LT1228
100MHz Current Feedback
Amplifier with DC Gain Control
FEATURES
DESCRIPTION
The LT®1228 makes it easy to electronically control the
gain of signals from DC to video frequencies. The LT1228
implements gain control with a transconductance ampli-
fier (voltage to current) whose gain is proportional to an
externally controlled current. A resistor is typically used
to convert the output current to a voltage, which is then
amplified with a current feedback amplifier. The LT1228
combinesbothamplifiersintoan8-pinpackage, andoper-
ates on any supply voltage from 4V ( 2V) to 30V ( 1ꢀV).
A complete differential input, gain controlled amplifier can
be implemented with the LT1228 and just a few resistors.
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Very Fast Transconductance Amplifier
Bandwidth: 7ꢀMHz
g = 10 × I
m
SET
Low THD: 0.2% at 30mV
Input
RMS
Wide I Range: 1µA to 1mA
SET
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Very Fast Current Feedback Amplifier
Bandwidth: 100MHz
Slew Rate: 1000V/µs
Output Drive Current: 30mA
Differential Gain: 0.04%
Differential Phase: 0.1°
High Input Impedance: 2ꢀMΩ, 6pF
Wide Supply Range: 2V to 1ꢀV
Inputs Common Mode to Within 1.ꢀV of Supplies
Outputs Swing Within 0.8V of Supplies
Supply Current: 7mA
TheLT1228transconductanceamplifierhasahighimped-
ancedifferentialinputandacurrentsourceoutputwithwide
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n
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n
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output voltage compliance. The transconductance, g , is
m
setbythecurrentthatflowsintoPinꢀ,I .Thesmallsignal
SET
g isequaltotentimesthevalueofI andthisrelationship
m
SET
Available in 8-Lead PDIP and SO Packages
holds over several decades of set current. The voltage at
Pin ꢀ is two diode drops above the negative supply, Pin 4.
APPLICATIONS
TheLT1228currentfeedbackamplifierhasveryhighinput
impedanceandthereforeitisanexcellentbufferfortheout-
putofthetransconductanceamplifier.Thecurrentfeedback
amplifiermaintainsitswidebandwidthoverawiderangeof
voltage gains making it easy to interface the transconduc-
tance amplifier output to other circuitry. The current feed-
back amplifier is designed to drive low impedance loads,
suchascables,withexcellentlinearityathighfrequencies.
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Video DC Restore (Clamp) Circuits
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Video Differential Input Amplifiers
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Video Keyer/Fader Amplifiers
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AGC Amplifiers
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Tunable Filters
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Oscillators
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
Frequency Response
6
Differential Input Variable Gain Amp
V
=
15V
S
L
R
= 100Ω
3
0
15V
4.7µF
I
= 1mA
SET
+
R3A
–3
10k
7
4
3
2
+
–
+
–6
1
8
g
R2A
10k
V
m
+
–
IN
–9
6
I
I
= 300µA
SET
V
–
5
CFA
OUT
–12
–15
–18
I
SET
R
F
–15V
R2
470Ω
R4
1.24k
R5
R3
100Ω
= 100µA
1M
4.7µF
10k
SET
+
100Ω
R1
270Ω
R
G
–21
–24
HIGH INPUT RESISTANCE
EVEN WHEN POWER IS OFF
–18dB < GAIN < 2dB
10Ω
R6
6.19k
100k
10M
100M
V
≤ 3V
IN
RMS
LT1228 • TA02
LT1228 • TA01
FREQUENCY (Hz)
1228fd
1