Radio - Wireless

Two-Transistor Regenerative Receiver Schematic Circuit Diagram

Rediscovering the Regenerative Receiver

In today’s era dominated by Twitter and smartphones, assuming that knowledge beyond the internet realm is common can be risky. The term ‘regenerative receiver’ might sound obscure, but surprisingly, Google records almost 80,000 hits for this search term. Even the German term ‘Audion,’ albeit lacking a lowercase prefix, garners substantial hits. While ‘iAudion’ also exists, it is less prevalent with only 12,000 hits. This indicates that the regenerative receiver, though not universally recognized, has not faded into obscurity. It’s a safe bet that some familiarity with the concept exists. For those still in the dark, a regenerative receiver is an uncomplicated yet remarkably sensitive type of radio receiver.

Two-Transistor Regenerative Receiver Schematic Circuit Diagram

Building the Regenerative Receiver on a Prototyping Board

In this Elektor version of the regenerative receiver, a user-friendly approach is adopted, making it easier for enthusiasts to build the circuit themselves. Instead of the conventional breadboard construction used before the advent of PCBs, this design employs an optimized component layout created with the Lochmaster 4 program. The assembly is intended for the Alex prototyping board, also known as UPBS-1, readily available from the Elektor Shop.

Understanding the Circuit and its Unique Features

This regenerative receiver design utilizes two BC548 transistors, typically associated with audio applications, yet proven effective for HF signals in the medium-wave broadcast band ranging from 0.5 to 1.6 MHz. The heart of the circuit lies in the variable capacitor C1 and coil L1, forming the parallel resonant circuit that determines the receiver frequency. What sets a regenerative receiver apart is its use of an active component, here T1, to implement a form of feedback teetering on the brink of oscillation. This technique reduces the load on the resonant circuit, heightens its selectivity, and enhances the receiver’s sensitivity. T1 serves a dual purpose by providing HF gain and demodulating the AM signals common in the MW band due to the nonlinear characteristic of the BE junction. T2 further amplifies the audio signal, which can be heard through headphones or a small loudspeaker connected via coupling capacitor C6.

Assembly and Aerial Construction

Constructing the circuit is relatively straightforward, thanks to the optimized layout for the prototyping board. Assembling the aerial requires a bit more finesse, but with basic hobbyist tools, it’s easily manageable. Components’ dimensions, as specified in the list, can be obtained from a home improvement shop. The PCB is positioned between the variable capacitor on the left and the potentiometer on the right, with a lath cross attached behind it. The aerial loop consists of 20 turns of enameled copper wire wound on the cross, with a tap at the end of the fifth turn. The exact configuration is not overly critical, allowing room for experimentation. The receiver, drawing 1.4 mA from a 9 V battery, achieves a frequency range of 0.4 to 1.4 MHz. Adjusting the feedback with potentiometer P1 optimizes reception quality, delivering surprisingly impressive results.

Enhancements and Applications

Compared to superheterodyne receivers, the regenerative receiver stands its ground in reception quality. If you prefer an amplifier instead of headphones, you can modify the circuit accordingly. Additionally, for those lacking a suitable variable capacitor rated at around 500 pF, the VCAP4 can be purchased, allowing the connection of two 265-pF gangs in parallel. In Elektor’s lab, adjusting the aerial loop turns by three helped align the receiver frequency range with the MW band. This regenerative receiver not only exemplifies a loop aerial receiver but also serves as an excellent “father and son” project, offering ample opportunities for experimentation and learning. It’s crucial to remember that a loop aerial receiver is inherently directional, adding to its fascinating capabilities.

Internet Links

[1] en.wikipedia.org/wiki/Regenerative_circuit

[2] www.elektronik-radio.de/39994.html (in German)

[3] www.ak-modul-bus.de/ (in German)

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