LT1228
100MHz Current Feedback
Amplifier with DC Gain Control
U
DESCRIPTIO
EATURE
S
F
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Very Fast Transconductance Amplifier
Bandwidth: 75MHz
The LT1228 makes it easy to electronically control the gain
of signals from DC to video frequencies. The LT1228
implements gain control with a transconductance amplifier
(voltage to current) whose gain is proportional to an exter-
nally 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 oper-
ates 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.
gm = 10 × ISET
Low THD: 0.2% at 30mVRMS Input
Wide ISET Range: 1µA to 1mA
Very Fast Current Feedback Amplifier
Bandwidth: 100MHz
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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
Inputs Common Mode to Within 1.5V of Supplies
Outputs Swing Within 0.8V of Supplies
Supply Current: 7mA
The LT1228 transconductance amplifier has a high imped-
ancedifferentialinputandacurrentsourceoutputwithwide
output voltage compliance. The transconductance, gm, is
setbythecurrentthatflowsintopin5, ISET. Thesmallsignal
gm isequaltotentimesthevalueofISETandthisrelationship
holdsoverseveraldecadesofsetcurrent. Thevoltageatpin
5 is two diode drops above the negative supply, pin 4.
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O U
PPLICATI
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Video DC Restore (Clamp) Circuits
Video Differential Input Amplifiers
Video Keyer/Fader Amplifiers
AGC Amplifiers
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.
Tunable Filters
Oscillators
U
O
TYPICAL APPLICATI
Frequency Response
6
Differential Input Variable Gain Amp
V
= ±15V
= 100Ω
S
L
R
3
0
15V
4.7µF
+
I
= 1mA
SET
R3A
10k
–3
7
4
3
2
+
–
+
–6
1
8
g
R2A
10k
V
m
+
–
IN
–9
6
I
I
= 300µA
SET
–
5
CFA
V
OUT
–12
–15
–18
I
SET
R
–15V
R2
F
470Ω
R4
R5
10k
R3
100Ω
= 100µA
4.7µF
1.24k
SET
+
100Ω
R1
270Ω
R
G
–21
–24
HIGH INPUT RESISTANCE
EVEN WHEN POWER IS OFF
–18dB < GAIN < 2dB
10Ω
R6
100k
1M
10M
100M
6.19Ω
V
≤ 3V
IN
RMS
LT1228 • TA01
FREQUENCY (Hz)
LT1228 • TA02
1