AN1229
Application note
SD2932 RF MOSFET for 300 W FM amplifier
Introduction
This application note gives a description of a broadband power amplifier operating over the
frequency range 88 - 108 MHz using the new STMicroelectronics RF MOSFET transistor
SD2932.
Table 1. Typical achievable performances
Parameter Performance
Device 1 X SD2932
Frequency 88-108 MHz
Vdd 50 V
Idq 200 mA
Pout 300 W
Gain >19 dB
Input return loss < -11 dB
Drain efficiency >70%
July 2007 Rev 3 1/7
www.st.com
Amplifier design AN1229
1 Amplifier design
1.1 Input matching network
Typical input gate-to-gate impedance of SD2932 at 100 MHz is Zin = Rs + jXs = 2 - 2.6 j,
and can also be expressed as the combination of parallel resistance and reactance using
the following formulas:
Equation 1
2
Xs
⎛⎞
⁄ 4.14jΩ–==
P
-------
+• 5.38Ω==
⎝⎠
Rs
X
S
⎛⎞
-------
⎝⎠
R
S
Rp Rs 1
Equation 2
XPR
Therefore, in order to achieve good input matching performances over the frequency range
88-108 MHz the unbalanced 50 Ω is to be transformed into an impedance with a value as
close as possible to Rp of 5.38 Ω.
From the circuit schematic given in Figure 6 , we can see that the input matching network is
based on a two section balun (1:1 balun in cascade with a 9:1 balun transformer) which
transforms the unbalanced 50 Ω to a balanced 5.56 Ω (2 x 2.78 Ω / 9:1 ratio). The first
section, a 5" long - 50 Ω coaxial cable and the second section, a two 3.9" long - 25 Ω flexible
coaxial cables with ferrite core NEOSIDE, are connected as described: a 10 nH inductor
(L1) is connected between the two gates to compensate SD2932 input parallel reactance
Xp.
1.2 Input matching network tuning
Figure 1. Input Impedance of 1:1 balun in cascade with 4:1 balun
-10
-15
-20
S11 (dB)
-25
-30
-35
0 50 100 150 200 250 300 350
Frequency (MHz)
2/7
AN1229 Amplifier design
Figure 2. Input Impedance of 1:1 balun in cascade with 9:1 balun
0
-5
-10
-15
-20
S11 (dB)
-25
-30
-35
0 50 100 150 200 250 300 350
Frequency (MHz)
SD2932 input matching network was tuned in order to achieve the best compromise in
terms of power gain (Gp) and input return loss (Rtl) over the frequency range 88 - 108 MHz.
Best results were achieved by adding a 10 pF chip capacitor (C1) between RFIN and the
1 nF blocking capacitor (C2).
1.3 Output matching network
The output impedance of each side is a combination of the output capacitance Coss (
195 pF) and the optimum load resistance which can be determined as follows:
Equation 3
0.85 Vd d•()
------------------------------------ -
Rp
2 Pout•
2
0.85 2500•
------------------------------ -
2 150W•
6.02Ω===
The total optimum load, seen by SD2932 (drain to drain), is 2 x 6.02 = 12.04 Ω. Therefore, a
simple two section balun (1:1 balun in cascade with a 4:1 balun transformer) is used to
transform the unbalanced 50 Ω to a balanced 12.5 Ω (2 x 6.25 Ω) which is very near to the
total optimum load resistance.
The first section, a 5" long - 50 Ω flexible coaxial cable, and the second section, two 5" long
- 25 Ω flexible coaxial cables, are connected as described in Figure 6.
