What is source and load impedance?
The BC549C isn't intended to be a low noise RF amplifier and as FvM has pointed out, the gain depends on what drives it and in particular what loads it. Consider the 4.7pF output capacitor alone has a reactance (think of it as series resistance) of almost 17K at 2MHz. Even at 28MHz, Xc is 1.2K.
I would think using a MMIC would be your easiest solution. They give lots of gain, are low noise and stable and best of all only need one load resistor and in/out coupling capacitors. They are quite old devices now but I used lots of MSA0135 in the past as general purpose gain blocks, they have a response from DC to > 1GHz with more than 20dB gain. The MSA0886 is inexpensive and will give you >30dB gain at 1Ghz and beyond with only 3dB noise figure.
Brian.
That design has a lot of problems which cannot be overcome by changing values. The load we referred to isn't the collector resistor, it's the impedance after the coupling capacitor. Ignoring resonances, the 4.7pF coupling capacitor and the impedances after it form an attenuator which probably loses more signal than the transistor amplifies. The trouble with that design is the need to keep capacitance across the resonator to a minimum so you retain as much tuning range as possible so you can't increase the coupling.
In that design I don't think a transformer will help for the reasons you mentioned. True, it will help to match the impedance between the transistor and resonator but there is no way a single transformer will work effectively over that frequency range or work well with such a complex impedance as it's load.
If you want to try a MMIC you can just substitute one for the transistor, it will certainly provide much more RF gain but I'm not sure the AF output will work as well.
Brian.
Can you please explain that in more detail Brian? How can the low level Rf input signal modulate the much more powerful oscillator, under which process does it happen?The incoming signal amplitude modulates the oscillator and the non-linearity of the FET converts that back to the low frequency you hear.
Brian.
Can you please explain that in more detail Brian? How can the low level Rf input signal modulate the much more powerful oscillator, under which process does it happen?
The SSB is amplitude variations but also frequency variations, so under which mechanism does this modulation happen?
Also The suspicion that the crystal operates as a filter as well, came from the fact that this is the only point where the incoming signal input is applied, but also from the fact that it cannot led AF pass through it when reflex mode is on.
I'll try to answer both posts together.
The circuit is a self oscillating mixer. If you removed the BC547 and all the components connected to it, you would have a JFET oscillator with a varicap to pull it slightly off frequency. By adding a tiny amount of received signal to it you get a mixing effect with the result being (Oscillator+signal frequency) and (oscillator-signal frequency) as products. The resulting amplitude also depends on the amplitudes of the two signals but as the oscillator has a fairly constant level, the result is the envelope of the received signal only.
The recovered signal(s) - don't forget this receives equally on both sides of the oscillator frequency, passes through the RF blocking components (see later) back to the base of the BC547 and then from it's collector to to AF output. The 4.7pF is a compromise, if you make it bigger you get more signal but reduce the tuning range, you want to keep anything in parallel with the varicap to a minimum so it has sole effect on the frequency. The value is in any case low enough that it doesn't pass audio frequencies.
Those blocking components, think of this: Suppose you have the output of a signal generator giving say 1V at 10MHz, how much is it reduced if you wire a 100nF capacitor and 3.3mH choke in series with it?
Regarding the best RF amp, you have conflicting requirements in that design. The BC547 stage has to carry the received frequency and also the recovered audio so it needs wide bandwidth, high gain and ideally low noise. It is almost impossible to satisfy all those requirements in one device. The BC547 is a poor choice for an RF amplifier anyway, it is intended for switching applications and has quite a high noise figure, especially when you consider the signal passes through it twice so you amplify the noise of the first pass on the second pass. Given the low level of the audio it would be better to use a good RF rated device in that position where the RF performance is more critical than audio.
Brian.
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