APPLICATION NOTE Gain Block Bias Networks Introduction Skyworks gain block amplifiers are InGaP/GaAs HBT integrated circuits. They use a Darlington-pair transistor configuration with bias and feedback resistors properly selected to determine the gain, input and output impedances and bias parameters. A schematic representation of the amplifier is shown in Figure 1. IS VS C4 R1 L1 RF OUT RF In 50 Ω RF Q1 RF IN VD C1 C2 RF Out 50 Ω Figure 2. Amplifier with Bias Network Q2 RB RBIAS Figure 1. Darlington Pair Amplifier Abstract As shown in Figure 2, the amplifier is a two-port device. C1 and C2 provide DC blocking in the RF input and output paths, respectively. RF ground is made through the remaining package pins. The RF output port is also used to provide bias to the amplifier through an external bias network (C4, R1 and L1). The values of the bias decoupling network components are selected based upon the frequency of operation and supply voltage available. This application note provides details on the proper selection of these components. Theory of Operation Skyworks gain blocks are Darlington feedback amplifiers which feature a constant gain-bandwidth product for use as an RF gain block. They use a circuit topology in which two transistors are combined to form a configuration known as a Darlington pair. This transistor pair behaves like a single transistor with a current gain equivalent to the product of the current gain of the two transistors. Darlington transistors are connected in a common emitter configuration, while sharing the same collector contact. The emitter current of the first transistor is applied to the base of the second transistor. This gives a high current gain (written b or Hfe), and takes less space than two discrete transistors in the same configuration. As RF gain blocks, Darlington pairs are offered as integrated packaged devices, usable from very low frequency to over 10 GHz. They exhibit good broadband matching into 50 W and tight performance distributions. The designed DC voltage that appears at the RF output of the amplifier, known as VD, is fixed by the base-emitter voltages of Q1 and Q2, and by the voltage divider formed by RF and RB. The values of resistors RF and RB are selected to establish the necessary feedback to set the input and output impedances of the amplifier, as well as to set the base voltages and currents of the Darlington pair. Consequently, the amplifier is controlled by the current supplied to the output node. Thus, the amplifier is biased using a fixed current source rather than a fixed voltage source. The simplest current source is a resistor (R1) connected to a voltage source (VS) as shown in Figure 2. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200942 Rev. B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 25, 2009 1 APPLICATION NOTE • Gain Block Bias Networks Figure 3 shows a simplified DC equivalent circuit of the Darlington pair gain block. The voltage that appears at the point labeled VD, which is the RF output of the amplifier, is the series sum of the forward voltages of the base-emitter junctions of the transistors and the voltage drop across RF. The forward voltage of each of the base-emitter diodes is exponentially related to the current flowing through them, and is also affected by temperature. Skyworks offers a series of gain block amplifiers in 3 different package styles (see Figure 4). These products differ by such parameters as gain, output power, frequency range and supply current draw, among others. Proper selection of a gain block depends upon the specific requirements of the application. Table 1 shows a list of gain blocks currently offered. Since the current through each of the base-emitter diodes directly controls the collector current for each of these transistors (these paths are not shown in this diagram), it is clearly important to control the bias current through each of the base-emitter junctions with a well-regulated current source. If a regulated voltage source is connected to the VD point in place of current source, the current through the transistors in the amplifier can vary widely in response to comparatively small changes in supply voltage. Additionally, due to the I-V characteristics of the base-emitter junction of each transistor, a small change in voltage or temperature can result in a large change in current. NOTE: For purposes of this application note, the SKY65015-92LF will be referenced. The information provided is common to other Skyworks gain block part numbers. Refer to the data sheets for specific part information. VD RF VBE Q1 RB RBIAS VBE Q2 Figure 4. Gain Block Package Styles Figure 3. Simplified DC Equivalent Circuit of Darlington Pair Gain Block 2 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com February 25, 2009 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200942 Rev. B APPLICATION NOTE • Gain Block Bias Networks Table 1. Skyworks Gain Block Part List Test Frequency (GHz) Gain Typ. (dB) Quiescent Current Typ. (mA) VCC (V) Noise Figure Typ. (dB) OIP3 (dBm) P1 dB (dBm) Part Number Package Frequency Range (GHz) SKY65013-70LF SOT-89 LF-7 