Sean Gold William Jett Introduction Three current feedback amplif
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Fast Current Feedback Amplifiers Tame Impedance Loads Design Note
Sean Gold William Jett
Introduction Three current feedback amplifiers (CFAs) available from Linear Technology considerably ease task driving impedance loads. This Design Note reviews capabilities LT®1206, LT1207 LT1210 CFAs addresses some design issues encountered when using them. These CFAs fast capable delivering high levels current. They readily compensated reactive loads fully protected against thermal short-circuit faults. Table summarizes their electrical characteristics. Driving Transformer-Coupled Loads Transformer coupling frequently used step transmission line signals. Voltage signals amplified this constrained local supply voltages, amplifier's rated current rather than voltage swing usually limits power delivered load. Amplifiers with high output current drive therefore appropriate transformercoupled systems. Figure shows transformer-coupled application ADSL which LT1210 drives twisted pair. transformer turns ratio allows just over reach load full output. Resistor acts primary side backtermination also prevents large currents from flowing coil. overall frequency response flat within from 500Hz 2MHz. Distortion products 1MHz
Table Fast Current Feedback Amplifier Specifications
PART NUMBER LT1206 LT1207 LT1210 NUMBER CFAs BANDWIDTH (MHz) RATED OUTPUT CURRENT 0.25 0.25 SUPPLY RANGE SLEW RATE (V/µs) (NOTE ILIM/CLOAD ILIM/CLOAD ILIM/CLOAD 1000 THERMAL RESISTANCE (°C/W) (NOTE PDIP TO-220 TO-220 SUPPLY CURRENT (mA) POWER SHUTDOWN
4.7µF* 100nF 2.5W 4.7µF* -15V 100nF *TANTALUM MIDCOM 671-7783 EQUIVALENT
DN132
LT1210
2.5W
Figure Twisted Pair Driver ADSL. Voltage Gain About Input Corresponds Full Output
below 70dBc total output power 0.56W (load plus termination), rising -56dBc 2.25W. removed, amplifier will load about maximum output power will increase blocking capacitor should used this case. Bridging used increase output power transferred transformer. Differential operation also promotes cancellation even-order distortion. Figure shows differential application using LT1207 bridge driver HDSL. dual configured gain ten,
registered trademarks Linear Technology Corporation.
Note Slew rate depends circuit configuration capacitive load. Note packages measured with part mounted 2.5mm thick copper board with 5000mm2 area.
07/96/132
delivering 10VP-P signal nominal load impedance. output signal amplitude remains flat over 8MHz bandwidth. Driving Capacitive Loads devices Table combine high output current required slew large capacitances with appropriate frequency compensation. CFAs described here C-Loadamplifiers stable with capacitive loads 10,000pF.
C-Load trademark Linear Technology Corporation.
0.1µF 4.7µF 68.1
LT1207
0.1µF
LT1207
Figure Bridge Driver HDSL
45pF
4fCLK 2MHz
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977
0.1µF
FROM TRANSMIT 192kHz LOWPASS FILTER
0.1µF 4.7µF
good example difficult capacitive load clock driver charge-coupled device (CCD). These devices require precise multiphase clock signals initiate transfer light-generated pixel charge from charge reservoir next. Noise, ringing overshoot clock signal must avoided. problems complicate clock generation. First, CCDs present input capacitance (typically 100pF 3300pF) which directly proportional number sensing elements (pixels). Second, CCDs often require clock's amplitude exceed logic supply. amplifying filter Figure addresses these issues. Both CFAs LT1207 configured third-order Gaussian lowpass response with 1.6MHz cutoff frequency (one section shown). This transfer function produces clean clock signals with controlled rise fall times. Figure shows LT12O7's quadrature outputs driving 3300pF loads that simulate image sensor. Ringing overshoot notably absent from clock signals, which have rise fall times approximately 300ns.
QUADRATURE INPUTS
4.7µF 68.1
*MIDCOM 671-7108, TRANSPOWER SMPT-308 SIMILAR DEVICE 4.7µF
DN132
FILTERED CLOCK OUTPUTS
Figure Clock Driver Waveforms
500kHz 180pF 91pF
10µF
0.1µF
1,16 0.1µF 0.1µF
SIMULATED ARRAY LOAD
LT1207
3300pF
74HC74 QUADRATURE OUTPUT OTHER SECTION 510k -10V
10µF
DN132
Figure Clock Driver
literature Current Feedback Amplifiers, call 1-800-4-LINEAR. applications help, call (408) 432-1900, Ext. 2456
LT/GP 0796 155K PRINTED
LINEAR TECHNOLOGY CORPORATION 1996
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