LT1228
100MHz Current Feedback
Amplifier with DC Gain Control
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FEATURES
DESCRIPTIO
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
combines both amplifiers into an 8-pin package, and
operates on any supply voltage from 4V (± 2V) to 30V
(±15V). A complete differential input, gain controlled
amplifier can be implemented with the LT1228 and just a
few resistors.
■
Very Fast Transconductance Amplifier
Bandwidth: 75MHz
gm = 10 × ISET
Low THD: 0.2% at 30mVRMS Input
Wide ISET Range: 1µA to 1mA
■
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: 25MΩ, 6pF
■
Wide Supply Range: ±2V to ±15V
The LT1228 transconductance amplifier has a high imped-
ancedifferentialinputandacurrentsourceoutputwithwide
output voltage compliance. The transconductance, gm, is
setbythecurrentthatflowsintoPin5, ISET. Thesmallsignal
gm isequaltotentimesthevalueofISETandthisrelationship
holds over several decades of set current. The voltage at Pin
5 is two diode drops above the negative supply, Pin 4.
■
Inputs Common Mode to Within 1.5V of Supplies
■
Outputs Swing Within 0.8V of Supplies
Supply Current: 7mA
■
■
Available in 8-Lead PDIP and SOIC Packages
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APPLICATIO S
■
Video DC Restore (Clamp) Circuits
■
The LT1228 current feedback amplifier has very high input
impedance and therefore it is an excellent buffer for the
output of the transconductance amplifier. The current feed-
back amplifier maintains its wide bandwidth over a wide
range of voltage gains making it easy to interface the
transconductance amplifier output to other circuitry. The
current feedback amplifier is designed to drive low imped-
ance loads, such as cables, with excellent linearity at high
frequencies.
Video Differential Input Amplifiers
■
Video Keyer/Fader Amplifiers
■
AGC Amplifiers
■
Tunable Filters
■
Oscillators
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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Frequency Response
TYPICAL APPLICATIO
6
V
= ±15V
= 100Ω
S
L
Differential Input Variable Gain Amp
R
3
0
I
= 1mA
15V
SET
4.7µF
+
–3
R3A
10k
7
4
3
2
–6
+
–
+
1
8
–9
g
R2A
10k
V
m
+
–
IN
I
I
= 300µA
SET
6
–12
–15
–18
–
5
CFA
V
OUT
I
SET
R
–15V
R2
F
470Ω
= 100µA
R4
1.24k
R5
10k
SET
R3
100Ω
–21
–24
4.7µF
+
100Ω
R1
270Ω
R
G
HIGH INPUT RESISTANCE
EVEN WHEN POWER IS OFF
–18dB < GAIN < 2dB
10Ω
100k
1M
10M
100M
R6
6.19k
V
≤ 3V
RMS
FREQUENCY (Hz)
IN
LT1228 • TA02
LT1228 • TA01
1228fc
1