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enter image description here

enter image description here enter image description here

This circuit is a radio receiver.

The above picture is quoted from here and here. My question is in the diagram.

It looks like a transformer symbol.

Is it even essential? Can't we use a transformer and just use the inductor (highlighted in blue in the picture) as our antenna?

We can just use an op-amp as amplifier instead of a transformer to amplify voltage, right? It seems the author of the video does not use a transformer, right?

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    \$\begingroup\$ Photo may look like an "iron rod", but it is ferrite (powdered iron fixed in a ceramic rod). Yes, it is an inductor, having low losses at low-MHz radio frequencies. It is tuned to resonance with parallel capacitors. It also has characteristics of an antenna. If you don't like ferrite, you can probably substitute a hula-hoop-sized loop of wire inductor having less than ten turns. \$\endgroup\$
    – glen_geek
    Commented Sep 1, 2022 at 15:24
  • \$\begingroup\$ Circuit#1 also to be found in the UTC7642 data sheet. Unfortunately the circuit used in the 2nd picture does not use a transformer --> see this found here. This probably means you should decide which circuit path you are following and whether you use the term transformer or ferrite rod. \$\endgroup\$
    – Andy aka
    Commented Sep 1, 2022 at 15:37
  • \$\begingroup\$ @glen_geek, about the "hula-hoop-sized loop of wire inductor having less than ten turns", does it still have to be coiled in ferrite rod or can just leave it hanging hollow like most of the antenna? Some of the antenna i see has just loops without even the need for ferrite core. \$\endgroup\$ Commented Sep 1, 2022 at 17:31
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    \$\begingroup\$ yes, wire wound in air with no magnetic material. This is also a large inductor. Adding a parallel capacitor (perhaps a variable capacitor) makes a resonant circuit that selects one station's frequency in favour over other stations at a different frequency. The large diameter of the coil makes it sensitive to AC magnetic fields, whereas a tiny coil of the same inductance is far less sensitive to external AC magnetic fields. \$\endgroup\$
    – glen_geek
    Commented Sep 1, 2022 at 18:10
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    \$\begingroup\$ @glen_geek: Which sort of hula hoop do you mean - the one you twirl around your waist, diameter ~70cm, or the one you eat, diameter ~1cm? I'm guessing the former, but thought it worth clarifying since there's quite a difference in size :) \$\endgroup\$
    – psmears
    Commented Sep 2, 2022 at 9:52

4 Answers 4

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Consider what is attached to the other side of the transformer.

If the rod by itself ("loopstick antenna") is sensitive enough for the receiver chip, you might not have an aerial connected at all. (This does seem to be pretty standard for AM radios I've seen.) In that case, it does nothing, and can probably be left out.

If you do use an aerial, its impedance will be extremely high (essentially the reactance of its self-capacitance), so it must be reduced via transformer or other matching network to suit the receiver's input resistance. Presumably, the aerial side has more turns, giving it a higher impedance (inductance of both sides, or turns counts, are not given on the screenshot, so this is just a guess).

Transformers have other features as well, though they aren't used here: for example, galvanic isolation, or to create a double-tuned circuit (only one side is resonant here). So that leaves just the above explanation.

There may be other requirements specific to the IC, which the datasheet or related documentation can explain (granted, perhaps not very well: in which case, asking about that separately might be a good idea).

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One thing to know is that transformers do not amplify. The term amplifier is usually applied to a circuit or device where there is the capability of a net power gain (see this discussion), the product of the output voltage and current can be greater than the product of the input voltage and current. In a transformer the output power is always less than the input power (in an ideal transformer they would be the same, but real ones have losses).

The 'transformer' in the schematic is a loopstick antenna. It is two coils of wire on a ferrite rod. The reason it is wired as a transformer is for impedance matching. Sometimes you will see it done with a single coil that is tapped near one end to create an auto-transformer. Matching the antenna to the RF amplifier input impedance can also be done with a capacitive divider, or an LC network.

