Singel State Transistor

SIMPLE PCB BASED S-PARAMETER EXTRACTION METHOD FOR RF AMPLIFIER CIRCUITS S. C. Choi, J. E. Youm and S. W. Hwang Member Dept. of Electronic and Computer Engineering, Korea University Anam, Sungbuk, Seoul 136-701, Korea Phone: +82-2-927-6114, Fax: +82-2-927-6114 Email: [email protected]

Keywords: S-parameters, de-embedding, printed circuit board (PCB), RF amplifier Abstract A new, simple S-parameter extraction method of RF transistor and amplifier is proposed and demonstrated. The method is based on one-step layout and fabrication of a single reference PCB and the complicated extra calibration kit is not needed. It is shown that the S-parameters of the transistor can be accurately extracted by using the measurement results of the reference board with the transistor and the equivalent circuits of the input and the output stage. More importantly, it is also shown that the measurement results of the reference board without the transistor can be replaced by full software-based equivalent circuit modeling.

I. Introduction Vendors distributing commercial RF amplifiers and transistors do not usually open the information about the internal design structure and the package. Since the evaluation board including matching circuits is presented only at the same bias and frequency given in the datasheet, the given information does not help RF engineers design the systems at different biases or frequencies. In other words, totally new parameters such as the Sparameters, the noise figure, and other nonlinear parameters, under the changed condition, should be extracted and used in design. Various methods to extract the S-parameters have been introduced [1-3]. However, it is hard for field engineers to use those methods, since they are composed of the combinations of complex equations and the extra calibration kit such as TRL or SOLT. These calibration kits are accurate but very hard to use. We propose and demonstrate a simple PCB-based technique for the extraction of the S-parameters of an unknown transistor amplifier. A simple PCB layout for the transistor is made. Two PCBs with the same layout are fabricated. One is used as a reference board with a 100 Ω resistor mounted on the position of the transistor. The other is the board with the transistor. The former is used to verify if the equivalent circuit model of the input and the output stage is accurate, and the later is used for extraction of the Sparameters of a transistor. The extracted S-parameters are shown to predict the performance of a two-stage amplifier circuit consisting of two transistors. One important thing is that the measured results of the reference board with a 100 Ω resistor match with the results calculated from the model parameters of the transmission line and

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passive components [5]. This suggests that the measurement of the reference board with a100 Ω resistor can be skipped and complete software-based board analysis can be used in the parameter extraction.

II. Extraction of the S-parameters of an Unknown Transistor Figure 1 shows the layout of a PCB for the extraction of the S-parameters of an unknown transistor. The board is composed of three sections: the input stage, the amplification stage, and the output stage. Figure 2 shows the flow chart of our parameter extraction process. In step 1, the PCB layout shown in Fig.1 is constructed. This board includes a T-type matching block and bias circuits as well as a transistor. In step 2, the equivalent circuit of the input and the output stage is obtained. The structure of the input stage of the PCB used in this paper is symmetrical with that of the output stage and the analysis of only the input stage is performed. Figure 3 shows the circuit of the input stage used in the board Fig. 1. In step 3, the reference board in Fig. 1 including the input and the output stage is fabricated and measured to verify if the equivalent circuit model in step 2 is accurate. The S-parameters are obtained by 2-port measurement after connecting the gate pad and the drain pad with a resistor (100 Ω). Figure 4 shows the return loss (S11) and the transmission loss (S21) of the reference board. The difference between the simulation results and the measured results is less than 10-4 (–40 dB) in magnitude and than ±15° in phase. This suggests that this step 3 can be omitted in our parameter extraction process and the results fully based on the equivalent circuit can be utilized. In step 4, 2-port S-parameters of each section are calculated and measured. The Sparameters of the input and the output stage are calculated from the equivalent circuit of which the accuracy has been verified in step 3. The S-parameters of the board with the transistor are obtained by full 2-port measurement. In step 5, all the S-parameters are converted to the ABCD matrices. Then the ABCD matrix of the transistor can be extracted by the following formula. Here, [ABCD_tr], [ABCD_in], [ABCD_out ], and [ABCD_full] are the ABCD matrices of the transistor, the input stage, the output stage, and the board with the transistor, respectively. [ABCD_tr] = [ABCD_in]-1 [ABCD_full] [ABCD_out ]-1 Figure 5 represents the S-parameters of the ATF-34143 transistor [4] extracted by using the proposed method and by using the transistor model given by the vendors. The gate bias in the transistor model is adjusted until the DC current-voltage (I-V) characteristic of the transistor model is best matched with the measured I-V of the actual transistor.

III. Design of Multi-stage Amplifiers Using the Extracted Parameters First of all, a single stage amplifier using ATF-34143 is designed by using the extracted parameters. The frequency of the circuit is arbitrarily chosen (1.6 GHz). Figure 6 shows the S-parameters of the circuit. The measured results are in almost agreement with the results calculated by using the extracted parameters within maximum 3 dB for S11 and ±0.3 dB for S21 and S12, except for S22.

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Secondly, a 2-stage amplifier is designed again by using the extracted S-parameters of the transistor. Figure 7 shows that the measured results are in agreement with the calculated results within maximum 2 dB for all the S-parameters. A similar parameter extraction is performed for the commercial amplifier (AH1) [6] to demonstrate that our method can be applied quite generally. Matching is performed at the frequency of 1.55 GHz. At this frequency, matching circuit is not described in the datasheet. Figure 8 shows that the measured S-parameters of the circuit are again in agreement with the calculated results except a little shift of the matching frequency.

