RF Amplifier for NXP Contactless Reader IC’s, MFRC500, MFRC530, MFRC531, CLRC632, SLRC400

RF Amplification System
The block diagram in Fig 1 shows the complete RF amplifier system placed between the NXP contactless reader IC and the antenna. The system consists of a transmitting path (blue) and a receiving path (red).

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(1) This solution implements an active amplifier and filter for the receiver part
Fig 1. Block Diagram of RF Amplifier Solution

The main part of the RF amplifier stage in the transmitting path is built around a class A/B RF amplifier working in a four-quadrant operation. It delivers the amplified current to the antenna to generate a higher magnetic field. A filter network before this RF amplifier stage acts as an EMC filter in order to attenuate higher frequency components to form a sinusoidal waveform out of the square wave signal coming from the contactless reader IC. The receiver path is also accomplished by two parts, consisting on a 13.56 MHz oscillator and a dual operational amplifier (OA). The oscillator acts as a band-stop filter which decreases the 13.56 MHz carrier, such that the sideband levels can be better amplified by the amplifier OA 2. OA 1 acts as a buffer amplifier which decouples the signal from the antenna to the band-stop filter.

EMC Filter
The NXP contactless reader IC offers the output pins TX1 and TX2 which deliver square wave signal shapes, where TX2 is phase delayed for 180° compared to TX1. These signals are converted to sinusoidal waveforms by the EMC filters. The EMC filter is directly connected to GND and TX1 and to GND and TX2 of the NXP contactless reader IC. It consists of a series inductance and a parallel capacitance as shown in Fig 2.
NOTE: The amplitude of the signal will be increased because of the resonance effect of the filter. This is desirable since it is the input signal for the A/B power amplifier. The amplification of the voltage does not solely depend on the values for the coil and the capacitor but also on the value of the input impedance of the A/B power amplifier. When a 12V power supply is used, the peak-to-peak value for this signal must not exceed 10V to prevent chipping.

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Fig 2. EMC Filter

The filter is a low pass filter with a cut-off frequency of about 15MHz, which transforms the rectangular signals coming from TX1 and TX2 into sinusoidal signals. The value of the parallel capacitor is calculated with a predefined value for the cut-off frequency and the coil.

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Assuming a value of 560nH for the inductor and 15.8MHz for the cut-off frequency the required capacitance is determined to be 181pF.

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Table 1. Components of EMC Filter
Component Value
C01, C02 Typically 0402, 0603 or 0805 SMD parts with low tolerance (< ±2%). NPO is required. The voltage limit has to be considered.
L01, L02 Typically a small inductance with high Q for general applications. The frequency range and the maximum allowed current have to be considered. This inductance should be magnetically shielded.
NOTE: Please refer to the application note [6] for details.