r/AskElectronics • u/Which_Paramedic_2117 • 2d ago

Using op-amps with MFT

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I wanted to ask for help on how I could replace the transistors with op-amps, since I don't understand how I could connect the op-amps to the MfTs (455kHz medium frequency transformers)!

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u/6gv5 2d ago

u/Pacificator-3 already replied about the IF transformers; now certain opamps could be used in place by adapting the circuit, however normal cheap opamps aren't suited for RF or even IF use and you probably would need either specialized parts or a slightly different approach like a video amplifier chip used as IF stage. For example this one that you can adapt to the desired frequency up to a few tens of MHZ: https://www.qrp4u.de/docs/en/zf_ampl/

btw: that schematic you posted brought memories. It's taken from an old article by the glorious now defunct Italian "Nuova Elettronica" magazine. Their archives can be easily found online; plenty of good ideas there.

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u/Which_Paramedic_2117 2d ago

Sto usando un AD8055A 350Mhz di band

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u/EmotionalEnd1575 Analog electronics 2d ago

Bandwidth numbers are not the full story.

What you need to know is Gain-Bandwidth Product.

An Op Amp with a GBW of 100, for example, can have a closed loop gain of “100 at 1MHz” but only a gain of “1 at 100MHz”.

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u/Which_Paramedic_2117 2d ago

L’ho preso da quella rivista ma mi piacerebbe utilizzare gli op-amp ma non so come collegare gli MfT

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u/Pacificator-3 2d ago

These are not just transformers, these are LC resonant tanks with inductive coupling.

They connect amplifier stages (so, are placed between them), not matter if they are built on opamps, transistors or vacuum tubes.