help with economical radio receiver
The last three transistors are AF amplifiers but I am not confident of how the first stage works.
When I remove the tuned circuit and feed an audio signal on c2 the audio is amplified.
When the tuned circuit is in place and I feed an audio signal to c2 the audio is still amplified but much less (the tuned circuit keeps mostly the RF)
Is this stage an audio amplifier, a detector or what? What is the principle of operation, I would like to know?
Hello,
The first stage is a detector where the BE diode is used as "crystal" detector, but you get hfe*ib as output at the emitter, the emitter voltage follows the envelope of the input signal. .
Due to the hfe of the transistor, the output impedance is relatively low, though you have reasonable input impedance so that you don't load the tuning circuit too much. Therefore it also provides power gain (not voltage gain).
What is the thing that forces it to behave as a detector? is it the resonant circuit?
What I mean is that for example say I want to remove the resonant circuit and add an rf preamplifier at the input. Then Include the resonant circuit at the beginning of the rf amplifier. Will this stage around T1 still behave as a detector, or the detection will be done then by the first added rf preamplifier transistor?
Yes, because L1 is shorting the audio signal.
Both. The RF is rectified at the T1 BE diode and the AF buffered by T1 as well. C3 achieves a full RF level across the rectifier.
So, is C3 the one that is responsible for "instructing" the transistor to rectify the RF? I.e. if C3/R2 were missing this would behave as an audio amplifier like the rest of the transistors?
My intention is to include a front end RF amplifier to this circuit but I want to make sure that this amplifier will only amplify the RF signal and not rectifying it. So I am confused if I need to place the ferrite rod at the beginning or after the RF amplifier?
It is the BE junction diode of T1 that does the rectification. So the Base of T1 does carry both RF and LF current. Because of you need LF current in the base also, the coupling capacitor C2 must be large to allow LF current. If you would change this capacitor to a value that only passes the RF current, you will not have your LF current in the emitter (hence no good detection).
If you want to drive it from another RF AM source, you need a LF path to ground (that doesn't short circuit the RF signal of course).
You are correct, when you remove C3, it will no longer rectify efficiently. The parallel resistor is to provide a DC bias path for T1.
---------- Post added at 17:10 ---------- Previous post was at 17:05 ----------
If you want RF amplification, you need to put it in between T1 and the ferrite rod with capacitor tuning. You may need matching to avoid that the RF amplifier loads the Ferrite LC circuit too much (reducing sensitivity). What frequency will you use?
This is basically for the AM MW band, I would be very helped if you could suggest a circuit s an rf amplifier for this.