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I'm not really sure why, but I have seen many different RF circuits in which the input to a transistor is taken from a center tap instead of across the entire secondary, such as this example:

Tapped input

Why is the input taken from the tap instead of across the entire secondary?, I don't understand the need for the tap.

Sometimes also the output might be connected to the tap instead of the bottom side of the primary like this example:

Output Tap

And in some extreme cases, like this IF amplifier, only the tap is used, the outer end of the primary is left disconnected.

Super Het

What is the logic behind this?

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Impedance conversion.

Tapping the top of a tuned cct tends to load it, reducing its Q, broadening the bandwidth where you want a narrowband to reduce interference. In your first example the FET is a high resistance load but it still has input and Miller capacitances which you don't want across the tuned circuit.

Tap at 1/3 of the inductor, and you transform the equivalent impedance across the whole tuned cct by 3^2 = 9.

Same in the 2nd cct : while the transistor approximates a current source, it adds a high but not infinite resistance across the coil : by connecting it to a tap, the impedance across tho whole coil is increased. (Also the transformer steps up the resulting voltage, increasing gain).

In the third circuit I suspect it's merely a way to get one of several step-up ratios (and impedance transformations) out of a standard transformer. I see part of a second one : was that connected differently?

Sometimes it can be used to convert from a standard impedance like 50 or 75 ohms from an antenna or transmission line to a higher one for tuning.

"Impedance conversion" by adding resistors would add Johnson noise generated by the resistor : impedance conversion by transformer does not, as well as stepping up the voltage when an impedance increase is desired.

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  • \$\begingroup\$ Thank you for your answer, it makes a lot of sense, I have updated the third circuit so you can see a bit more where the signal is coming from and where its going to, the circuit is part of a DIY superheterodyne AM radio reciever, as you can see, only the taps are used and the bottom side of the primaries are left floating. I believe the transformers are IF transformers with a parallel capacitor included which apparently is not shown, they are tuned to 455 KHz. \$\endgroup\$ – S.s. Jun 3 at 21:29
  • \$\begingroup\$ Regarding the input transformer, which comes from the antenna, having the input from the tap lowers the signal, isn't that undesireable? wouldn't you want the largest signal coming from the antenna? \$\endgroup\$ – S.s. Jun 3 at 21:36
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    \$\begingroup\$ No, you want the best impedance match into the amplifier input (Q2 base) which is not a very high impedance. Source impedance = load impedance is the condition for maximum power transfer (per the max power transfer theorem) to get the most RF power into the amplifier. \$\endgroup\$ – Brian Drummond Jun 3 at 22:09

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