The Method of Measuring Large-Signal S-Parameters of High Power Transistor With Normal Condition

Transcription

1 The Method of Measuring Large-Signal S-Parameters of High Power Transistor With Normal Condition Ung Hee Park*, Seok Kyun Park**, Ik Soo Chang ** * FTRI, ** Sogang university Abstract In this paper, a measurement method for scattering parameters (S-parameters) of microwave high-power transistor with normal condition is presented. In an existing measurement method for S 12 and S 22, they are respectively measured by reflection coefficient at collector port and transmission coefficient at base port when input signal is injected into collector port. They are incorrect value because transistor is abnormally operating. A proposed method in this paper directly finds out S-parameters of high-power transistors measuring overall gain, phase and S-parameters of input and output matching network when transistor operates normally. Using this method, we obtained S-parameters for MRF581 of Motorola for small signal amplifier transistor and a commercial high power transistor MRF6402 of Motorola, respectively. I. Introduction High power amplifiers (HPAs) are very important portion in wireless communication systems. To obtain the optimum output performance, the characteristics of high power amplifiers such as scattering parameters (S-parameters) or load impedance with which we can get optimum output power are essential. In the case of designing amplifiers for a small signal amplification, S-parameters are useful and sufficient one to design amplifiers [1]. For a design of large signal amplifiers, it is not useful parameters any more, because the method of measuring S-parameter has some problem to cause it to make uncertain S-parameters for high power transistor. That is, S 11 and S 21 of S-parameters are respectively measured by reflection coefficient at base port and transmission coefficient at collector port when input signal is injected into base port. They are correct value because transistor is normally operating. But S 12 and S 22 of S-parameters are respectively measured by reflection coefficient at collector port and transmission coefficient at base port when input signal is injected into collector port..[2][3] In this paper, a measurement method of S-parameters of microwave high-power transistor with normal condition using port reduction technique is presented. This -1-

2 method finds out S-parameters of high-power transistors by overall gain, overall phase of the measurement system and S-parameters of input and output matching network when transistor operates normally unlike an existing method which measures S 12 and S 22 when transistor operates abnormally. The used transistors to show the validity of this method were MRF581 of Motorola for small signal amplifier and MRF6402 of Motorola for high power transistor, respectively. MRF581 was measured at 1000Mhz and biased in class A and MRF6402 was measured at 1855Mhz and was biased in class AB II. The Theory of Measuring Method of S-parameters The principle of measuring S-parameters with normal condition using port reduction technique can be derived like below. As an illustration, consider the block diagram of Fig. 1. Where [S T ] is the S-parameter matrix of DUT(transistor), [S i ] and [S o ] are S-parameter matrix of input matching network and output matching network respectively. The S-parameter matrix of 4-port network whose boundary is dashed line is given by (2.1) If we partition this 4 x4 matrix into 2x2matrix; where, (2.2) (2.3) The S-parameter matrix of TR is represented by -2-

3 Fig. 1 The block diagram for measuring S-parameters (2.4) Which reduced to (2.5) From the equation (2.5) and (2.2), we obtain Using this result in (2.6) gives (2.6) Substituting (2.7) in (2.2) gives (2.7) (2.8) Therefore, the overall S-parameter between port 1 and port 2, S all is given by (2.9) The symbols, S all and S -1 T, in (2.9) mean that (2.10) From (2.11) and (2.3), (2.11) -3-

4 (2.12) where, Using (2.12) and (2.3) gives (2.13) From (2.9) (2.14) Equating S 21 on both sides in (2.14) gives (2.15) By using (2.15), we can yield the equation (2.16). (2.16) Therefore, if we know four different values of overall S 21all and S-parameter of matching network for the S 21all, then we can make a new matrix whose parameters are S 11t, S 21t, S 22t, as shown in equation (2.17). We can solve the final equation (2.18) without difficulty, knowing S-parameters of matching network (S 21i, S 22i, S 11o, -4-

5 S 21o) and overall network including matching network (S 21all). Finally, we can find out the S-parameters of DUT, S 11t,S 21t,S 12t and S 22t (2.17) From (2.17), (2.18) III. Measurement Method Fig.2 simplified structure used as a measurement system for MRF6402 Fig.2 shows simplified structure of the measurement system. As you can see, drive amplifiers need to drive high power amplifier. Output power of drive amplifier is -5-

