Oscillators Circuit Diagrams

Emitter-Follower Audion Schematic Circuit Diagram

A shortwave audion receiver using only two transistors and a single 1.5 V battery — that must be the ideal entry level into shortwave receiver technology. Just add an active PC loudspeaker for very convincing performance. A special feature is the audion circuitry that uses a BC558C PNP transistor working in emitter follower mode. This function works thanks to the few picofarads of internal capacity between the transistor’s base and emitter. This produces a capacitive voltage divider, enabling the transistor to operate as a three-point oscillator, also known as a Hartley oscillator. Only a minute amount of emitter current is required to go into oscillation.

The trimpot (trimmer potentiometer) is used to adjust the audion for AM reception so that it does not quite oscillate (immediately before oscillation sets in), for CW (telegraphy with keyed carrier) and SSB (single-sideband) reception it is set slightly higher.

Emitter-Follower Audion Schematic Circuit Diagram

Decoupling and amplification of the audio signal is handled by the second transistor. The signal on the output connector K1 is at line level, with an output impedance of about 1 kΩ.

Either of the two antenna connections ANT1 and ANT2 can be used. A good ground (earth) connection is essential for this circuit, in which case a short indoor wire antenna of less than a meter in length connected to ANT1 will be sufficient to pull in countless broadcast stations. For DX (long distance) reception an external antenna is better, for example, an aerial ‘long wire’ of around ten meters (30 ft.) in length. In this case, the ANT2 connection must be used. The coupling to this input is slightly weaker in order to reduce resonance and offset any reaction (feedback). As a general rule, the longer the antenna, the smaller the value of coupling capacitor C1.

The BC558C is manufactured in a plastic TO-92 case. When looking at the flat side with the leads pointed downward, the three leads emerging from the transistor are, from left to right, the collector, base, and emitter leads.



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