Why is a receiving circuit in e.g. RFID made resonant with a parallel capacitance instead of a series capacitance?

Question

Take for example the schematic below of a inductive receiving circuit in RFID, where we want to maximize the voltage over the load resistor R4 (R3 is the resistance of the inductive coil, and the voltage source represents the signal that is received and induced in the coil)

^{simulate this circuit – Schematic created using CircuitLab}

The voltage is now V2*R4/(R3+sL1+R4), which in my opinion can easily be maximized by adding a series resonant capacitance, such that the impedance is real at resonance (and further maximized by increasing R4). However, I read that voltage maximization for circuits as this one is done by adding a parallel capacitance. When doing the calculations, I always obtain a voltage that is lower than the one for the simple series resonant circuit I present here.

EDIT: when considering the measuring device to have an infinite input impedance, the measured voltage for the series resonant circuit equals the applied voltage V2. For the parallel resonant circuit, it equals 1/(sRC) times the applied voltage V2. Is it possible that the eventual best solution is depending on the size of R and C and the operating frequency (whether their product is larger or smaller than 1)?

Answer 1

That's because the model you are assuming does not well replicate an rfid receiver.

The EM wave that couples with the coil produces some varying voltage at the coil's terminals, so the voltage generator, with a series resistor if you want, should be in parallel with the coil. Now the best you can do is adding a parallel capacitor so that the LC parallel resonates at the right frequency provoking a very high voltage gain (not power gain).

Answer 2

The RFID receive coil is induced with a voltage from the transmit coil. This voltage is generated in series with the receive coil and, when you add a "so-called" parallel capacitor you are actually making a highly resonant LC low pass filter: -

^{simulate this circuit – Schematic created using CircuitLab}

It's still a series circuit but you are taking the voltage across the capacitor i.e. there can be very significant voltage amplification. We call it parallel thru laziness I suspect and I am as guilty of this as anyone.