6 27dBm. Output power of DUT, however, is too high to measure S-parameters directly, so we used directional coupler (30dB) at output port. In case of small signal amplifier, we can measure the S-parameters directly with network analyzer. Circulator was used to prevent drive amplifier from being affected by reflected signal from DUT while it is tuned and to check if the output impedance of drive amplifier is 50 ohm. To be concrete, we can know the output impedance indirectly by measuring the output power of port 3 terminating port 2 with 50 ohm. That is, the bigger the difference between output impedance of drive amplifier (Z 0 ) and 50 ohm is, the more the output power reflected from port 2 will be. Tuner was composed of 3-dB directional coupler and variable sliding short. It is possible for this tuner to realize every point of smith chart. The measurement process is like this. First, S 21a11 of overall network that includes only input matching and output matching network in Fig.2 can be obtained by compensating S-parameter of drive amplifier and output coupler. And then S-parameter S 21i, S 22i, S 11o, S 21o of the matching network must be measured. We should repeat this procedure four times for four different S 21a11. With this data, we can obtain the required S-parameter of DUT solving matrix equation of (2.18). IV. Experimental Results In order to illustrate the capabilities of our system, we present experimental results obtained from the MRF581 of Motorola for small signal amplifier at fo=1000mhz in class A and the high power transistor (Motorola MRF6402: BJT) at fo=1855mhz. It's typical output power is 4.5W. It biased in class AB (V CE =26V, V BE =0.78V, IcQ=40mA). Table 1 shows the measured S 21i, S 22i, S 11o, S 21o, and S 21a11 of matching network for MRF581. Table.2 shows the S-parameter of MRF581 obtained solving the equation (2.18). To show the validity of the S-parameters in table 2, we compared the S-parameters in table 2 with S-parameters of MRF581measured with network analyzer. There are the measured S-Parameters in Table 3. You can see the measured S 21i, S 22i, S 11o, S 21o and S 21a11 of matching network for MRF6402. Table.5 shows the S-parameter of MRF 6402 obtained solving the equation (2.18). To verify the validity of the S-parameters in table 5, we compare the gain characteristic of amplifier by matching network using tuner with that of amplifier by matching network using the obtained S-parameters in table.5. Fig. 3 shows the characteristic of amplifier by matching network of tuner. Fig.4 shows the characteristic of amplifier by matching network using the obtained S-parameters. As you can see, there was only 0.8dB difference between two data. When designing the amplifier, we used the design technique using constant gain circle of small signal amplifier. As the constant gain contour of high power transistor is not a -6-

7 circle, the difference between Fig.3 and Fig.4 is an allowable error [4]. V. Conclusions In this paper, a novel measurement method of scattering parameters (S-parameters) for large-signal active device in normal operating condition is proposed. Fig.3 The characteristics of amplifier using tuner matching network Fig.4 The characteristics of amplifier by matching circuit using obtained S-parameter -7-

8 Table 1. Measured S-parameter of matching network for MRF581 Table 2. Measured S-parameters of MRF581 using a proposed method in this paper Table 3. S-parameters of MRF581measured with network analyzer Table 4. Measured S-parameter of matching network for MRF6402 Table 5. Obtained S-Parameter of MRF6402 using the method proposed in this -8-

9 paper (V CE =26V,V BE =0.78V I CQ =40mA in class AB bias operation) Usually, S 12 and S 22 of S-parameters are respectively measured with network analyzer by reflection coefficient at collector port and transmission coefficient at base port when input signal is injected into collector port. This is a setup which don't make transistor operate normally. So S 12 and S 22 aren't correct value. But the method proposed in this paper only use the measured data while transistor operates normally. Using this method, we obtained the S-parameter of MRF581 and MRF6402 of Motorola respectively biased in class A and class AB and verified the validity of this method respectively comparing the S-parameters obtained from the proposed method in this paper with S-parameters measured with network analyzer and comparing the gain characteristic of amplifier by matching network using tuner with that of amplifier by matching network using the obtained S-parameters. Reference [1] Guillermo Gonzalez, "Microwave Transistor Amplifiers Analysis and Design", New Jersey, Prentice Hall, pp , 1997 [2] S.R.Mazumder, "Two-Signal Method of Measuring the Large-Signal S-parameters of Transistor", IEEE Trans. Microwave and Tech., Vol. MTT-26, No.6, pp , June 1978 [3] Van Valkenburg, "Reference Data for Engineers: Radio, Electronics, Computer Communications", SAMS Publishing, 8th, pp 12-17, 1995 [4] Guillermo Gonzalez, "Microwave Transistor Amplifiers Analysis and Design", New Jersey, Prentice Hall, pp ,