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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