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Drivers for GaAs FET Switches and Digital Attenuators Rev. V5 Application Note S2079 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. Introduction Many of M/A-COM's GaAs FET switches and digital attenuators cannot operate directly with simple TTL or CMOS logic, but instead require external circuits to provide appropriate control voltages. This application note, an update of M539, Drivers for GaAs FET MMIC Switches and Digital Attenuators , provides information on M/A-COM's SW-109 and SWD-119 drivers and other commercially available digital logic IC's for control of switches and digital attenuators. GaAs FET’s GaAs MMIC control devices such as switches and digital attenuators typically employ Field Effect Transistors (FET’s). The most common FET is the n- channel depletion mode device, which has low source- to-drain resistance in the absence of a gate bias, and allows a current IDSS to flow. With the application of a negative gate bias voltage, the electric field below the gate causes the conduction channel to narrow, increasing the source-to-drain resistance. The gate voltage that creates a high enough resistance to reduce the source-to-drain current to (typically) 1 - 2 percent of IDSS is known as the pinch-off voltage. For M/A-COM FET’s,. the pinch-off voltage is typically –2.5 volts. If the transistor is biased at the extremes, (0 V and –5 V typically), on and off switching results, providing the basis for both GaAs MMIC switches and digital attenuators. Switch Circuit Topology In switches, FET's are arranged in both series and shunt configurations. The series FET's provide a through-path for the on state, while the shunt FET's provide isolation for the off state. The operation of the switch requires that series FET's and shunt FET's associated with each switch state have opposite (or complementary) conduction states and therefore opposite (or complementary) gate biases. For example, Figure 1 illustrates the operation of a typical dual control SPST GaAs MMIC switch. If the RF to RF1 path is on and the RF to RF2 path is off, then FET's Q2 and Q4 are biased on, while Q1 and Q3 are biased off. Digital attenuators use series/shunt stages with circuit components that form fixed attenuator pads, corresponding to digital attenuation bits, switched in or out of the transmission path, either individually or in combination. Switches require complementary bias voltages for each state, while digital attenuators require complementary bias voltage to activate each bit. Figure 2 shows a 3-bit digital attenuator. Applying the correct bias voltage and its complement to any stage switches the pad for that stage into the RF signal path. RF Common RF1 RF2 Control "A" Control "B" Q2 Q3 Figure 1: Typical Dual Control Switch (SW-239, etc) Control A Control B RF Common to RF1 RF Common to RF2 -5 V 0 V On Off
0 V -5 V Off On
Typical complementary logic control voltages: Logic low Logic high 0 V to –2 V @ 20 μA max. -5 V to 40 μA typ. To –8 V @ 200 μ A max. Dual Control Switch Truth Table Figure 2: Digital Attenuator Based on Switched Pads (AT-230) VC1 RF 1 VC1 16 dB Pad 8 dB Pad VC2 VC2 4 dB Pad VC3 VC3 RF 2
Drivers for GaAs FET Switches and Digital Attenuators Rev. V5 Application Note S2079 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. Built-in Drivers Some of M/A-COM's newer switches and attenuators feature simplified control using CMOS (0 V, 2.7 V) or TTL (0 V, 5 V) logic, with no need for negative control voltages. The Appendix to this application note lists some popular M/A-COM switches. The SW-277, SW-349, SW-394, and SW-399 include level shifting components for compatibility with positive CMOS or TTL control voltages, but these switches still requi re complementary control logic. The SW-335, SW65-0xxx series and related switches incorporate a CMOS driver circuit in the same package, along with the GaAs switching elements, for true single line control. Many future switches from M/A- COM will likely incorporate driver circuitry and switching elements together in small, low cost plastic packages. The AT-226, AT-264, and AT -242 digital attenuators feature internal level shifting to provide control with a single CMOS input line for each attenuation bit. The AT65-0xxx series miniature digital attenuator modules incorporate CMOS driver circuitry to accomplish this. SWD-109 & SWD-119 Drivers M/A-COM's SWD-109 and quad-channel SWD-119 provide the complementary c ontrol voltages necessary for driving GaAs FET switches and digital attenuators using a single control input per bit. Both the SWD-109 and SWD-119 incorporate buffering stages so that the drivers will switch with either standard TTL or CMOS logic level input. The devices employ standard CMOS analog fabrication techniques for low power consumption. The devices consist of input buffers, inverters to generate complementary logic values, voltage translators, and output buffers, all designed to allow the designer the flexibility to optimize switch and attenuator performance. To design a board with RF switches and attenuators, consider that modulation of the source-drain resistance in the FET's