LT1228 Datasheet, PDF (1/20 Pages) Linear Technology – 100MHz Current Feedback Amplifier with DC Gain Control

FEATURES
s _{Very Fast Transconductance Amplifier}
Bandwidth: 75MHz
g_m = 10 × I_SET
Low THD: 0.2% at 30mV_RMS Input
Wide I_SET Range: 1µA to 1mA
s _{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
s _{Wide Supply Range: ±2V to ±15V}
s _{Inputs Common Mode to Within 1.5V of Supplies}
s _{Outputs Swing Within 0.8V of Supplies}
s _{Supply Current: 7mA}
APPLICATI S
s _{Video DC Restore (Clamp) Circuits}
s _{Video Differential Input Amplifiers}
s _{Video Keyer/Fader Amplifiers}
s _{AGC Amplifiers}
s _{Tunable Filters}
s _Oscillators
LT1228
100MHz Current Feedback
Amplifier with DC Gain Control
DESCRIPTIO
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.
The LT1228 transconductance amplifier has a high imped-
ance differential input and a current source output with wide
output voltage compliance. The transconductance, g_m, is
set by the current that flows into pin 5, I_SET. The small signal
g_m is equal to ten times the value of I_SET and 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.
TYPICAL APPLICATI
Differential Input Variable Gain Amp
R3A
10k
+
V_IN R2A
10k
–
R3
100Ω
15V
4.7µF
3₊ 7
g_m
2_–
5
4
–15V
R2
100Ω
+
4.7µF
R4
1.24k
R6
6.19Ω
1₊
6
CFA
V_OUT
I_SET
8_–
R_F
R5
470Ω
10k
R1
R_{G HIGH INPUT RESISTANCE}
270Ω 10Ω _{EVEN WHEN POWER IS OFF}
–18dB < GAIN < 2dB
V_IN ≤ 3V_RMS
LT1228 • TA01
Frequency Response
6
3
0
I_SET = 1mA
V_S = ±15V
R_L = 100Ω
–3
–6
–9
–12
I_SET = 300µA
–15
–18
I_SET = 100µA
–21
–24
100k
1M
10M
100M
FREQUENCY (Hz)
LT1228 • TA02
1