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Download DN132 - Fast Current Feedback Amplifiers Tame Low Impedance Loads
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advertisement Fast Current Feedback Amplifiers Tame Low Impedance Loads – Design Note 132 Sean Gold and William Jett Introduction Three current feedback amplifiers (CFAs) now available from Linear Technology can considerably ease the task of driving low impedance loads. This Design Note reviews the capabilities of the LT®1206, LT1207 and LT1210 CFAs and addresses some design issues encountered when using them. These CFAs are fast and capable of delivering high levels of current. They can be readily compensated for reactive loads and are fully protected against thermal and short-circuit faults. Table 1 summarizes their electrical characteristics. Driving Transformer-Coupled Loads Transformer coupling is frequently used to step up transmission line signals. Voltage signals amplified in this way are not constrained by local supply voltages, so the amplifier’s rated current rather than its voltage swing usually limits the power delivered to the load. Amplifiers with high output current drive are therefore appropriate for transformercoupled systems. Figure 1 shows a transformer-coupled application for ADSL in which an LT1210 drives a 100Ω twisted pair. The 1:3 transformer turns ratio allows just over 1W to reach the load at full output. Resistor RT acts as a primary side backtermination and also prevents large DC currents from flowing in the coil. The overall frequency response is flat to within 1dB from 500Hz to 2MHz. Distortion products at 1MHz are 15V + 4.7µF* VIN 100nF RT 11Ω 2.5W + LT1210 T1 – 1 + 4.7µF* RL 100Ω 2.5W 3 100nF 845Ω –15V 274Ω *TANTALUM T1: MIDCOM 671-7783 OR EQUIVALENT DN132 F01 Figure 1. Twisted Pair Driver ADSL. Voltage Gain is About 6; 5V P-P Input Corresponds to Full Output below – 70dBc at a total output power of 0.56W (load plus termination), rising to –56dBc at 2.25W. If RT is removed, the amplifier will see a load of about 11Ω and the maximum output power will increase to 5W. A DC blocking capacitor should be used in this case. Bridging can be used to increase the output power transferred to a transformer. Differential operation also promotes the cancellation of even-order distortion. Figure 2 shows a differential application using an LT1207 as a bridge driver for HDSL. The dual CFA is configured for a gain of ten, , LTC and LT are registered trademarks of Linear Technology Corporation. Table 1. Fast Current Feedback Amplifier Specifications NUMBER OF CFAs BANDWIDTH (MHz) RATED OUTPUT CURRENT (A) SUPPLY RANGE (V) SLEW RATE (V/µs) (NOTE 1) THERMAL RESISTANCE θJA (°C/W) (NOTE 2) SUPPLY CURRENT (mA) LOW POWER OP/ SHUTDOWN LT1206 1 60 0.25 ±5 to ±15 ILIM/CLOAD to 900 DD = 25, PDIP = 100 SO = 60, TO-220 = 5 20 Yes LT1207 2 60 0.25 ±5 to ±15 ILIM/CLOAD to 900 SO = 40 2 × 20 Yes LT1210 1 35 1 ±5 to ±15 ILIM/CLOAD to 1000 DD = 25, SO = 40 TO-220 = 5 30 Yes PART NUMBER Note 1: Slew rate depends on circuit configuration and capacitive load. Note 2: θJA on SO packages measured with part mounted to a 2.5mm thick FR4 2oz copper PC board with 5000mm2 area. 07/96/132 www.BDTIC.com/Linear delivering a 10VP-P signal to the nominal 35Ω load impedance. The output signal amplitude remains flat over an 8MHz bandwidth. Driving Capacitive Loads The devices in Table 1 combine the high output current required to slew large capacitances with appropriate frequency compensation. All of the CFAs described here are C-LoadTM amplifiers and are stable with capacitive loads up to 10,000pF. C-Load is a trademark of Linear Technology Corporation. 5V 3 4 2 FROM TRANSMIT 192kHz LOWPASS FILTER + + 16 1/2 LT1207 – 4.7µF 0.1µF 1 68.1Ω 15 511Ω 14 L1* 7 –5V 0.1µF + 4.7µF 113Ω 2 5V 6 + 4.7µF 0.1µF 8 4 6 7 5 + 8 9 1/2 LT1207 – A good example of a difficult capacitive load is a clock driver for a charge-coupled device (CCD). These devices require precise multiphase clock signals to initiate the transfer of light-generated pixel charge from one charge reservoir to the next. Noise, ringing or overshoot on the clock signal must be avoided. Two problems complicate clock generation. First, CCDs present an input capacitance (typically 100pF to 3300pF) which is directly proportional to the number of sensing elements (pixels). Second, CCDs often require the clock’s amplitude to exceed the logic supply. The amplifying filter in Figure 3 addresses these issues. Both CFAs in the LT1207 are configured for a third-order Gaussian lowpass response with 1.6MHz cutoff frequency (one section is shown). This transfer function produces clean clock signals with controlled rise and fall times. Figure 4 shows the LT12O7’s quadrature outputs driving two 3300pF loads that simulate a CCD image sensor. Ringing and overshoot are notably absent from the clock signals, which have rise and fall times of approximately 300ns. 511Ω 68.1Ω 12 11 A QUADRATURE INPUTS B 9 FILTERED C CLOCK OUTPUTS D *MIDCOM 671-7108, TRANSPOWER SMPT-308 OR SIMILAR DEVICE –5V 0.1µF + 4.7µF Figure 4. CCD Clock Driver Waveforms DN132 F02 Figure 2. Bridge Driver for HDSL 20V 45pF 5V 4 4fCLK 2MHz + 500kHz 3 2 1 10 11 12 13 1CK 1D 1Q 5 A 1k 1k 180pF 1Q 1k 3 91pF 6 2 1,16 + 1/2 LT1207 – 14 74HC74 2CK 2D 2Q 2Q 4 9 8 0.1µF 10µF QUADRATURE OUTPUT TO B OTHER SECTION 15 C 13 0.1µF SIMULATED CCD ARRAY LOAD 10 3300pF 0.1µF 10µF + 510k –10V 1k DN132 F03 Figure 3. CCD Clock Driver For literature on our Current Feedback Amplifiers, call 1-800-4-LINEAR. For applications help, call (408) 432-1900, Ext. 2456 Linear Technology Corporation LT/GP 0796 155K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1996 www.BDTIC.com/Linear