I am trying to make a radio receiver for 800 kHz AM radio signal.
I connected an antenna with an LC filter circuit which is then connected to a voltage divider biased BJT amplifier whose output is connected with a diode and speaker.
Will this reciever work? Will I be able to listen to 800 kHz radio signal if all the components are connected and their values are chosen properly?
Will you please give me any suggestions for this circuit to make it work?
(My drawing)
Will this reciever work?
No, not as shown...because the transistor is not biased properly.
However, it is a good start:
Another answer suggests re-arranging the order so that the diode detector comes first, followed by an audio amplifier that drives an audio transducer. This has the merit that an audio amplifier would probably have more gain than would a radio-frequency amplifier. However, a diode detector deals poorly with tiny signals from an antenna, providing little audio signal to the following audio amplifier.
Here's a suggested way to bias the transistor so that its base sees the 800 kHz signal on top of a DC bias from R1 & R3 (leftmost circuit). The transistor amplifies 800 kHz signal first, then drives a diode detector through a coupling capacitor to headphones. A loudspeaker has such low resistance that transistor gain would be difficult to achieve...high impedance headphones would allow more gain:
simulate this circuit – Schematic created using CircuitLab
On the right, a ferrite rod is used as RF selector AND antenna. This is often done in transistor radios. A link winding on the rod having fewer turns than the main winding provides a lower impedance signal to the transistor, which gives higher gain.
It would be convenient to take the ferrite rod from another radio, along with its variable tuning capacitor...everything is already optimized so that the transistor sees a good RF signal between base and the bias resistors R8, R6. A bypass capacitor C9 provides an AC ground reference for radio-frequency signals.
DC base bias current flows through the small link winding on the ferrite rod.
Edit:
The left circuit requires antenna wire connected to C2 & L1 and a connection to GROUND as well. At 800 kHz frequency, most any length wire antenna will add capacitance which adds to C2, affecting resonant frequency.
The ferrite rod transformer of the rightmost circuit needs neither antenna nor ground.
Bias resistors R1 & R3 at left circuit should be chosen on the high side so that they don't load down L1,C2 resonance...more than tens of kilohms.
To achieve high gain, both Rheadphone and R2 ideally would be above 1000 ohms. D1 would best be germanium or schottky rather than silicon. This also applies to right circuit.
The one in the photo is a draft project of an am- receiver (m is the modulation index) ... it should work .... once all the components are sized ...
Both the input and output have DC bias problems. On the input the coil is a DC short across R3 and the transistor base so the transistor would be biased below cutoff. The diode and speaker on the output will limit the collector voltage to around one diode drop, approximately 0.6 V.
When amplifying an AC signal such as audio or RF the transistor bias needs to be isolated from any DC paths in the input and load circuits, typically with series ‘blocking’ capacitors or transformers.
Also the input impedance of a CE amp with voltage divider bias is relatively low while an LC tank circuit has a high impedance, so there needs to be impedance matching. This can be done a number of ways such as tapping the amplifier input down on the coil (connected to a tap close to the grounded end of the coil).
Likewise the amplifier will have a relatively high output impedance compared to a speaker, so in addition to DC blocking the output you need to either use a higher impedance load such as high impedance headphones or add a power amplifier to drive a speaker.
There are other improvements that could be made, I would recommend doing some research on receivers and amplifiers, amateur radio books are often a good resource for this and many can be found online.
Addendum:
Trying to design a receiver from scratch is going to be problematic, if it doesn't work you don't know if it's because it's a poor design or if you've built it wrong. I would start with a known working design, get that working and go on from there. One thing you might look into is reflex receivers, they use one transistor to amplify both RF and audio. Eventually you can work your way up to superheterodyne receivers or even software defined radio.
You keep using the term "speaker" - it will not work, because common speakers are like an 8-36Ω resistor. They will be unnecessarily quiet. High impedance headphones are needed for "listening" to the demodulator diode - or an amplified speaker or headphones with an amplifier (good bluetooth models that support wired operation are fully amplified when turned on).
On the RF side, you'd use a double variable capacitor for tuning and have one section tune the antenna tank, and the other section tune a collector tank. That way the amplifier would be selective. Or you can lower the Q of the antenna tank and make it wideband - then it won't need tuning, but you lose selectivity.
You'd want another section as an output amplifier. Connecting the demodulator diode directly to the speaker or headphone driver coil is a pessimization that unnecessarily cripples the performance of the circuit.
