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In the circuit below, there is a subcircuit that connects between C4, D1 and the VCC (5V), with Q1 and D2 as the central component, this circuit seems to me to be a Gyrator, I know that Gyrators are a choice to replace coils, however here it replaces a variable capacitor.

How is this circuit characterized? is there a standard that defines it, is there any paper that defines it clearly?

TDA7000 FM Receiver - with gyrator

source: https://www.py2bbs.qsl.br/tda7000_fm_receiver.php

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    \$\begingroup\$ D1 itself is the variable capacitor. All the circuitry upstream of R8 is just to generate a specific range of DC voltages. Why it is so complex ... temperature compensation maybe? no idea, there should be a white paper describing that cct somewhere (it's a 30+ year old circuit so good luck finding it) \$\endgroup\$ Apr 10 '20 at 10:13
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    \$\begingroup\$ TDA7000 description and internal block diagram here || As Brian notes - D1 is a varicap . Q1 and associated cct provides a fixedish DC voltage across R7+P2. This is tapped off by P2 wier and used to DC bias (tune) the varicap. | TDA7000 pin6 provides the other end of the varicap voltage - this is internal connected to a VCO and internal varicaps. SO it seems that the circuitry in question is part of s stable DC tuning supply. \$\endgroup\$
    – Russell McMahon
    Apr 10 '20 at 11:47
  • \$\begingroup\$ I was confused by this, the circuit seemed to be nothing more than a voltage regulator, but I feared I was being innocent in this analysis. So, as I understand it, the function is just to keep the voltage as stable as possible so that the tuning does not vary with the temperature. \$\endgroup\$
    – Delfino
    Apr 12 '20 at 23:55
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TDA7000 description and internal block diagram here

As Brian notes - D1 is a varicap - a diode with voltage variable capacitance.

Q1 and associated circuitry provides a fixedish DC voltage across R7+P2.
This is tapped off by P2 wiper and used to DC bias (tune) the varicap.

TDA7000 pin6 provides the other end of the varicap voltage - this is internal connected to a VCO and internal varicaps. SO it seems that the circuitry in question is part of a stable DC tuning supply.

enter image description here

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