LT1228
100MHz Current Feedback
Amplifier with DC Gain Control
FEATURES
n
DESCRIPTION
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.
n
n
n
n
n
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
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
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 SO Packages
APPLICATIONS
n
n
n
n
n
n
Video DC Restore (Clamp) Circuits
Video Differential Input Amplifiers
Video Keyer/Fader Amplifiers
AGC Amplifiers
Tunable Filters
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.
TYPICAL APPLICATION
Differential Input Variable Gain Amp
15V
R3A
10k
R2A
10k
7
g
m
1
4
5
I
SET
4.7µF
R4
1.24k
R6
6.19k
R5
10k
8
6
3
0
–3
Frequency Response
I
SET
= 1mA
V
S
= ±15V
R
L
= 100
+
3
2
4.7µF
V
IN
+
CFA
GAIN (dB)
+
–
+
–
–6
–9
–12
–15
–18
–21
–24
100k
I
SET
= 100µA
1M
10M
100M
LT1228 • TA02
6
R
F
470Ω
V
OUT
I
SET
= 300µA
–15V
–
+
R3
100Ω
R2
100Ω
R1
270Ω
RG
10Ω
HIGH INPUT RESISTANCE
EVEN WHEN POWER IS OFF
–18dB < GAIN < 2dB
V
IN
≤ 3V
RMS
LT1228 • TA01
FREQUENCY (Hz)
1228fd
1
LT1228
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
I
OUT
–IN
+IN
V
–
1
2
3
4
g
m
+ –
8
7
6
5
GAIN
V
+
V
OUT
I
SET
Supply Voltage .......................................................
±18V
Input Current, Pins 1, 2, 3, 5, 8 (Note 8) ..............±15mA
Output Short Circuit Duration (Note 2) ......... Continuous
Operating Temperature Range
LT1228C...................................................0°C to 70°C
LT1228I ................................................–40°C to 85°C
LT1228M
(OBSOLETE)
...................... –55°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
Junction Temperature
Plastic Package................................................. 150°C
Ceramic Package
(OBSOLETE)
......................... 175°C
Lead Temperature (Soldering, 10 sec)...................300°C
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 150°C,
θ
JA
= 100°C/W (N)
T
JMAX
= 150°C,
θ
JA
= 150°C/W (N)
J8 PACKAGE
8-LEAD CERDIP
T
JMAX
= 175°C,
θ
JA
= 100°C/W (J)
OBSOLETE PACKAGE
ORDER INFORMATION
LEAD FREE FINISH
LT1228CN8#PBF
LT1228IN8#PBF
LT1228CS8#PBF
LT1228IS8#PBF
LT1228MJ8
LT1228CJ8
TAPE AND REEL
LT1228CN8#TRPBF
LT1228IN8#TRPBF
LT1228CS8#TRPBF
LT1228IS8#TRPBF
LT1228MJ8#TRPBF
LT1228CJ8#TRPBF
PART MARKING
LT1228CN8
LT1228IN8
1228
1228I
LT1228MJ8
LT1228CJ8
PACKAGE DESCRIPTION
8-Lead Plastic DIP
8-Lead Plastic DIP
8-Lead Plastic SO
8-Lead Plastic SO
OBSOLETE PACKAGE
8-Lead CERDIP
8-Lead CERDIP
–55°C to 125°C
0°C to 70°C
TEMPERATURE RANGE
0°C to 70°C
–40°C to 85°C
0°C to 70°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on nonstandard lead based finish parts.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. Current Feedback Amplifier, Pins 1, 6, 8. ±5V ≤ V
S
≤ ±15V, I
SET
= 0µA,
V
CM
= 0V unless otherwise noted.
SYMBOL
V
OS
PARAMETER
Input Offset Voltage
Input Offset Voltage Drift
I
IN+
I
IN–
e
n
i
n
Noninverting Input Current
Inverting Input Current
Input Noise Voltage Density
Input Noise Current Density
T
A
= 25°C
l
ELECTRICAL CHARACTERISTICS
T
A
= 25°C
CONDITIONS
l
l
MIN
TYP
±3
10
±0.3
±10
MAX
±10
±15
±3
±10
±65
±100
UNITS
mV
mV
µV/°C
µA
µA
µA
µA
nV/√Hz
pV/√Hz
1228fd
T
A
= 25°C
l
f = 1kHz, R
F
= 1k, R
G
= 10Ω, R
S
= 0Ω
f = 1kHz, R
F
= 1k, R
G
= 10Ω, R
S
= 10k
6
1.4
2
LT1228
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. Current Feedback Amplifier, Pins 1, 6, 8. ±5V ≤ V
S
≤ ±15V, I
SET
= 0µA,
V
CM
= 0V unless otherwise noted.
