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Old 20th November 2010, 01:02 AM   #1
neazoi is offline neazoi  Greece
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Default Needed: single transistor audio amplifier with positive feedback

hello,
I am trying to amplify as much as possible the very few microwatts out of a radio detector.
I would like to use just one transistor if possible so I believe the best to do is to apply some positive feedback from the output to the input to further amplify the signal.

Is there any such schematic out there using a single transistor and positive feedback loop??
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Old 20th November 2010, 01:07 AM   #2
Bigun is offline Bigun  Canada
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Try doing some research via Google, e.g. check this link out for regenerative radio circuit - I think it's what you are trying to build.

Regenerative circuit - Wikipedia, the free encyclopedia

http://www.electronics-tutorials.com...o-receiver.htm
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Old 20th November 2010, 01:58 AM   #3
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Here you go, the super-regen:-

designing superregenerative receivers

w
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Old 20th November 2010, 12:07 PM   #4
DF96 is offline DF96  England
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The OP was asking about positive feedback for amplifying the output from a radio detector. The regen uses positive feedback for amplifying the input to the detector. The superregen does this even more, and includes the detector too. There is also the reflex, which uses one device to first amplify the RF then amplify the audio on a second pass.

Could the OP give us a bit more detail on what he is trying to do? I would have thought an op-amp would give more gain than a transistor, and less risk of instability.
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Old 20th November 2010, 12:40 PM   #5
sandyK is offline sandyK  Australia
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BTW, whatever happened to Tunnel Diodes ? They made great little TV audio super regen detectors .
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Old 20th November 2010, 01:12 PM   #6
neazoi is offline neazoi  Greece
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Quote:
Originally Posted by DF96 View Post
The OP was asking about positive feedback for amplifying the output from a radio detector. The regen uses positive feedback for amplifying the input to the detector. The superregen does this even more, and includes the detector too. There is also the reflex, which uses one device to first amplify the RF then amplify the audio on a second pass.

Could the OP give us a bit more detail on what he is trying to do? I would have thought an op-amp would give more gain than a transistor, and less risk of instability.
That is exactly df96! The regen/superregen/reflex uses feedback to amplify the radio or the radio and audio together. I do not want radio feedback. I leave the radio part to another circuit. I just need an AUDIO amplifier that uses positive feedback for ultra high gain, which I will put after the detector. For simplicity reasons (since my design is minimal) I would like it to use just a single transistor.

I have found many regens but this is just not what I am trying to do. It must not be that hard to feed some positive output signal back to the input of a single transistor, but I just do not know how to do it.
I think this would be of interest to many radioamateurs building minimal designs.
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Old 20th November 2010, 03:47 PM   #7
Elvee is offline Elvee  Belgium
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There are a number of fundamental impossibilities there: you cannot use a reflex stage to amplify twice the same frequency range, and you cannot use positive feedback on a single active stage without a "prop", such as a transformer for phase inversion.
But even if you allow a transformer, it would be silly, because it would be difficult to ensure the stability of such a stage, and a supplementary transistor is much less expensive than any transformer.

60 years ago, it might have been worth exploring the possibility, but not anymore.
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Old 20th November 2010, 04:22 PM   #8
jcx is offline jcx  United States
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another view is that positive feedback just "unmasks" gain that is inherent in the device - in triodes there is internal negative feedback since the electrons "see" and respond to the plate V - it is possible (and was necessary in early tube circuits) to use positive feedback to partially cancel the triode's internal negative feedback over a restricted frequency range to obtain higher useful gain from the tubes

bjt transistors have much lower internal feedback and don't benefit much from positive feedback - although multistage amps may use positive feedback to make loop gain available from a stage otherwise using 100% local feedback

Last edited by jcx; 20th November 2010 at 04:25 PM.
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Old 20th November 2010, 04:48 PM   #9
DF96 is offline DF96  England
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You can get some positive feedback with a single transistor and without a transformer by having some feedback from collector to emitter. Put a small resistor in the emitter circuit - by itself this will give negative feedback. Then add a resistor plus capacitor from the collector to the emitter, to give positive feedback. Personally, I would use an opamp.

Positive feedback does not "unmask gain", it actually adds gain. You can see this from any oscillator, where the power gain approaches infinity - it amplifies thermal noise right up to useful levels. The device did not have hidden gain this big!
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Old 20th November 2010, 04:49 PM   #10
neazoi is offline neazoi  Greece
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Quote:
Originally Posted by Elvee View Post
There are a number of fundamental impossibilities there: you cannot use a reflex stage to amplify twice the same frequency range, and you cannot use positive feedback on a single active stage without a "prop", such as a transformer for phase inversion.
But even if you allow a transformer, it would be silly, because it would be difficult to ensure the stability of such a stage, and a supplementary transistor is much less expensive than any transformer.

60 years ago, it might have been worth exploring the possibility, but not anymore.
why do you need another stage or transformer? A transistor amplifier outputs two signals. One from it's collector, which is 180degrees out of phase from the input signal and one from it's emitter, which is in phase with the input signal. Couldn't I get a portion of the signal using a capacitor/resistor combination from the emitter and fed it back to the input, thus positive feedback?
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