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

RF Power A/B Amplifier
The 13.56MHz amplifier in Fig 3 is working in a four-quadrant operation using a pushpull A/B amplifier and 12V power supply (approximately 200mA required from the DC source).

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Fig 3. Push-pull amplifier in A/B operation

The capacitors C1 to C4 are used to block the DC current and should have a value of 100nF to offer low resistance in the 13.56MHz region. D1 to D4 are 1N4007 diodes and biasing the transistors Q1 to Q16. The resistors RE1 to RE4 build voltage dividers, biasing the input signals on a 6V potential, which is half the supply voltage of this circuit. The 47Ω resistors R1 to R16 synchronize the currents through the transistors, which may differ due to manufacturing tolerances. Moreover, they match the output impedance of the amplifier stage to the desired antenna impedance (approximately 25Ω for each path) and they provide a current feedback which acts as temperature compensation.

The collector currents of 2N3904 (NPN) and 2N3906 (PNP) transistors used in this application are limited to 200mA. That’s why four of them are connected in parallel for each stage, to increase the overall output current and withstand shorts and open loops.

Table 2. Components of A/B Power Amplifier
Component Value
C1…C4 DC blocker, 100nF; (Ceramic NP0, tolerance ≤ ± 2%)
RE1…RE4 330Ω (Small 0402, 0603 or 0805 SMD parts)
R1…R16 47Ω (e.g. MRS25, 0.6W)
D1…D4 1N4007
Q1…Q4, Q13…Q16 2N3904 NPN Transistors or equivalent
Q5…Q12 2N3906 PNP Transistors or equivalent
VCC 12V Power supply

 

Antenna Design & Matching
The sample antenna used in this application note is shown in Fig 4. The outline of the rectangular antenna is approximately 10cm x 10cm. In order to match the antenna to desired impedances some calculations for external passive components have to be made.
The antenna must be connected to a network analyzer by using an appropriate test fixture that does not influence the antenna parameters. The analyzer must be calibrated (open, short and load calibration) and the test fixture compensated (electrical delay) according to the instrument manual before each measurement.
Settings on the network analyzer:
–S11, Chart: Smith Z
–Start frequency: 1MHz
–Stop frequency: above self-resonance frequency of the antenna

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Fig 4. 2-coil antenna

The series equivalent properties as shown in Fig 5 of the antenna have to be measured first, whereas
Rs Equivalent resistance at f = 1MHz
La Equivalent inductance at f = 1MHz
Rp Equivalent resistance at the self-resonance frequency
fres Self-resonance frequency of the antenna
RDC DC resistance

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Fig 5. Parallel equivalent circuit for the 2-coil antenna

Measurement results for the sample antenna:

   LA = 1.2364μH RDC = 189mΩ fres = 23.75MHz RP = 7.1345kΩ    

The parasitic capacitance of the antenna is calculated as follows: RFID Mifare 4K S70 Printing Cards,Mifare Classic 4K cards,NXP Mifare 4k Offset Printing Cards,Mifare 4K Access Control Cards,

   At an operating frequency of 13.56MHz the skin effect has an impact on the system and further calculations. The k-factor, which stands in relation to carrier frequency and selfresonance frequency, is introduced to correct the results.

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Thus, the parallel equivalent resistance is determined to be: Mifare 4k Pre-printed cards,Mifare 4k Printing Cards,Contactless Smart Card,Mifare Classic Cards,Mifare 1K S50 Card,

The Q-factor of the antenna is either calculated by the series equivalent resistance Rs or by the parallel equivalent resistance Rp’. 

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The total series equivalent resistance is calculated by adding the DC resistance to the series resistance RS:  

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 A Q-factor of 78 for the sample antenna is too high for proximity reader applications. A range of 8 to 15 is recommended in order to meet the ISO/IEC 14443-2 (2000) [7] specification and to achieve best results for high data rate operations. Therefore, an additional external damping resistor has to be added:

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NOTE: For a symmetrical antenna the values of the capacitors will double and the values of the resistors and inductors will be divided by two. Hence, the parallel equivalent resistance for one half of the symmetrical antenna is determined as follows:

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Fig 7 shows the matching network for the antenna. It consists of one serial and one parallel capacitor for each branch. The values to be tuned are CM1 and CM2 in order to get defined matching impedances (25Ω for each branch). The values of these components can be estimated according to following equations:

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NOTE: These calculated values for CM1 and CM2 are first order approximations since the measurement of the antenna parameters cannot be done accurately. This is due to the fact that the GND layer on the bottom side of the antenna builds an additional parasitic capacitor which influences the measured values. However, it should help during antenna tuning. The proper values have to be determined manually by testing and measuring.
HINT: Start with the next lower value of the calculation of the conductance and then increase until the desired impedance is achieved.

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Fig 7. Matching network with C1 and C2

Since both amplifier branches are tuned to 25Ω and the antenna is symmetrical, the differential input impedance can be estimated to be 50Ω. More detailed information about antennamatching and design can be found in [6].