Trying to make an Long Wave radio converter: about impedance transformation in HF and audio circuits



Published
I tell in this video something about “impedance transformation”.

That has a lot to do with radio circuits, especially when you want to connect a HF coil with a parallel capacitor (=tank circuit, tuned to a certain HF frequency, be it on Shortwave or not, say between 20 KC and 19 MC) to tune in to certain radio stations. Say Radio stations between 100 KC up to 17 MC.

When the impedance of the amplifier to which you connect your radio coil/tank circuit/tuned circuit (say between 100 KC and 17 MC) via that (necessary) DC blocking capacitor (range 100 pF-470 pF) is too low that will not work good and/or there will be problems.

Read: ‘’damping” : especially on the higher frequencies. To give an idea: connect a 10 K resistor parallel to a L-C coil, that will damp the frequency response very substantially. In some cases the radio signal will be completely lost. You will hear nothing.

These issues don’t pop up (in general) in the audio band, going from 20 Hz to approx. 18/20 KHZ. Always use a series capacitor of 1 uF non polar in these cases, so to send the audio signal into the base of a tone correction circuit transistor (NPN, base) or at the output of a tone correction circuit (collector, NPN transistor).

Output capacitor: 1 uF non polar. Please note (!) for audio the showed stage with the BC 547 b transistor needs a 1 uF non-polar output capacitor from the collector to .. etc. Of course not the 50 pF! (50 pF/100 pF coupling capacitor only refers to HF circuits, 50 KC-17 MC).

With the higher amplification of say a passive tone control circuit with the help of good quality NPN transistors (Hfe = 300, say the BC 547 b), all frequencies in the audio band can be pushed up or pushed down or amplified better, etc.

Say with the classic Baxandall circuit. By the way: there are approx. 40 or much more circuits on my YT Channel about audio, frequency filters, Baxandall, audio tone correction, etc. Search via the “looking glass” on my YT Channel. Type in keywords.

With a standard NPN/PNP Silicon or NPN/PNP Germanium transistor the input impedance is in the 5 K – 10 K range and many circuits often do not work.

Question: why do you see so many radio circuits published in books of the 1970’s and 1980’s that work so properly when they use standard NPN/PNP/Silicon/Germanium transistors?

Answer: Many of these circuits are specialized and adapted (very specially developed) to work with Germanium or Silicon transistors.

So: not with FETS (= Field Effect Transistors). FETS resemble in a kind of way the triode tube with its endless impedance on its grid. Read: Gate in case of a FET.

Of course there is a big difference about how a coil of 20 KC (KHZ) looks like, compared to a coil of 17 MC. We are talking about “meters” in difference (wavelength), when you don’t see that, study the basics of radio technology first. Say wavelength (meters) in relation to frequency, be it audio or HF or VHF or in the GHZ range.

Conclusion: the video shows a good kind of universal setup (HF or Audio) with a FET that has an “endless” impedance on its input.

NB: important: the output capacitor in the AF (=audio) circuit (from the collector to a possible second amplifier stage) must be 1 uF non polar and the capacitor on the node of the 1 K resistor (emitter of the say BC 547 b) must be 10 uF for audio applications. That gives the best results in audio.

When you are interested in my video's: go to the “looking glass” on my Channel trailer. Type there the keywords that you want/need like: “power supply”, audio amplifier, capacitor tester, oscillator, radio, shortwave, shortwave radio, or any keyword you like to find the essential info & schematics on my YT channel.

You can contact me via email when you have questions about my published circuits. I always ask for a picture of what you have made + a problem description; I ask to communicate.

My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4


When you search, search always “NEWEST FIRST” to get the right overview. You can also search via the “looking glass” on my Channel trailer via keywords like ”audio”, “radio”, “amplifier”, “filter”, “Shortwave”, “transistor”, “FET”, “oscillator”, “generator”, “switch”, “schmitt trigger” etc; so the electronic subject you are interested in. My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there.

https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=Ko+Tilman.

I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc.

Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that these things happen. Upload 21 October 2021.
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Audio
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