To compensate for the output capacitance Coss of SD2932 , a 40 nH inductor (L2) is
connected between the two drains. This LC network (L2 & Coss) is a high pass filter with a
resonance frequency calculated at 10 % below the minimum operating frequency:
Equation 4
C
OSS
C
-------------- -=
OSS
2
(per side)
180p F
-----------------
2
90pF==
Equation 5
Frequency of resonance
0.9 88MHz 80MHz=•=
Equation 6
L2 Coss 2 pi F••()••
2
1 L2 44nH=→=
3/7
Amplifier design AN1229
Figure 3. Power gain vs. frequency
22
21
20
19
Power Gain (dB)
18
17
85 90 95 100 105 110
Pout = 300W Vdd = 50V Idq = 200 mA
Frequency (MHz)
Figure 4. Drain efficiency vs. frequency
80
78
76
74
72
70
68
66
Drain Efficiency (%)
64
62
60
85 90 95 100 105 110
Pout = 300W Vdd = 50V Idq = 200 mA
Frequency (MHz)
Figure 5. Drain efficiency vs. frequency
0
-2
-4
-6
-8
-10
-12
-14
-16
Return Loss - Rtl (dB)
-18
-20
85 90 95 100 105 110
4/7
Pout = 300W Vdd = 50V Idq = 200 mA
Frequency (MHz)
AN1229 SD2932 typical performances and conclusion
2 SD2932 typical performances and conclusion
Figure 3, Figure 4 and Figure 5 show power gain, efficiency and input return loss over the
frequency range 88 - 108 MHz at a constant output power of 300 W and a drain supply
voltage of 50 V and a quiescent current of 200 mA. Typical performances are as follows:
Table 2. Typical performances
Parameters Min Max
Gp 19.3 dB 19.6 dB
R
-18 dB -11 dB
TL
Eff 71% 73%
Finally, in this report we have demonstrated ST’s SD2932 MOSFET transistor excellent
performance as a wideband 300 W - 50 V push-pull amplifier for FM applications.
Figure 6. 88-108 MHz circuit schematic
Table 3. 88-108 MHz circuit components list
Symbol Description
PCB 1/32” Woven fiberglass 0.0030 Cu, 2 side, er
T1 50 Ω Flexible coax cable OD 0.006”, 5” long
T2/ T3 9:1 Transformer, 25 Ω flexible coax cable OD 0.1” 3.9”. ferrite core NEOSIDE
T4 / T5 4:1 Transformer, 25 Ω flexible coax cable OD 0.1” 5.0” long.
T6 50 Ω flexible coax cable OD 0.1” 5.0” long.
FB1 VK200
C1 10 pf Ceramic capacitor
C2/C3/C4/C7/C8 1 nF Chip capacitor
C5/C6 1 nF ATC chip capacitor
5/7
Revision history AN1229
Table 3. 88-108 MHz circuit components list (continued)
Symbol Description
C9 470 pF ATC chip capacitor
C10 100 nF chip capacitor
R1 56 Ω Resistor
R2/R4 10 Ω Chip resistor
R3 10 K Ω Resistor
R5 5.6 K Ω Resistor
R6 10K Ω 10 Turn trim resistor
R7 3.3 K Ω/ 1 W Resistor
R8 15 Ω/ 1 W Resistor
D1 6.8 V Zener diode
L1 10 nH inductor
L2 40 nH inductor
L3 70 nH inductor
3 Revision history
Table 4. Revision history
Date Revision Changes
21-Jun-2006 2 Minor text changes
30-Jul-2007 3 The document has been reformatted
6/7
AN1229
Please Read Carefully:
Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the
right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any
time, without notice.
All ST products are sold pursuant to ST’s terms and conditions of sale.
Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no
liability whatsoever relating to the choice, selection or use of the ST products and services described herein.
No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this
document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products
or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such
third party products or services or any intellectual property contained therein.
UNLESS OTHERWISE SET FORTH IN ST’S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED
WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS
OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT.
UNLESS EXPRESSLY APPROVED IN WRITING BY AN AUTHORIZED ST REPRESENTATIVE, ST PRODUCTS ARE NOT
RECOMMENDED, AUTHORIZED OR WARRANTED FOR USE IN MILITARY, AIR CRAFT, SPACE, LIFE SAVING, OR LIFE SUSTAINING
APPLICATIONS, NOR IN PRODUCTS OR SYSTEMS WHERE FAILURE OR MALFUNCTION MAY RESULT IN PERSONAL INJURY,
DEATH, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE. ST PRODUCTS WHICH ARE NOT SPECIFIED AS "AUTOMOTIVE
GRADE" MAY ONLY BE USED IN AUTOMOTIVE APPLICATIONS AT USER’S OWN RISK.
Resale of ST products with provisions different from the statements and/or technical features set forth in this document shall immediately void
any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever, any
liability of ST.
ST and the ST logo are trademarks or registered trademarks of ST in various countries.
Information in this document supersedes and replaces all information previously supplied.
The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners.
© 2007 STMicroelectronics - All rights reserved
STMicroelectronics group of companies
Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan -
Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America
www.st.com
7/7
Loading...