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  • \$\begingroup\$ Nitpick: in a power amplifier there is net power gain, but this is not necessary for a current or a voltage amplifier, which a transformer might be. \$\endgroup\$ Commented Sep 2, 2022 at 9:12
  • \$\begingroup\$ @VladimirCravero Yeah, terminology gets a bit complicated. I added a link to some discussion on the topic. My main point is that a transformer is not generally considered to be an amplifier. \$\endgroup\$
    – GodJihyo
    Commented Sep 2, 2022 at 13:56
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    \$\begingroup\$ While this is all correct when talking about ideal devices, I would like to note that it's not impossible to achieve power amplification with a transformer: by driving the core right to the edge of saturation, you can make use of the nonlinear saturation characteristics to amplify an AC signal. This was used occasionally as an alternative to vacuum tube amplifiers, and is referred to as a magnetic amplifier. \$\endgroup\$
    – Hearth
    Commented Sep 2, 2022 at 19:21
  • \$\begingroup\$ A transformer can be used to amplify a voltage which comes from a low-impedance source and will be used to drive a high-impedance load, or to amplify current which is supplied by a high-impedance source and will be used to drive a low-impedance load. While the transformer doesn't actually amplify power, it can increase the amount of useful power delivered to the load. \$\endgroup\$
    – supercat
    Commented Sep 2, 2022 at 21:20
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Video clearly talks about LC tank circuit, inductors and capacitors, and shows the formula for resonant frequency.

$$f_R = \frac {1} {2 \pi \sqrt {LC}}$$

This is the circuit from the referenced data sheet. It does show a transformer used, so it is clear where you got your misunderstanding, but it is there mainly as an inductor, not as a transformer.

enter image description here

It's also clear why the manufacturer used it (because they are easier to obtain than making an inductor). But the selected transformer characteristics are not in the data sheet (aside from inductance), so it is completely understandable why the youtuber went the way they did.

This is the circuit, the youtuber implemented. Notice the transformer has been simplified to an inductor symbol. In the video, he then has to search for an appropriate trimmer capacitor. This is closer to the classic AM radio, with wire wrapped around a ferrite rod to form the inductor L and a parallel plate air capacitor connected to a dial form a capacitor C to tune to frequencies in the AM frequency band. His trimmer capacitor will have less range, but that is not the point of his video.

enter image description here

From Wikipedia.

enter image description here

As used (by the youtuber), the copper wrapped iron rod, would be better referred to as an inductor or choke.

Any transformer/inductor AND capacitor connected in parallel form a resonant frequency, tank circuit or tunable circuit to select a specific frequency via the resonant frequency formula, but proper selection of L and C, will select a frequency in the AM radio band from 540 kHz to 1600 kHz.

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  • \$\begingroup\$ Hi, thanks for replying. About the resonant equation, that you posted which is affected by L and C only, does the L (inductance) resistance have a significant effect on the resonant frequency or we can simply ignore it :)? \$\endgroup\$ Commented Sep 2, 2022 at 4:11
  • \$\begingroup\$ I built an AM radio as a child where the inductor was the tunable component by way of a threaded iron core that could be slowly inserted or removed from the windings by turning it. Either way, it seems like clarity on a tuning mechanism is omitted from the original circuit diagram. \$\endgroup\$ Commented Sep 2, 2022 at 4:18
  • \$\begingroup\$ The inductor has resistance, which decreases the received signal or transmitter carrier, but gain in the amplifier counters this. \$\endgroup\$ Commented Sep 2, 2022 at 5:56
  • \$\begingroup\$ The 2nd circuit in your answer is actually the receiver circuit he used and not the transmitter. \$\endgroup\$
    – Andy aka
    Commented Sep 2, 2022 at 13:50
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Transformers are used with RF for many tasks.
Filtering, impedance adapting, oscillators ...

At RF high frequencies, the op-amp is not really the best choice for amplifying, even if some can do it ...

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    \$\begingroup\$ Hi, but transformer as voltage amplifier is not really essential for this chip right because this chip is very sensitive to small input voltage picked by by antenna.The video does not seem to use transformer right? \$\endgroup\$ Commented Sep 1, 2022 at 15:28
  • \$\begingroup\$ The configuration you show is a "band-pass filter". It allows only "one" frequency to be amplified after by the integrated circuit which has also probably the function of demodulating the "signal" ... TA7642 is "ONE CHIP AM RADIO CIRCUIT" \$\endgroup\$
    – Antonio51
    Commented Sep 1, 2022 at 15:33

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