27-03-2014, 02:40 PM
AN1425 RF Amplifier for NXP Contactless NFC Reader ICs
RF Amplifier for NXP.pdf (Size: 209.27 KB / Downloads: 19)
General
Some of NXP contactless NFC reader IC’s are designed for handheld devices and low
power consumption which results in shorter read range performance. An RF amplifier
can extend the usage of such devices to meet requirements for higher transmission
performance and communication distance.
This application note is intended to give a practical guide to extend and optimize the
transmission power performance and communication distance of NXP contactless NFC
reader IC’s. The document describes the design and dimension of antennas and RF
amplifier circuits.
The amplifier circuit in this document is designed for a supply voltage of 5V (according to
USB 2.0 specifications). For higher power supply voltages, the circuit as well as the
components have to be accordingly adapted.
How to use this document
The application note gives a practical guide to design antennas, calculate the matching
components as well as implement an RF amplifier for NXP contactless NFC reader IC. It
gives a guideline for complete RF circuit design, an introduction to the overall antenna
design theory for RFID systems as well as a description of the transmitter matching
resistance. Finally the matching procedure is described using a reference antenna which
is connected to the amplifier circuit.
Block Diagram
The amplifier solution is designed to communicate in four different reader/writer operating
modes:
• Reader/Writer mode for communication with devices compliant to ISO/IEC14443A/B
• Reader/Writer mode for communication with devices compliant to MIFARE
• Reader/Writer mode for communication with devices compliant to FeliCa
• NFCIP-1 mode for communication to NFC devices
Antenna Matching
Depending on the antenna PCB (red box in Fig 1), the necessary antenna matching (blue
box Fig 1) consists of a symmetric arrangement of an EMC – filter (L0 and C0) plus a
serial and parallel tuning capacitors, from the reader chip point of view. To regulate the
quality factor of the antenna, the resistor Rq is added.
The capacitors and the resistors are used to both achieve the required 13.56MHz
resonance frequency, and a quality factor for appropriately signal shaping according to
ISO/IEC 14443.
The following equations in chapter 4.1.3 are used to calculate the matching components.
Please keep in mind, that these values, slightly modified, are also required for fine tuning
the components. This is necessary because of the direct influence of the amplifier circuit
onto the phase shifting of the two signals on both sides of the antenna matching circuit.
The maximum output is reached when the antenna is first tuned to 13.56MHz and the
two ends of the antenna tuning circuit, connected to the amplifier, oscillate (in relation to
GND) with 180 degree phase shift.
Amplifier Circuit
The amplifier circuit (refer to Fig 11) consists of a symmetric arrangement of emitter
amplifiers followed by a filter. The resistor Rb at the transistor base controls the input
current. The components Ce and Re in the emitter path of the transistor stabilize the
biasing of the transistor. Re limits the current and Ce acts in combination with Rb as a low
pass. The antenna is connected to the collector and AC- decoupled connected to the
power supply.