® DEM-OPA660-4G EVALUATION FIXTURE FEATURES APPLICATIONS ● EASY AND FAST PERFORMANCE TESTING ● COMPONENTS INCOME CONTROL ● SHOWS OPTIMIZED BOARD LAYOUT ● CIRCUIT DESIGNS ● PERFORMANCE CHECKS ● REPLACES SELF-MADE BOARDS DESCRIPTION The unassembled demo board DEM-OPA660-4G contains three different configurations of the OPA660 building blocks OTA and buffer stage: the Diamond Transistor and Buffer (DEM-OPA660-41G), the Current-Feedback Amplifier (DEM-OPA660-42G), and the Direct-Feedback Amplifier (DEM-OPA660-43G). It is designed for engineers who want to test the various possibilities of the OPA660AU for themselves. application. For more information about applications with OPA660, please refer to the application notes AN-179 “Video Operational Amplifier,” AN-180 “Ultra High-Speed ICs,” and AN-181 “Diamond Transistor OPA660,” as well as the OPA660 data sheet. Unlike the DEM-OPA660-1GC to -3GC, which are assembled and tested versions of the individual configurations in DIL packaging, the DEM-OPA660-4G offers the three circuits for the SO package. An unassembled DIP version of the configurations is available under the part number DEM-OPA660-5G. The board can be easily broken into three parts to design custom circuits as required by the particular International Airport Industrial Park • Mailing Address: PO Box 11400 Tel: (602) 746-1111 • Twx: 910-952-1111 • Cable: BBRCORP • © 1992 Burr-Brown Corporation • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Telex: 066-6491 • FAX: (602) 889-1510 • Immediate Product Info: (800) 548-6132 LI-418 Printed in U.S.A. December, 1992 TEST FIXTURE: DIAMOND TRANSISTOR AND BUFFER RECOMMENDED COMPONENT VALUES GAIN R1 R4 R6 R7 IQ RQC 1 100Ω 200Ω 25Ω 50Ω 20mA 250Ω 2 100Ω 51Ω 75Ω 50Ω 20mA 250Ω 10 100Ω 51Ω 475Ω 50Ω 20mA 250Ω Description This printed circuit board allows easy and fast performance testing of the OPA660AU building blocks OTA and buffer stage. The voltage-controlled current source or Operational Transconductance Amplifier (OTA) can be viewed as an “ideal” transistor. Like a transistor, it has three terminals: a high-impedance input (base), a low-impedance input/output (emitter), and the complementary current source or sink (collector). The OTA, however, is self-biased and bipolar. The transconductance of the OTA and the buffer amplifier can be adjusted by the external resistor RQC, allowing bandwidth, quiescent current, and gain trade-offs to be optimized. The low-frequency gain of a common emitter amplifier is determined by the following equation: R5 5 6 1 1 gm R4 + RL ~ R4 where RL = R2 + R3 The voltage buffer is an open-loop buffer with gain slightly less than unity, which is ideal for interstage buffering. Figure 1 shows the schematic diagram of the board layout and the recommended power supply bypassing. Figure 3 illustrates the OTA transfer characteristics vs input voltage and IQ, and the performance curve Total Quiescent Current vs RQC shown in Figure 2 defines the resistor value for a certain IQ. BUF In RL G= TEST FIXTURE: CURRENT-FEEDBACK AMPLIFIER Description When the OTA and buffer sections are combined, these sections of the OPA660 can be interconnected in a currentfeedback amplifier configuration. Current-feedback amplifi- R6 BUF Out R7 +5V –5V R2 1 OTA Out R1 OTA In 8 R3 3 2 C1 R4 C2 470p 470p 10n 10n 2.2µ 2.2µ RQC IN4004 7 4 FIGURE 1. Block Diagram of the Test Fixture’s Diamond Transistor and Buffer. 10 OTA Output Current (mA) Total Quiescent Current (mA) 100 30 10 3.0 5 IQ = 5mA 0 IQ = 10mA –5 IQ = 20mA –10 1.0 100 300 1.0k 3.0k –60 10k FIGURE 2. Total Quiescent Current vs RQC of the Diamond Transistor and Buffer. –20 0 20 FIGURE 3. OTA Transfer Characteristics. ® DEM-OPA660-4G –40 OTA Input Voltage (mV) RQC — Resistor Value (Ω) 2 40 60 FIGURE 4. Silk Screen and Board Layouts of the Diamond Transistor and Buffer. ers have nearly constant bandwidth for varying closed-loop gains. The reason is that the user can adjust the open-loop gain of the current-feedback amplifier by changing the feedback network, without affecting the open-loop pole. Figure 5 shows the block diagram of the Current-Feedback Amplifier test fixture. The size of R3 is equal to the characteristic impedance of the transmission line minus the output resistance of the amplifier. Figure 6 illustrates the silk screen and double-sided layout. TEST FIXTURE: DIRECT-FEEDBACK AMPLIFIER Description The low-frequency gain of a noninverting current-feedback amplifier is determined by the following equation: G=1+ R4 The demo board layout allows easy and fast performance testing of the OPA660AU in the so-called Direct-Feedback Amplifier configuration. We named this structure DirectFeedback Amplifier due to its short feedback loop across the complementary current mirror. The currents at the collector and emitter flow in the same direction. The output current of the OTA is noninverting. The additional current flowing from the collector across R3 and through R5 causes a voltage drop and counteracts the base-emitter voltage. The reduced voltage difference, however, causes reduced collector flow. It functions like double feedback, and the low-frequency gain is adjusted according to the following equation: R5 The flat frequency response can be adjusted by changing the size of R4. The size of R4 determines the transconductance (gm) of the OTA and the open-loop gain of the amplifier. RECOMMENDED COMPONENT VALUES GAIN R1 R2 R4 R5 IQ RQC 1 150Ω 220Ω 300Ω — 20mA 250Ω 2 150Ω 220Ω 270Ω 270Ω 20mA 250Ω 10 47Ω 56Ω 200Ω 22Ω 20mA 250Ω G=1+ R3 2R5 +5V C1 R1 In R2 5 1 –5V R3 6 1 Out 8 3 470p 470p 10n 10n 2.2µ 2.2µ RQC R4 2 R5 IN4004 7 4 FIGURE 5. Block Diagram of the Test Fixture’s Current-Feedback Amplifier. ® 3 DEM-OPA660-4G FIGURE 6. Silk Screen and Board Layouts of the Current-Feedback Amplifier. R2 5 1 6 R4 BUF Out R6 Either or +5V 1 R7 Out R1 In –5V R2 8 3 R3 R8 2 470p 470p 10n 10n 2.2µ 2.2µ RQC IN4004 R5 7 4 FIGURE 7. Block Diagram of the Test Fixture Direct-Feedback Amplifier. Using an emitter compensation technique parallel to R5, it is possible to achieve both excellent pulse responses and bandwidths of up to more than 500MHz at 1.4Vp-p output voltage. The RC combination parallel to R5 increases the closed-loop gain at high frequencies. The PDS of the OPA660 shows an application circuit with gain of 3, as well as presenting a bandwidth diagram and small- and large-signal pulse responses. conduct 10mA (30mA peak). If input voltages can exceed the power supply voltages by 0.7V, then the input signal current must be limited. The buffer output is not current-limited or protected. If the output is shorted to ground, current of up to 60mA could flow. Momentary shorts to ground (a few seconds) should be avoided, but are unlikely to cause permanent damage. The same cautions apply to the OTA section when connected as a buffer. The subsequent buffer amplifier decouples the relatively high-impedance collector when driving low-impedance load resistances. The board layout for the Direct-Feedback Amplifier configuration is illustrated in Figure 8. BASIC CONNECTIONS Figure 9 shows basic connections required for operation. Power supply bypass capacitors should be located as close as possible to the device pins. Solid tantalum capacitors are generally best. A resistor (25Ω to 200Ω) in series with the buffer and/or B input may help to reduce oscillations and peaking. APPLICATION INFORMATION The OPA660 operates from ±5V power supplies (±6V maximum). Do not attempt to operate with larger power supply voltages, as permanent damage may occur. The inputs of the OPA660 are protected by internal diode clamps. These protection diodes can safely, continuously ® DEM-OPA660-4G 4 QUIESCENT CURRENT CONTROL PIN temperature holds the transconductance, gm, of the OTA relatively constant with temperature. The quiescent current of the OPA660 is set with the resistor RQ connected from Pin 1 to –VCC. It affects the operating currents of the buffer and OTA sections, thus controlling both the bandwidth and AC behavior and the transconductance of the OTA section. TEST CONFIGURATION When testing the AC parameters of RF components, impedance matching is necessary at the input and output of the DUT. Double termination of the transmission cables between the signal and DUT and DUT and analyzer is the cleanest way to drive, since reflections are absorbed on both ends of the cable. The output resistance between the amplifier’s output and the OUT socket should be equal to the characteristic impedance minus the output impedance of the amplifier. In turn, the input of the DUT should be terminated by the characteristic cable impedance. Figure 11 shows a typical test configuration. RQC = 250Ω sets approximately 20mA total quiescent current at 25°C. With a fixed 250Ω resistor, process variations could cause this current to vary from approximately 16mA to 26mA. It may be appropriate in some applications to trim this resistor to achieve the desired quiescent current or AC performance. With a fixed RQC resistor, the quiescent current increases with temperature (see typical performance curve, Quiescent Current vs Temperature). This variation of current with FIGURE 8. Silk Screen and Board Layouts of the Direct-Feedback Amplifier. RQC = 250Ω sets roughly IO = 20mA 1 +5V(1) 8 470pF IO Adjust 1 8 C E 2 7 V+ =+5V B 3 6 Out V– = –5V 4 5 In 10nF 2 7 RQC 250Ω + +1 3 6 2.2µF Solid Tantalum –5V(1) 4 1 5 10nF 470pF + 2.2µF Solid Tantalum NOTE: (1) VCC = ±6V absolute maximum. FIGURE 9. Basic Connections. FIGURE 10. Pin Configuration. ® 5 DEM-OPA660-4G 50Ω 50Ω In Out 50Ω Network Analyzer DUT 50Ω 50Ω • • • Generator FIGURE 11. Test Configuration. ORDERING INFORMATION ABSOLUTE MAXIMUM RATINGS: OPA660AU Power Supply Voltage ........................................................................ ±6V Input Voltage(1) ........................................................................ ±VCC, ±0.7V Operating Temperature .................................................. –40°C to +85°C Storage Temperature .................................................... –40°C to +125°C Junction Temperature ................................................................... +150°C Lead Temperature (soldering,10s) ............................................... +300°C MODEL DEM-OPA660-4G DESCRIPTION TEMPERATURE RANGE Layouts for all applications using SO packages, unassembled –25°C to +85°C NOTE: (1) Inputs are internally diode-clamped to ±VCC. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of 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. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. ® DEM-OPA660-4G 6