by input RF can lead to output compression and intermodulation distortion. Although GaAs FET switches and attenuators will operate well with nominal 0 V and -5 V for control, carefu l selection of the control voltages in the ranges of - 8 V < VFEToff < - 5 V, and 0 V < VFETon < 2 V can improve the maximum RF level (P1dB). With proper selection of positive and negative supply voltage, the SWD-109 and SWD-119 can both provide output control voltages in these ranges. Another consideration in design with switches and attenuators is the elimination of crosstalk that can arise from RF leakage onto control lines. Most board designers take care of this by adding capacitance to ground on the control lines, shunting any RF energy to ground. The SW-109 and SWD-119 output buffers can drive load capacitance up to 25 pF. Other Circuits as Drivers You can use TTL and CMOS logic IC's to drive GaAs FET switches and attenuators. An ideal driver would run from a single supply voltage, consume little current, and introduce very little switching delay. One driver technique that works well floats the channel of the FET's on the MMIC switch above ground potential through the addition of pull-up resistors and DC blocking and bypass capacitors. As shown in Figure 3, the circuit takes a voltage of 0 VDC, applied to either control port, and shifts it to -5 VDC at the attached FET gates to turn them off. A voltage of +5 VDC shifts to 0 VDC at the FET gates to turn them on. Figure 3: GaAs SPDT Switch with CMOS Driver Control A Control B RF Common to RF 1 RF Common to RF 2 TTL Low TTL High ON OFF TTL High TTL Low OFF ON RF Common +5 VDC Q2 Q3RF1 Q1 Q4 RF2 +5V Control "B" Control "A" +5V TTL Control +5 VDC VCC GND CD54HCT04
Drivers for GaAs FET Switches and Digital Attenuators Rev. V5 Application Note S2079 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. Additional Notes: Application Note M537, GaAs MMIC Based Control Components with Integral Drivers defines performance parameters for switches and attenuators. 1. See Application Note M517, MASW6010 GaAs SPDT Switch Performance and Driver Circuit Techniques for additional information on designing with the SWD-109 and SWD-119 drivers. 2. For pin assignments and supply voltages for the SWD-109 and SWD-119 single/quad drivers, see the SWD-109/119 data sheet, available on the M/A- COM web site at www.macom.com. 3. See Application Note M521, Positive Voltage Control of GaAs MMIC Control Devices for more information on floating attenuators above ground potential. 4. See Application Note M539, Drivers for GaAs MMIC Switches and Digital Attenuators for more information on compression and intermodulation distortion and the operation of the SW-109 and SWD-119 drivers. 5. See manufacturers' data sheets and application notes for additional information on digital logic IC's. 6. Please contact your M/A-COM sales representative for information on the latest switches and attenuators. Appendix Popular M/A-COM GaAs FET switches In the following tables, switching speed is the time from the 50 percent point of the c ontrol voltage rise or fall to the occurrence of 90 percent ( on) or 10 percent (off) of the switched RF level. Part No. Type Package Switching Speed SW-212 SPST FP-13 6 ns SW-214 SPST terminated FP-13 6 ns SW-226 SPDT terminated CR-2 6 ns SW-227 SPDT CR-2 6 ns SW-239 SPDT SOIC-8 4 ns SW-259 SPST terminated SOIC-8 8 ns SW-276 SPDT CR-2 35 ns SW-279 SPDT SOIC-8 35 ns SW-289 SP4T SOIC-14 6 ns SW-337, 338 SPDT terminated SOIC-8 10 ns SW-3911 SPDT SOT-26 42 ns SW-3921 SPDT SOT-26 20 ns SW-3951 SPDT SOT-26 8 ns SW-419 SP4T SOIC-24 16 ns SW-4251 SPDT SOT-26 ~ 20 ns SW-437 SPDT SOT-363 ~ 8 ns SW-439* SPDT MSOP-10 ~ 34 ns Dual Control Negative Bias 1. Contains no shunt FET’s, hence can operate with positive control voltages without ground pull-up components if provided with DC blocking capacitors on all RF lines. Dual Positive Control, Requires Positive Supply (Includes pull-up components on-chip) Part Number Type Package Switching Speed SW-277 SPDT SOIC-8 35 ns SW-349 SPST terminated SOIC-8 2 μs SW-394 SPDT SOIC-8 36 μs SW-399 SPST SOT-26 110 μs Part Number Type Package Switching Speed SW05-0311 SPST, TTL/CMOS in CR-9 150 ns SW10-0312 SPDT, TTL/CMOS in CR-9 150 ns SW10-0313 SPDT, TTL/CMOS in CR-9 150 ns SW65-0014 SPST, TTL/CMOS in SOIC-24 50 ns SW65-0114 SPST, TTL/CMOS in SOIC-24 50 ns SW65-0214 SP3T, TTL/CMOS in SOIC-24 50 ns SW65-0313 SP2T, TTL/CMOS in SOIC-16 50ns SW65-0314 SP4T, TTL/CMOS in SOIC-24 50 ns SW65-0440 SP4T, absorptive, TTL/CMOS in QSOP-24 50 ns SW-110 SPDT, TTL/CMOS in CR-9 35 ns SW-311 SPST, TTL.CMOS in CR-9 12 ns SW-312 SPDT, TTL/CMOS in CR-9 7 ns SW-313 SPD, TTL/CMOS in CR-9 18 ns SW-335 SPDT, TTL /CMOS in SOIC-8 200 ns SW-224 SPDT, TTL in TO-5-4 150 ns SW- 225 SPDT, TTL in FP-13 150 ns Single Control, Integral Driver, Positive Supply Part Number Type Package Switching Speed SW-205 SPDT, TTL in DI-1 20 ns SW-206 SPDT, CMOS in DI-1 40 ns SW-215 SPST, TTL in DI-1 20 ns SW-216 SPDT, CMOS in DI-1 40 ns SW-217 SPDT, TTL in DI-1 20 ns SW-233 SPDT, TTL in FP-16 20 ns SW-236 SPDT, CMOS in FP-16 40 ns Single Control, Integral Driver, Requires Positive And Negative Supplies