Radio - Wireless

Two-Transistor Regenerative Receiver Schematic Circuit Diagram

Regenerative? What’s that? In the age of Twitter and smartphones, you can’t assume that things not related to the Internet are still generally known — or maybe they are? Amazingly, Google delivers nearly 80,000 hits for the search term ‘regenerative receiver’ and much more for the German name Audion, even though it lacks a lower-case as a prefix (‘iAudion’ also exists, but it yields only 12,000 hits). From this, we can conclude that this type of receiver is not entirely unknown nowadays, even if some of the search results have nothing to do with radio circuits. In any case, it’s a reasonable assumption that you all have the idea of what ‘regenerative’ means. If not: it’s a super-simple but nevertheless sensitive type of radio receiver. If you want to know more, check the references. The Wikipedia entry on this topic [1] is also quite extensive. The author is a fan of the many HF experiments and projects dreamed up by the well-known Elektor author Burkhard Kainka. Based on Kainka’s regenerative receiver circuit, the author developed an especially simple but high-performance version using modern components — a regenerative receiver using only garden-variety transistors, but with good reception characteristics. The author has published a version of this design on his website [2].

Two-Transistor Regenerative Receiver Schematic Circuit Diagram

 

The Elektor version, which is perhaps a bit easier to build yourself, is described here. Instead of breadboard construction, which was common in the days before PCBs were invented, the component layout of the design presented here has been optimized with the Lochmaster 4 program for assembly on an Alex prototyping board (a.k.a. UPBS-1 and available from the Elektor Shop). First a few words about the circuit. A noteworthy feature is that both transistors are type BC548 in the 0815 package. Apparently, types that are actually designed for audio use are adequate for use with HF signals in the medium-wave broadcast band from 0.5 to 1.6 MHz, which is what we’re interested in here.

Variable capacitor C1 and coil L1 from the usual parallel resonant circuit that determines the receiver frequency. The special feature of a regenerative receiver is that an active component, or more precisely the gain of an active component, is used to implement a form of feedback that is adjusted to the point where the circuit is just on the edge of oscillation. This reduces the load on the resonant circuit and increases its selectivity, and the high gain makes the receiver fairly sensitive. The active component, in this case, is T1. The feedback is provided by P1 and the tap on L1. Here T1 does double duty: it provides HF gain and (thanks to the nonlinear characteristic of the BE junction) it demodulates the AM signals commonly transmitted in the MW band. T2 provides additional gain for the audio signal. A small loudspeaker or (preferably) headphones can be connected to coupling capacitor C6. The headphones should have a high impedance to improve matching. For this reason, it’s a good idea to connect the two earphones in series.

Assembling the circuit is straightforward thanks to the layout for the prototyping board. A bit more dexterity is required for making the aerial, but even here you don’t need much more than the usual hobbyist tools. You can have planks and laths sawn to the dimensions given in the components list in any home improvement shop for a small charge. Fit the PCB in the middle of the baseboard between the variable capacitor on the left and the potentiometer on the right. Screw the lath cross to the baseboard behind the PCB as shown in the photo of the prototype (note: in the prototype the author fitted a trimpot directly on the PCB instead of using an external potentiometer; this is also shown in the component layout for the Elex board). Wind 20 turns of enameled copper wire on the cross, with a tap at the end of the fifth turn. The exact arrangement is not as critical as it might appear to an HF novice. The prototype built in the Elektor lab drew 1.4 mA from a 9 V battery. The measured frequency range with this construction was 0.4 to 1.4 MHz. The reception quality is surprisingly good, once you get the hang of adjusting the feedback with potentiometer P1.

In terms of reception, the regenerative receiver can hold its own against superheterodyne receivers. If you want to connect an amplifier instead of headphones, you can replace R6 by a potentiometer with the positive end of C6 connected to its wiper. If you can’t scare up a suitable variable capacitor rated at 500 pF or so, you can purchase the VCAP4 from [3] and connect the two 265-pF gangs in parallel. Incidentally, in the Elektor lab it was necessary to reduce the turns count of the aerial loop by three in order to roughly match the receiver frequency range to the MW band. In our opinion, this regenerative receiver is not only a good example of a loop aerial receiver, but also a good candidate for a ‘father and son’ project where you can try out lots of things and learn from them. And don’t forget that a loop aerial receiver is a directional receiver!

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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