SYMBOL
R
IN
C
IN
PARAMETER
Input Resistance
Input Capacitance (Note 3)
Input Voltage Range
CONDITIONS
V
IN
= ±13V, V
S
= ±15V
V
IN
= ±3V, V
S
= ±5V
V
S
= ±5V
V
S
= ±15V, T
A
= 25°C
l
l
l
ELECTRICAL CHARACTERISTICS
MIN
2
2
±13
±12
±3
±2
55
55
55
55
TYP
25
25
6
±13.5
±3.5
69
69
2.5
MAX
UNITS
MΩ
MΩ
pF
V
V
V
V
dB
dB
dB
dB
V
S
= ±5V, T
A
= 25°C
l
CMRR
Common Mode Rejection Ratio
V
S
= ±15V, V
CM
= ±13V, T
A
= 25°C
V
S
= ±15V, V
CM
= ±12V
V
S
= ±5V, V
CM
= ±3V, T
A
= 25°C
V
S
= ±5V, V
CM
= ±2V
V
S
= ±15V, V
CM
= ±13V, T
A
= 25°C
V
S
= ±15V, V
CM
= ±12V
V
S
= ±5V, V
CM
= ±3V, T
A
= 25°C
V
S
= ±5V, V
CM
= ±2V
V
S
= ±2V to ±15V, T
A
= 25°C
V
S
= ±3V to ±15V
V
S
= ±2V to ±15V, T
A
= 25°C
V
S
= ±3V to ±15V
l
l
l
Inverting Input Current Common
Mode Rejection
2.5
l
l
l
10
10
10
10
µA/V
µA/V
µA/V
µA/V
dB
dB
PSRR
Power Supply Rejection Ratio
Noninverting Input Current Power
Supply Rejection
60
60
80
10
0.1
50
50
5
5
nA/V
nA/V
µA/V
µA/V
dB
dB
kΩ
kΩ
V
V
V
V
Inverting Input Current Power Supply V
S
= ±2V to ±15V, T
A
= 25°C
V
S
= ±3V to ±15V
Rejection
A
V
R
OL
V
OUT
Large-Signal Voltage Gain
Transresistance, ∆V
OUT
/∆I
IN–
Maximum Output Voltage Swing
V
S
= ±15V, V
OUT
= ±10V, R
LOAD
= 1k
V
S
= ±5V, V
OUT
= ±2V, R
LOAD
= 150Ω
V
S
= ±15V, V
OUT
= ±10V, R
LOAD
= 1k
V
S
= ±5V, V
OUT
= ±2V, R
LOAD
= 150Ω
V
S
= ±15V, R
LOAD
= 400Ω, T
A
= 25°C
l
l
l
l
l
l
55
55
100
100
±12
±10
±3
±2.5
30
25
300
65
65
200
200
±13.5
±3.7
65
6
500
3500
10
100
3.5
3.5
15
45
0.01
0.01
0.04
0.1
20
125
125
11
V
S
= ±5V, R
LOAD
= 150Ω, T
A
= 25°C
l
I
OUT
I
S
SR
SR
t
r
BW
t
r
Maximum Output Current
Supply Current
Slew Rate (Notes 4 and 6)
Slew Rate
Rise Time (Notes 5 and 6)
Small-Signal Bandwidth
Small-Signal Rise Time
Propagation Delay
Small-Signal Overshoot
R
LOAD
= 0Ω, T
A
= 25°C
l
mA
mA
mA
V/µs
V/µs
ns
MHz
ns
ns
%
ns
%
DEG
%
DEG
V
OUT
= 0V, I
SET
= 0V
T
A
= 25°C
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 400Ω
T
A
= 25°C
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 100Ω
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 100Ω
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 100Ω
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 100Ω
0.1%, V
OUT
= 10V, R
F
=1k, R
G
= 1k, R
L
=1k
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 1k
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 1k
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 150Ω
V
S
= ±15V, R
F
= 750Ω, R
G
= 750Ω, R
L
= 150Ω
l
t
S
Settling Time
Differential Gain (Note 7)
Differential Phase (Note 7)
Differential Gain (Note 7)
Differential Phase (Note 7)
1228fd
3
LT1228
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. Transconductance Amplifier, Pins 1, 2, 3, 5. ±5V ≤ V
S
≤ ±15V,
I
SET
= 100µA, V
CM
= 0V unless otherwise noted.
