DESIGN APPLICATION NOTE --- AN011 SXT-289 Balanced Amplifier Configuration

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1 DESIGN APPLICATION NOTE --- AN11 Abstract Increasing the data rate of communications channels within a fixed bandwidth forces an increase in amplifier linearity. Modulation and coding schemes are often used to provide the increase, but the corresponding system requirements for instantaneous amplifier bandwidth and linearity have previously been hard to achieve. Stanford Microdevices high efficiency GaAs Heterojunction Bipolar Transistor (HBT) MMICs offer the linearity and output power needed to satisfy these requirements. Combining two GaAs HBT amplifiers in a balanced configuration will improve the output third order intercept performance and 1 compression point each by 3, and provide redundancy so the system will continue to operate if a device fails. This application note will provide information about using the SXT- 289 amplifiers in a balanced configuration. Figure 1: SXT-289 Balanced Circuit Schematic Introduction The balanced amplifier configuration is a practical method for obtaining a broadband amplifier with output power equal to twice that of a single amplifier. The output third order intercept performance is also improved by 3. The arrangement of the balanced amplifiers is shown in Figure 1. Two 3-, 9 o hybrid couplers are used. The input coupler divides the input power equally between the two output ports. The output coupler recombines the output signals from the amplifiers. The reflected signals from the amplifier s input and output due to mismatch are coupled to 5 Ohm loads, thereby achieving excellent input and output return loss. The linearity performance is limited by the P1 or TOI match inside the couplers as well as the bandwidth of the coupler. Some other advantages of a balanced amplifier are: Redundancy - if one of the amplifier stages fails, the balanced amplifier will still operate with reduced gain and linearity. Input and output VSWR dependent on the coupler. Individual amplifiers can be matched for specific parameters - IP3, P1, noise figure, etc. Easily cascaded - each unit is isolated by the coupler. High degree of stability. SXT-289 Z=5W, pf RF IN SMT 9 Coupler Anaren 1W pf 59.1F 1pF 59 RF OU.1 F 1pF 18pF SMT 9 Coupler Anaren 1W Z=5, SXT pf The information provided herein is believed to be reliable at press time. Stanford Microdevices assumes no responsibility for inaccuracies or omissions. Stanford Microdevices assumes no responsibility for the use of this information, and all such information shall be entirely at the user s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. Stanford Microdevices does not authorize or warrant any Stanford Microdevices product for use in lifesupport devices and/or systems. Copyright 2 Stanford Microdevices, Inc. All worldwide rights reserved. 1

2 SXT-289 Application Circuit The SXT-289 is a high efficiency GaAs Heterojunction Bipolar Transistor (HBT) MMIC. These amplifiers are designed for use as driver devices for infrastructure equipment in the MHz cellular, ISM, MDS, WLL, PCS and Wideband CDMA applications. A summary of measured performance on a single amplifier and balanced amplifier is shown in Table 1. It is tuned for output IP3 performance at 2.2 GHz. The balanced amplifier circuit has approximately 3 improvement in P1 and IP3 over the single amplifier over the band of the coupler. DESIGN APPLICATION NOTE --- AN11 The performance is extended in the balanced amplifier configuration to the bandwidth of the coupler. Over the GHz frequency band of the coupler: P1 is greater than 26 m Output IP3 is greater than 43 m Input/Output Return Loss greater than 18. The S-parameters, Output IP3, and P1 for the single vs. balanced circuit are shown on the following page. Table 1: Summary for Single and Amplifier Symbol Parameters: Test Conditions: Z = 5 Ohms, T = 25 C = 11 ma /stage, V = 5V D I D Units Single SXT Typ. Balanced SXT Typ. P 1 Output Power at 1 Compression m m m m m IP3 Third Order Intercept Point Tone Spacing = 1MHz, Power out per tone =1 m m m m m m S 21 Small Signal Gain S 11 Input Return Loss S 22 Output Return Loss NF Noise Figure ( Z = 5 Ohms) s

3 DESIGN APPLICATION NOTE --- AN11 SXT-289 Application Performance Data, I CC = 11 ma/stage, T = 25 C, R bias = 27 Ohm, V S = 8V 25 S21 () S12 () S11 () S22 () IP3 (m) 45 4 P1 (m)

4 PCB Pad Layout The PCB pad layout for the balanced circuit configuration is shown below. Fifty ohm transmission lines are used to connect all of the RF ports. It is critical to maintain identical electrical phase lengths of the paths for both amplifier stages. The SXT-289 requires a shunt capacitor at the output (C match in Figure 2) to tune for optimal output IP3 performance. The DC blocking capacitors are chosen to be very low impedance at the application frequency band. The chokes are chosen to be high impedance at the operating frequency. The bias circuit uses a 39 Ohm feedback resistor (Rb1) and a 18 Ohm (Rb2) resistor from input to ground to provide consistent DC performance over Figure 2: Evaluation Board Layout DESIGN APPLICATION NOTE --- AN11 process variation and operating temperature. An RF choke is used to isolate the resistor network from the RF input so gain and noise figure are not degraded. Good RF gounding is another important consideration in the PCB layout. It is critical to place plated vias directly under the ground leads of the SXT-289 device to minimize ground inductance and for heat dissipation. The surface mount hybrid couplers also require a ground plane with a dense population of plated vias under the part to insure proper electrical performance. The coupler s termination resistor should be placed as close as practically possible to the hybrid as shown below. Cb1 RF IN Rb1 Rb2 Cb2 Cb3 Rload X1 IC1 Cmatch X1 Rload Rb2 Cmatch STANFORD MICRODEVICES SOT-89 Bal. Eval Board ECB- Rb1 Cb3 Cb2 Cb1 RF OUT Notes: Scale: approx. 2:1 Board Material: GETEK ML2C Dielectric Thickness: 31 mils Copper Cladding: 1 oz. (both sides) Plating: tin/lead (both sides) 4

5 DESIGN APPLICATION NOTE --- AN11 Parts List (SXT-289 Balanced Circuit) Qty Ref. Des. Part No. Manufacturer Description Package 2 IC1 SXT-289 Stanford Microdevices GaAs HBT Amplifier SOT-89 2 X1 1W135-3 Anaren Hybrid Coupler- 3, 9 SMT 2 Cmatch ROHM.5 pf Capacitor 63 4 ROHM 39 pf Capacitor 63 2 Cb1 ROHM.1 uf Tantalum Cap Size A 2 Cb2 ROHM 1 pf Capacitor 63 2 Cb3 ROHM 18 pf Capacitor 63 2 Rb1 ROHM 39 Ohm Resistor 63 2 Rb2 ROHM 18 Ohm Resistor 63 2 Rload ROHM 51 Ohm Resistor 63 2 ROHM 27 Ohm Resistor 21 4 TOKO Inductor 63 Conclusion This application note has shown a practical method for obtaining a broadband amplifier with 3 improvement in both 1 compression and output third order intercept performance. By combining two identical amplifiers in a balanced configuration, the bandwidth of the amplifier is extended to the frequency band of the hybrid couplers. The input and output VSWR of the amplifier is also improved by its dependence on the coupler. This technique can be applied to other Stanford Microdevices amplifiers and transistors to obtain similar improvements in bandwidth, VSWR, P1 and output IP3 performance. We hope that this application note and the devices offered by Stanford Microdevices will assist you in achieving your design goals. 5