Regenerative Detector

Early radio sets used regenerative detectors e.g. as regenerative audions. Due to the regeneration the bandwidth of the LC circuit apparently seems to be narrower than without regeneration. However, the LC circuit as such does not change its bandwidth but the reduced bandwidth observed results from the feedback due to regeneration. If the regenerative detector is modelled as an equivalent LC circuit, this equivallent circuit will have a higher quality Q, but not the physically LC circuit.

This is described in a German thread "Why is a set with one LC circuit only able to separate transmissions?"

The Ingenieur's usual method is to look at the problem in the frequency domain, and so most of the thread covers these ideas. Additionally, also a time domain approach using differential equations is given.

Here, an English version of the time domain treatment is presented.

Abstract

In vintage radio publications and literature there are common statements on the effects of applying positive feedback, also called regeneration, to a tuned radio frequency circuit that suggest that damping due to losses in the tuned circuit is mitigated and selectivity is improved. Recently, articles have been posted on www radiomuseum.org [4] challenging these statements. In aforementioned articles, it is shown experimentally and theoretically in the frequency domain that these common statements are misleading or false. In this article here, we will take an alternate look at the underlying physics in the time domain using differential equations describing the behavior of a tuned circuit with and without feedback.

Regeneration with LC Circuit

Thanks to Dipl.-Phys. Jochen Bauer who is the author.

Regards,

Dietmar

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In the above paper on regenerative tuned circuits, a "black box" feedback device was used to provide proper feedback to the tuned circuit. This feedback device was not further elaborated on. As a supplement to this previous paper, the article presented here elaborates on regenerative tuned circuits employing inductive feedback by a so called "tickler coil". It's main goal is to provide the reader with a basic understanding of how inductive feedback needs to be employed in a circuit to achieve proper regeneration or even oscillations.

For a more detailed overview, the abstract section of the paper is quoted below.

 

Abstract

In this paper, lossy tuned circuits with feedback provided inductively by a so-called "tickler coil" used as regenerative amplifiers or oscillators are analyzed in the time domain using differential equations. It is shown, that for the most important cases, the changes in the behavior of the tuned circuit due to the application of feedback can readily be seen from the structure of the resulting differential equations without the need to actually solve them. It turns out that providing feedback by a tickler coil placed in the plate (resp. drain) circuit of a pentode or field-effect transistor will alter the virtual loss resistance of the tuned circuit as desired while leaving the other parameters of the tuned circuit as they are. This applies to any voltage controlled current source driving the tickler coil. However, if the pentode or field-effect transistor is used in a voltage amplifier circuit with the tickler coil connected to the output, this setup will only provide proper feedback if the output impedance of the voltage amplifier is above a certain minimum. This applies to any voltage amplifier with the tickler coil connected to it's output. It is in contrast to many other applications where a voltage amplifier needs to have an output impedance as low as possible.

 

Attachments:

• InductiveFeedback (73 KB)

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