Increase gain in operational amplifiers for RFID reader

I'm having a little problem with a RFID circuit.

A few months ago I found a 125 KHz RFID tag reading circuit on the internet and decided to work with it to learn how these circuits work and to learn more about electronics in general.

The circuit is this: [login to view URL]

All information about the circuit is on the author's website: [login to view URL]

The circuit is supplied with 12V, but I decided to eliminate the power circuit and supply it with 5V, from a power supply.

In addition, I decided to improve it and I added decoupling capacitors (C2, C3, C6 and C7) and a pull down resistor (R3) on the digital output that produces the 125KHz wave which makes resonate the circuit. This pull-down resistor was added to eliminate any noise or fluctuation that could occur at that output.

The resultant circuit remains as follows: [login to view URL]

I sent it to manufacture at [login to view URL] and after soldering it was like this: [login to view URL]

I tested it with PVC cards and it worked perfectly. Then I tried it with key fobs and wristbands and it did not work. I have to say, I have another reader (a Chinese one, the RDM6300) and all my cards, key fobs and wristbands are read perfectly by this Chinese reader.

Then, I used my logic analyzer and PulseView software. This software is an open source tool for logic analyzers that has a decoder for the EM4100 protocol (the protocol used by all my RFID tags). I did this to make sure the digital signal to which goes to the microcontroller is good.

I discovered that the PVC card is decoded perfectly, but the signal of the key fobs and the wristbands did not. So the problem is in the analog signal that comes to my circuit. Specifically, that signal reaches my antenna (air core coil) from the antenna of the RFID tag in question (card, key fob or bracelet).

The problem is that a PVC card has a much larger antenna than a key fob or a wristband.

The antennas of the key fob and the wristbands are very small, and therefore does not produce an electromagnetic field large enough to generate signals of large amplitudes in the receiver.

To solve this, I must increase the gain in the amplifiers, with a corresponding factor, to be able to increase the signals received.

The Chinese reader (which is a different circuit, [login to view URL]) amplifies the signals enough to be able to read the RFID tags of anything: cards, key fobs, wristbands, ...

I need somebody to solve this problem on how to increase the gain in the amplifiers in order to increase the signals received.

I want a good solution and also a simulation of this circuit with that solution, so that I can see the gain in the amplification stage.

Ingeniería eléctrica Electrónica Diseño PCB

Nº del proyecto: #13300871

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