In this AM radio receiver circuit, the transistors (BC547 and BC557) serve as amplifiers to boost the weak radio signals received by the antenna. Here’s a detailed explanation of their roles and how they operate within the circuit:

BC547 (NPN) - RF Amplifier

Function:
The BC547 transistor acts as a radio frequency (RF) amplifier. It amplifies the weak RF signals received by the antenna and the LC circuit.

Operation:
1. Base Connection:
The base of the BC547 is connected to the output of the LC tuning circuit through a 100kΩ variable resistor. This allows the weak RF signal to be fed into the transistor.

2. Emitter Connection:
The emitter is connected to the ground. This establishes the common reference point for the transistor's operation.

3. Collector Connection:
The collector is connected to +3V through a 4.7kΩ resistor (R_C). This resistor limits the current flowing through the transistor and sets the operating point.

4. Signal Amplification:
When the RF signal is applied to the base, the transistor amplifies it. The amplified signal appears at the collector, which is connected to the base of the next transistor (BC557).

BC557 (PNP) - Audio Amplifier

Function:
The BC557 transistor acts as an audio frequency (AF) amplifier. It amplifies the detected audio signal for further processing by an external amplifier.

Operation:

1. Base Connection:
The base of the BC557 is connected to the collector of the BC547 through a 10kΩ resistor. This feeds the amplified RF signal into the BC557 for further amplification.
A 10kΩ resistor is also connected from the base to ground to ensure proper biasing.

2. Emitter Connection:
The emitter is connected to +3V through a 1kΩ resistor (R_E). A 100µF capacitor is placed in parallel with the 1kΩ resistor to stabilize the AC signal.
The emitter also connects to the external amplifier via a coupling capacitor (10µF to 100µF).

3. Collector Connection:
The collector is connected to ground.

4. Signal Amplification:
When the RF signal from the BC547 is applied to the base of the BC557, the transistor amplifies the audio component of the signal.
The amplified audio signal is then coupled through the capacitor to the external amplifier, which boosts the audio signal to drive a speaker.

Detailed Signal Path

1. Reception:
The antenna receives the AM signal, which is tuned by the LC circuit.

2. Detection:
The diode (1N4148) detects the amplitude variations in the signal, which correspond to the audio information.

3. First Stage Amplification (BC547):
The detected RF signal is fed into the base of the BC547.
The BC547 amplifies this signal, and the amplified RF signal appears at the collector.

4. Second Stage Amplification (BC557):
The amplified RF signal from the BC547’s collector is fed into the base of the BC557.
The BC557 amplifies the audio component of the signal.
The amplified audio signal is then passed through the coupling capacitor to the external amplifier.

5. Output:
The external amplifier further boosts the audio signal to drive a speaker, producing the sound corresponding to the received radio station.