JFET-Based Infinite Impedance Detector for AM-Demodulation

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This video shows my experiments with two simple variants of the so-called "infinite impedance". This type ofcircuit has been around for ages and was commonly used in vaccum tube circuits to demodulate AM-signals. Since vaccum tubes and JFETs are pretty similar, it was apparent that such a circuit could also be implemented usinf a BF256B JFET.
Here's the corresponding blog-article: baltic-lab.com/2023/02/jfet-b...
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Опубликовано:

7 фев 2023

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Комментарии : 17

@francoisdastardly4405 2 месяца назад

Fantastic demonstration ❤❤

@BalticLab 2 месяца назад

Thanks so much 😊

@amirb715 10 месяцев назад

the distortion in the baseband output highly depends on the source resistor and the amplitude and bias at the gate. It improved quite a bit in the end when the potentiometer was used instead of the 22k. It would be nice to see the THD or at least the FFT of the base band output and adjust the source resistor to minimize the THD. It's a FET so the second harmonic distortion is dominant

@BalticLab 10 месяцев назад

Good idea and good timing: Measuring THD is going to be a topic of a future video, so I will consider actually using the Infinite Impedance Detector as a device under test. Migth actually be interesting to even compare regular diode detector, with and without bias, and JFET-based options against each other in regards of THD performance. So thanks for your suggestion.

@Paul_VK3HN Месяц назад

Thats helpful, thanks!

@BalticLab Месяц назад

Glad you found the video helpful! P.S. Been subscribed to you since whenever I discovered your channel.

@Paul_VK3HN Месяц назад

@@BalticLab Haha, tnx!

@W1RMD Год назад

Great video! Thank you for sharing.

@Batmule Год назад

Nice and clear explanation. Thanks!

@ProdigalPorcupine Год назад

Valves is the terminology used in the UK (and probably some commonwealth countries). Tubes is the U.S. name, not sure about Canada. Fascinating video!

@medvidekkrupicka1404 Год назад

I am puzzled why we see both the halfwaves of the demodulated signal if, as the author said, the negative hafwave of the modulated signal does not change the conductivness of the JFET. I would be grateful for some explanation from someone knowing the right answer here.

@medvidekkrupicka1404 Год назад

Silly me. Of course you need only one HF halfwave to get the whole demodulated audio as it is all biased above zero level.

@markjacobsen302 Год назад

I have always been curious about this type of detector. I am currently in the process of building a communications receiver and at this point the detector is a diode type in which I have the diode slightly forward biased to overcome the problem of the .6 Volts of signal needed to get the diode to conduct. I never liked the low signal detect/no detect problem. So I am wondering how the infinite impedance detector performs at low signal input levels. Could you tell me about your findings regarding this aspect?

@BalticLab Год назад

Hey Mark, it's been a while since I tried a comparison between a biased diode detector and the infinite impedance detector. But the infinite impedance detector definitely performed a lot better in low-signal conditions than the biased diode detector. The second circuit in the video with the IF transformer, for instance, works well on 200 mVpp signal amplitude.

@iblesbosuok Год назад

Can I use 2SK19 or 2SK30 or BF245? Thank you

@BalticLab Год назад

Pretty much any n-channel JFET should work. The BF245 definitely works and I also tried a J113. You may have to adjust the source resistor, however, to bias the device into cut-off.

@iblesbosuok Год назад

@@BalticLab thank you very much, sir