2 12.5 40 5 5.5 29 12.5 SKY65014-70LF SOT-89 LF-6 2 16 70 5 4.8 36 18 SKY65015-70LF SOT-89 LF-6 2 18 70 5 4.2 35 17 SKY65016-70LF SOT-89 LF-3 2 20 40 5 4.8 27 14 SKY65017-70LF SOT-89 LF-6 2 20 100 5 4.5 35 20 SKY65013-92LF SC-88 LF-12 2 12.5 40 5 5.8 29 12.5 SKY65014-92LF SC-88 LF-9 2 15 70 5 5.4 36 15 SKY65015-92LF SC-88 LF-6 2 18 70 5 4.8 35 18 SKY65016-92LF SC-88 LF-3 2 20 40 5 5.4 27 20 SKY65013-214LF Micro-X LF-6 2 11.5 40 5 5.4 29 11.5 SKY65014-214LF Micro-X LF-6 2 13.5 70 5 4.6 36 13.5 SKY65015-214LF Micro-X LF-6 2 16 70 5 5.2 24 16 SKY65016-214LF Micro-X LF-3 2 19 40 5 6.5 20 19 The selection of inductor, L1 is based upon its self-resonant frequency (SRF) and its impedance in the band of operation (Fo). The inductor’s SRF should be greater than Fo and its impedance value should be approximately 10 times greater than Zo (50 W). Table 3 shows values for inductor L1 based upon the frequency of operation. The bill of materials for operation at 2 GHz for the SKY65015-92LF gain block amplifier circuit as shown in Figure 2 is shown in Table 2. Capacitors C1 and C2 are selected to provide DC blocking for the RF input and output. Capacitor C4 is used for a bias decoupling capacitor. This capacitor provides suppression of low frequency RF signals appearing across the DC bias lines. Table 2. SKY65015-92LF Bill of Materials Part ID Description Qty. Size 1 C1, C2, C4 Capacitor 3 0603 2 L1 Inductor 1 0603 3 R1 Resistor 1 0603 Value Units Product Number 47 nF GRM188R71E473K Murata 33 nH 0603CS-33NX_LU CoilCraft W Manufacturer See Current Limiting Res. Table Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200942 Rev. B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 25, 2009 3 APPLICATION NOTE • Gain Block Bias Networks Table 3. Recommended Chokes by Application Band Frequency(1) (GHz) Inductance (nH) SRF(2) (GHz) Impedance (XL)(3) Mfg. Part Number Manufacturer 0.4 220 0.82 553 0805CS-221X_L_ CoilCraft 0.8 120 1.10 603 0805CS-121X_L_ CoilCraft 0.9 110 1.10 622 0805CS-111X_L_ CoilCraft 1.2 91 1.33 686 0805CS-910X_L_ CoilCraft 1.6 47 1.70 472 0805CS-470X_L_ CoilCraft 1.8 56 1.90 633 0603CS-56NX_L_ CoilCraft 1.9 47 2.00 561 0603CS-47NX_L_ CoilCraft 2.1 39 2.20 514 0603CS-39NX_L_ CoilCraft 2.2 33 2.30 456 0603CS-33NX_L_ CoilCraft 2.4 27 2.80 407 0603CS-27NX_L_ CoilCraft 3 22 4.60 414 LQW18AN22NJ00 Murata 3.6 22 4.60 497 LQW18AN22NJ00 Murata 4.2 22 4.60 580 LQW18AN22NJ00 Murata 4.9 18 5.50 554 LQW18AN18NJ00 Murata 6 15 6.00 565 LQW18AN15NJ00 Murata 1. Evaluation board L1 nominal value = 33 nH (2.2 GHz). At high microwave frequencies, where no lumped element inductors are available, a ¼ wave length line with a stub may be used. 2. Self-resonant frequency 3. Choke selection is based upon SRF > FO and impedance (XL = 2*p*F*L) value 10 times greater than ZO (50 W). For extreme wideband operation 2 chokes may be combined, i.e. (33 nH and 110 nH). In most electronic systems, the power supply produces a fixed voltage, rather than a fixed current to the devices comprising the system. The selection of R1 is based upon the system supply voltage available and the value of VD that occurs when the correct supply DC current is present, as well as the power dissipated by the resistor. The resistor value is derived from the following equation: R = (VS-VD)/IS. The minimum resistor power dissipation rating is based upon the power dissipated across the resistor (Pdiss = V2 * R1). It is recommended to use resistors rated for at least twice the calculated value of Pdiss; otherwise, use parallel or a series split of several resistors that are each easily capable of dissipating their dissipated power levels. For the evaluation circuit as supplied by Skyworks, the default resistor size is ‘0603’ with a power rating of 1000 mW. Check the manufacturer’s power rating when using smaller sized resistors. Table 4. Current Limiting Resistor Values VS (V) R1 Resistance (W) Mimimum Resistor Power Dissipation Rating (mW) Product Number Manufacturer 4.4 0.5(1) – ERJ-3GEY0R00V Panasonic 5 8.2 125 ERJ-3GEYJ8R2V Panasonic 6 24 250 ERJ-3GEYJ240V Panasonic 7 36 500 ERJ-3GEYJ360V Panasonic 8 51 500 ERJ-3GEYJ510V Panasonic 9 68 1000 ERJ-3GEYJ680V Panasonic 10 82 1000 ERJ-3GEYJ820V Panasonic 12 110 1000 ERJ-3GEYJ111V Panasonic 1. At VS = 4.4 V, the series resistance (R1) is the DC resistance of the bias inductors (L1, L2 and L3) and trace lines. 4 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com February 25, 2009 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200942 Rev. B APPLICATION NOTE • Gain Block Bias Networks Low Frequency Operation No L2, L3 19 S21 (dB) 18 The modified bill of materials for extreme low frequency performance of the SKY65015-92LF is shown in Table 5. Gain (|S21|) performance using the bill of materials from Table 2 and Table 5 is shown in Figure 6. The ferrite bead and inductor L3 improve the frequency response below 300 MHz and reduce the variation in bias current, at the cost of degraded performance at 1 GHz and the monetary cost of the components themselves. 