IV. Conclusions We have proposed and demonstrated a new, simple S-parameter extraction method of RF transistor amplifier. The method is based on one-step layout and fabrication of a single reference PCB and no complicated extra calibration kit is needed. The Sparameters of the transistor have been shown to be accurately extracted by using the measurement results of the reference board with the transistor and the equivalent circuits of the input and the output stage. More importantly, it has been also shown that the measured results of the reference board without the transistor can be replaced by full software-based equivalent circuit modeling.

Acknowledgement This work is supported by the Brain Korea 21 project in 2004.

References [1] Hanjin Cho, Dorothea E. Burk, “A three-step method for the de-embedding of highfrequency S-parameter measurements”, IEEE Transactions on Electron Devices, Vol. 38 , pp. 1371 - 1375 , 1991. [2] E. P. Vandamme, D.M.M.-P Schreurs, C. Van Dinther, “Improved three-step deembedding method to accurately account for the influence of pad parasitics in silicon on-wafer RF test-structures”, IEEE Transactions on Electron Devices, Vol. 48, pp.737 – 742, 2001. [3] F.M. Ghannouchi, A. Brodeur, F. Beauregard, “A de-embedding technique for reflection-based S-parameters measurements of microwave devices”, Conference on Precision Electromagnetic Measurements, Digest, pp.119 – 120, 1994. [4] Agilent, Inc., ATF-34143. [Online], Available: “http://www.semiconductor.agilent. com/spg/doc/wirelessDesignTools/atf34143.zap” [5] S-parameters of the passive components are provided by Murata, Inc., [Online], Available: http://www.murata.com/designlib/mcsil.html [6] WJ Communications, [Online], Available: http://www.wj.com/rf_com/ponents/ amplifiers/AH1.asp

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Input

stage

output stage

Transistor

Figure 1 PCB layout used to extract the s-parameters of a transistor

1. PCB layout of the reference board

2.Equivalent circuit modeling of the input and the output stage

3. Verification of the accuracy of the input and the output circuit by fabrication of the reference board (optional)

4. Fabrication and measurement of the reference board with the transistor and then extraction of all the S-parameters of each section

5. ABCD matrices conversion and calculation

Figure 2 Flow chart describing the parameter extraction procedure

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RF in

MLOC TL14

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Port MLIN P1 TL3 Num=1

MLOC TL15

MLOC TL16

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Re f

S2P SNP1

1

MLIN TL7

MLIN TL8

MLOC TL17

To gate

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Re f

MLIN TL9

S2P SNP2

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MLIN S2P TL10 SNP7

MTAPER Taper1

Port P2 Num=2

Ref

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MLIN TL11

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2

Re f

S2P SNP3

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MLIN TL12

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Ref

S2P SNP11 Port P3

Vgg Figure 3 ADS equivalent circuit of the input stage

: simulated results × : measured results

(b)

S(4,3) S(2,1)

|S| = 1

S(3,3) S(1,1)

(a)

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

freq (500.0MHz to 3.000GHz)

freq (500.0MHz to 3.000GHz)

Figure 4 (a) Return loss (S11) and (b) transmission loss (S21) of the reference board of Fig. 1 when a 100 Ω resistor is mounted on the pads for the transistor.

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: The proposed method × : ADS model |S| = 1

(a)

(b)

S(4,3) S(2,1)

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-4

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S(4,4) S(3,3) S(2,2) S(1,1)

S22

S11

freq (500.0MHz to 3.000GHz)

freq (500.0MHz to 3.000GHz)

Figure 5 (a) Smith chart of S11 and S22, (b) Polar plot of S21

dB(S(4,3)) dB(S(2,1))

dB(S(4,4)) dB(S(2,2))

-5 -10 -15 -20 -25 0.5

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dB(S(3,4)) dB(S(1,2))

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freq, GHz

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freq, GHz

Figure 6 S-parameters of a single stage amplifier fabricated using the ATF-34143: (a) S11, (b) S22, (c) S21, and (d) S12

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(a)

(b)

0 -10 -15 -20 -25 0.5

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freq, GHz 30

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dB(S(3,4)) dB(S(1,2))

dB(S(4,3)) dB(S(2,1))

(c)

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dB(S(4,4)) dB(S(2,2))

dB(S(3,3)) dB(S(1,1))

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: simulated results × : measured results

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2.0

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Figure 7 S-parameters of a two-stage amplifier fabricated using the ATF-34143: (a) S11, (b) S22, (c) S21, and (d) S12

(b) -5

-10

dB(S(4,4)) dB(S(2,2))

dB(S(3,3)) dB(S(1,1))

(a) 0 -20 -30 -40 0.5

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(c)15 10 5 0 -5 0.5

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(d)-20 dB(S(3,4)) dB(S(1,2))

dB(S(4,3)) dB(S(2,1))

1.5

-10 -15 -20 -25 -30 -35 0.5

: simulated results × : measured results

1.0

1.5

2.0

freq, GHz

Figure 8 S-parameters of an amplifier fabricated using AH1: (a) S11, (b) S22, (c) S21, and (d) S12

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