
1
LT1228
1228fc
■ Video DC Restore (Clamp) Circuits
■ Video Differential Input Amplifiers
■
Video Keyer/Fader Amplifiers
■ AGC Amplifiers
■ Tunable Filters
■
Oscillators
100MHz Current Feedback
Amplifier with DC Gain Control
■ Very Fast Transconductance Amplifier
Bandwidth: 75MHz
gm = 10 × ISET
Low THD: 0.2% at 30mVRMS Input
Wide ISET Range: 1A to 1mA
■ Very Fast Current Feedback Amplifier
Bandwidth: 100MHz
Slew Rate: 1000V/
s
Output Drive Current: 30mA
Differential Gain: 0.04%
Differential Phase: 0.1
°
High Input Impedance: 25M
, 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
■ Available in 8-Lead PDIP and SOIC Packages
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.
The LT1228 transconductance amplifier has a high imped-
ance differential input and a current source output with wide
output voltage compliance. The transconductance, gm, is
set by the current that flows into Pin 5, ISET. The small signal
gm is equal to ten times the value of ISETand this relationship
holds over several decades of set current. The voltage at Pin
5 is two diode drops above the negative supply, Pin 4.
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.
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
Frequency Response
FREQUENCY (Hz)
100k
–24
GAIN
(dB)
–15
–3
3
6
1M
10M
100M
LT1228 TA02
0
–6
–9
–12
–18
–21
ISET = 100A
VS = ±15V
RL = 100
ISET = 1mA
ISET = 300A
LT1228 TA01
–
+
–
+
R3A
10k
R2A
10k
R3
100
R2
100
4.7
F
R4
1.24k
R6
6.19k
R5
10k
ISET
R1
270
RG
10
RF
470
4.7
F
15V
gm
CFA
VOUT
1
8
6
3
2
7
5
4
–15V
+
–
VIN
HIGH INPUT RESISTANCE
EVEN WHEN POWER IS OFF
–18dB
< GAIN < 2dB
VIN ≤ 3VRMS
Differential Input Variable Gain Amp
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.