17 16 15 14 0 1 2 3 4 5 6 Frequency (GHz) Figure 6. SKY65015-92LF S21 vs. Frequency, IS =70 mA, TA = 25 °C IS VS With L2 Ferrite 20 For operation with signals below 300 MHz, Ferrite Bead (L2) and Inductor (L3) may be added. Figure 5 shows the schematic for low frequency applications below 300 MHz. Resistor R1 must also be added in series with the inductors to limit the power supply current when the power supply provides a constant voltage, as discussed above. L2 C4 L3 R1 L1 VD Input C1 Output C2 Figure 5. Amplifier with Low Frequency Bias Network Table 5. SKY65015-92LF Bill of Materials for Low Frequency Applications Part ID Description Qty. Size Value Units Product Number Manufacturer 1 C1, C2, C4 Capacitor 3 0603 47 nF GRM188R71E473K Murata 2 L1 Inductor 1 0603 33 nH 0603CS-33NX_LU CoilCraft 3 L2 Ferrite Bead 1 1810 1600 W FBMH4525HM162N-T Taiyo-Yuden 4 L3 Inductor 1 0805 110 nH nH 0805CS-111X_L_ CoilCraft 5 R1 Resistor 1 0603 Summary Skyworks gain block amplifiers are InGaP/GaAs HBT integrated circuits using the Darlington-pair transistor configuration which requires a constant DC current applied to the output port to bias the device. Factors such as voltage supply level, system characteristic impedance, frequency of operation and cost should W See Current Limiting Res. Table be taken into account when selecting the proper DC current limiting and RF decoupling bias components and RF DC blocking components in the signal path. The information provided in this application note addresses these tasks. Further information about specific gain block amplifiers may be found in the appropriate data sheets at www.skyworksinc.com. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200942 Rev. B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 25, 2009 5 APPLICATION NOTE • Gain Block Bias Networks Copyright © 2008, 2009, Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. These materials, including the information contained herein, are provided by Skyworks as a service to its customers and may be used for informational purposes only by the customer. Skyworks assumes no responsibility for errors or omissions in these materials or the information contained herein. Skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. Skyworks makes no commitment to update the materials or information and shall have no responsibility whatsoever for conflicts, incompatibilities, or other difficulties arising from any future changes. No license, whether express, implied, by estoppel or otherwise, is granted to any intellectual property rights by this document. Skyworks assumes no liability for any materials, products or information provided hereunder, including the sale, distribution, reproduction or use of Skyworks products, information or materials, except as may be provided in Skyworks Terms and Conditions of Sale. THE MATERIALS, PRODUCTS AND INFORMATION ARE PROVIDED “AS IS” WITHOUT WARRANTY OF ANY KIND, WHETHER EXPRESS, IMPLIED, STATUTORY, OR OTHERWISE, INCLUDING FITNESS FOR A PARTICULAR PURPOSE OR USE, MERCHANTABILITY, PERFORMANCE, QUALITY OR NON-INFRINGEMENT OF ANY INTELLECTUAL PROPERTY RIGHT; ALL SUCH WARRANTIES ARE HEREBY EXPRESSLY DISCLAIMED. SKYWORKS DOES NOT WARRANT THE ACCURACY OR COMPLETENESS OF THE INFORMATION, TEXT, GRAPHICS OR OTHER ITEMS CONTAINED WITHIN THESE MATERIALS. SKYWORKS SHALL NOT BE LIABLE FOR ANY DAMAGES, INCLUDING BUT NOT LIMITED TO ANY SPECIAL, INDIRECT, INCIDENTAL, STATUTORY, OR CONSEQUENTIAL DAMAGES, INCLUDING WITHOUT LIMITATION, LOST REVENUES OR LOST PROFITS THAT MAY RESULT FROM THE USE OF THE MATERIALS OR INFORMATION, WHETHER OR NOT THE RECIPIENT OF MATERIALS HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. Skyworks products are not intended for use in medical, lifesaving or life-sustaining applications, or other equipment in which the failure of the Skyworks products could lead to personal injury, death, physical or environmental damage. Skyworks customers using or selling Skyworks products for use in such applications do so at their own risk and agree to fully indemnify Skyworks for any damages resulting from such improper use or sale. Customers are responsible for their products and applications using Skyworks products, which may deviate from published specifications as a result of design defects, errors, or operation of products outside of published parameters or design specifications. Customers should include design and operating safeguards to minimize these and other risks. Skyworks assumes no liability for applications assistance, customer product design, or damage to any equipment resulting from the use of Skyworks products outside of stated published specifications or parameters. Skyworks, the Skyworks symbol, and “Breakthrough Simplicity” are trademarks or registered trademarks of Skyworks Solutions, Inc., in the United States and other countries. Third-party brands and names are for identification purposes only, and are the property of their respective owners. Additional information, including relevant terms and conditions, posted at www.skyworksinc.com, are incorporated by reference. 6 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com February 25, 2009 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200942 Rev. B