LT1228_15 Datasheet, PDF (1/22 Pages) Linear Technology – 100MHz Current Feedback Amplifier with DC Gain Control

LT1228
100MHz Current Feedback
Amplifier with DC Gain Control
FEATURES
DESCRIPTION
n _{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
n _{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
n _{Wide Supply Range: ±2V to ±15V}
n _{Inputs Common Mode to Within 1.5V of Supplies}
n _{Outputs Swing Within 0.8V of Supplies}
n _{Supply Current: 7mA}
n _{Available in 8-Lead PDIP and SO Packages}
APPLICATIONS
n _{Video DC Restore (Clamp) Circuits}
n _{Video Differential Input Amplifiers}
n _{Video Keyer/Fader Amplifiers}
n _{AGC Amplifiers}
n _{Tunable Filters}
n _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.
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 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 out-
put of the transconductance amplifier. The current feedback
amplifier maintains its wide bandwidth over a wide range of
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,
such as cables, with excellent linearity at high frequencies.
TYPICAL APPLICATION
Differential Input Variable Gain Amp
15V
4.7µF
R3A
10k
+
V_IN
–
R2A
10k
R3
100Ω
3₊ 7
g_m
2_–
5
4
–15V
R2
100Ω
+
4.7µF
R4
1.24k
R6
6.19k
1₊
6
CFA
V_OUT
I_SET
8_–
R_F
R5
470Ω
10k
R1
270Ω
R_G
10Ω
HIGH INPUT RESISTANCE
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
1228fd
1