RFID high frequency amplifier circuit design

Summary: analysis of the RFID high frequency amplifier circuit, on the basis of a new type of high-frequency amplifier circuit, and his works.

Keywords: RFID high frequency amplifier circuit;; design; resonant circuit

Radio frequency identification technology is in the early 1980s, developed an advanced recognition technologies, after more than ten years of development, has in all walks of life, especially the electronic information industry has been widely used.

Radio frequency identification is a non-contact automatic identification technology, automatic identification by radio frequency signal target object and gets the correlation data, identify work without manual intervention, can work in all kinds of bad environment. Radio frequency identification system by readers and transponders (tags). When his work, the reader through an antenna to send out a certain frequency RF signal, when the label into the magnetic field generated induced current gain energy, sends out its own coding and other information be reader read and decoded to a computer for processing [1]. HF power amplifier is a key part of the reader's main function is to label signal return signal power amplification.

1 works

Figure 1 circuit for radio frequency identification of high-frequency power amplifier schematic diagrams.

13.56 MHz input square-wave signal amplification by the power amplifier outputs a square wave signal, after impedance transformation network part in antenna load to generate high frequency output AC voltage, launched from the antenna. The other part by detection circuits demodulation out useful signal output [2].

　　

　　

Figure 2 is a high-frequency power amplifier circuit.

Various parameters are as follows:

　　

　　

　　

　　

2 cell circuit design

(1) select r amplifier as shown in Figure 3.

High-frequency resonant power amplifier circuit can work in A class, a class b or C class status.

Compared to C class resonance amplifier distortion, although not as A class and class b, class c applies to the input signal is relatively large, output power, high efficiency, energy conservation, therefore, high-power RF power amplifier circuits are often used [3].

Specific parameters are as follows:

① determine optimal load: power amplifiers

Set the saturation voltage transistors to 1 V, then:

　　

　　

② HF choke coil inductance calculation, choke coil inductance should be much greater than the equivalent load amplifier, take:

　　

　　

　　

　　

Collector-emitter Breakdown voltage URCEO ≥ 2VCC, namely:

URCEO≥18 V

So choose triode 3DA106A type.

　　

　　

(2) impedance transformation network as shown in Figure 4.

　　

　　

Selection of impedance transformation network mainly has two roles:

① filter to filter out high frequency pulse effect can the harmonic components just output the required signal frequency voltage and power.

② impedance matching role through oscillation circuit impedance adjustment allows the oscillation circuit rendered high-frequency power to get the best impedance values, so that high-frequency power amplifier with high efficiency output maximum power [4].

Through parallel resonant circuit realization L1C1, selected frequency filtering, LC resonant circuit working frequency, bandwidth is relatively narrow, usually selected frequency amplifier band Δ f and center frequency f0 ratio from a few marginally to percent, from Δ f/f 0 = 1%, then:

　　

　　

The corresponding quality factors:

　　

　　

Because through the load current is on RL:

　　

　　

Secondly consider impedance converter adopts the high-pass network will be 50 Ω load transform best for amplifier required load 10.7 Ω, then:

　　

　　

The complete circuit is the inductance L in L1 and L2 parallel total inductance

　　

　　

　　

　　

Meet the back to the time constant.

　　

　　

3 resonance circuit adjust and test methods

Resonant circuit adjustment has 2 methods: point by point and sweep method [5].

This article takes a point-by-point method for high frequency signal generator for signal sources, oscilloscopes or voltmeter, direct wiring test instrument. Test circuit shown in Figure 6.

　　

lign=center>

　　

The signal generator output frequency placed fi = 13.56 MHz output Vi = 15 V, variable capacitor C2, so that the loop occur resonance, i.e. UHF millivoltmeter instructions for maximum value, the loop is in resonance State, you can debug the resonant frequency.

Amplitude-frequency characteristic of debugging method for:

When the center frequency adjustment, you can test the resonant circuit of frequency characteristics, keeping the input signal, in the resonance frequency f0-point change in both sides, with a frequency signal oscilloscope or high frequency millivoltmeter detect the corresponding output voltage U0, calculated points of arbitrary frequency resonant function of units under that resonance curve, you can find on the curve from a 2 Δ f 0 £ ® 7 and 2 Δ f 0 £ ® 1.

If 2 Δ f 0 £ ® 7 narrow, you can adjust the damping resistor R make it smaller, thus increasing the input loss to Δ f 0 £ ® 7 variable width.

Reference documents

［1］Joseph J Carr.

RF circuit design[M]. McGraw-Hill　Companies，Inc．NewYork，2001．

[2] Zhang XING. RF analog circuit [M] £ ® Beijing: electronic industry press, 2002.

[3] Wu Sau Ling. high frequency circuits [M] Xi'an: Xidian University Press, 1999.

[4] Shen Wei Chi. high frequency circuit [M] Xi'an: Xidian University Press, 2002.

[5] has hung-Wen, high frequency circuit theory and analysis [M] Xi'an: Xidian University Press, 2002.