to your assertion:

A) Why is the value a round 'magic' industry-standard part value like ".01" ?

B) What is going to be the effect of tuning the Variable Capacitor from 535 to 1706 kHz (the frequency range of the MW or Medium Wave broadcast band) if the Xc value of the 'feedback' capacitor is so terrible critical (as the Xc value will change markedly from 535 to 1705 kHz)?

Is .01 uf Cap value critical? Answer: It isn't. The value can range over a better than a 10:1 range with little affect in circuit performance, as this is just a AC coupling and DC block capacitor. Did you note the tap down lower impedance-wise on the inductor? This tap to a selected impedance point has more impact in respect to 'Beta' (or maybe more correctly for this circuit; 'Alpha' as this circuit is configured in CB configuration.) A sharp eye would catch this and then know the true purpose of the .01 uF cap (DC block and AC pass of feedback signal from tap point on inductor).

Regards,

Jim (with +30 years in RF technology field I might add)