SYMBOL
V
OS
PARAMETER
Input Offset Voltage
Input Offset Voltage Drift
Input Offset Current
Input Bias Current
Input Noise Voltage Density
Input Resistance-Differential Mode
Input Resistance-Common Mode
Input Capacitance
Input Voltage Range
CONDITIONS
I
SET
= 1mA, T
A
= 25°C
l
l
ELECTRICAL CHARACTERISTICS
MIN
TYP
±0.5
10
40
0.4
MAX
±5
±10
200
500
1
5
I
OS
I
B
e
n
R
IN
T
A
= 25°C
l
T
A
= 25°C
l
f = 1kHz
V
IN
≈ ±30mV
V
S
= ±15V, V
CM
= ±12V
V
S
= ±5V, V
CM
= ±2V
V
S
= ±15V, T
A
= 25°C
V
S
= ±15V
V
S
= ±5V, T
A
= 25°C
V
S
= ±5V
V
S
= ±15V, V
CM
= ±13V, T
A
= 25°C
V
S
= ±15V, V
CM
= ±12V
V
S
= ±5V, V
CM
= ±3V, T
A
= 25°C
V
S
= ±5V, V
CM
= ±2V
V
S
= ±2V to ±15V, T
A
= 25°C
V
S
= ±3V to ±15V
I
SET
= 100µA, I
OUT
= ±30µA, T
A
= 25°C
I
SET
= 100µA
I
SET
= 0µA (+I
IN
of CFA), T
A
= 25°C
V
S
= ±15V , R1 = ∞
V
S
= ±5V , R1 = ∞
V
S
= ±15V, V
OUT
= ±13V
V
S
= ±5V, V
OUT
= ±3V
V
S
= ±5V
I
SET
= 1mA
V
IN
= 30mV
RMS
at 1kHz, R1 = 100k
R1 = 50Ω, I
SET
= 500µA
R1 = 50Ω, I
SET
= 500µA, 10% to 90%
R1 = 50Ω, I
SET
= 500µA, 50% to 50%
l
l
l
30
50
50
±13
±12
±3
±2
60
60
60
60
60
60
0.75
70
C
IN
20
200
1000
1000
3
±14
±4
100
100
100
1.00
–0.33
100
0.3
±14
±4
8
8
6
9
0.2
80
5
5
1.25
130
3
10
l
l
l
l
l
CMRR
Common Mode Rejection Ratio
PSRR
g
m
I
OUT
I
OL
V
OUT
R
O
Power Supply Rejection Ratio
Transconductance
Transconductance Drift
Maximum Output Current
Output Leakage Current
Maximum Output Voltage Swing
Output Resistance
Output Capacitance (Note 3)
Supply Current, Both Amps
Total Harmonic Distortion
Small-Signal Bandwidth
Small-Signal Rise Time
Propagation Delay
l
l
l
l
l
l
l
±13
±3
2
2
I
S
THD
BW
t
r
l
15
UNITS
mV
mV
µV/°C
nA
nA
µA
µA
nV/√Hz
kΩ
MΩ
MΩ
pF
V
V
V
V
dB
dB
dB
dB
dB
dB
µA/mV
%/°C
µA
µA
µA
V
V
MΩ
MΩ
pF
mA
%
MHz
ns
ns
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
A heat sink may be required depending on the power supply voltage.
Note 3:
This is the total capacitance at Pin 1. It includes the input capacitance
of the current feedback amplifier and the output capacitance of the
transconductance amplifier.
Note 4:
Slew rate is measured at ±5V on a ±10V output signal while operating
on ±15V supplies with R
F
= 1k, R
G
= 110Ω and R
L
= 400Ω. The slew rate is
much higher when the input is overdriven, see the Applications Information
section.
Note 5:
Rise time is measured from 10% to 90% on a ±500mV output signal
while operating on ±15V supplies with R
F
= 1k, R
G
= 110Ω and R
L
= 100Ω.
This condition is not the fastest possible, however, it does guarantee the
internal capacitances are correct and it makes automatic testing practical.
Note 6:
AC parameters are 100% tested on the ceramic and plastic DIP
packaged parts (J and N suffix) and are sample tested on every lot of the SO
packaged parts (S suffix).
Note 7:
NTSC composite video with an output level of 2V.
Note 8:
Back to back 6V Zener diodes are connected between Pins 2 and 3
for ESD protection.
1228fd
4
LT1228
TYPICAL PERFORMANCE CHARACTERISTICS
Small-Signal Bandwidth
vs Set Current
100
V
S
= ±15V
R1 = 100
TRANSCONDUCTANCE (µA/mV)
R1 = 1k
–3dB BANDWIDTH (MHz)
10
R1 = 10k
1
100
10
Transconductance Amplifier, Pins 1, 2, 3, 5
Small-Signal Transconductance
vs DC Input Voltage
10000
TRANSCONDUCTANCE (µA/mV)
1000
SET CURRENT (µA)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
V
S
= ±2V TO ±15V
I
SET
= 100µA
–55°C
25°C
125°C
Small-Signal Transconductance
and Set Current vs Bias Voltage
V
S
= ±2V TO ±15V
T
A
= 25°C
1
100
0.1
0.01
10
1.0
0.1
1.5
R1 = 100k
0.1
10
100
SET CURRENT (µA)
LT1228 • TPC01
1000
0.001
0.9
1.0
1.1
1.2
1.3
1.4
0
–200 –150 –100 –50
0
50
100 150 200
LT1228 • TPC03
BIAS VOLTAGE, PIN 5 TO 4, (V)
LT1228 • TPC02
INPUT VOLTAGE (mVDC)
Total Harmonic Distortion
vs Input Voltage
10
V
S
= ±15V
1000
Spot Output Noise Current
vs Frequency
V
S
= ±2V TO ±15V
T
A
= 25°C
COMMON MODE RANGE (V)
V
+
–0.5
–1.0
–1.5
–2.0
Input Common Mode Limit
vs Temperature
V
+
= 2V TO 15V
OUTPUT DISTORTION (%)
SPOT NOISE (pA/√Hz)
1
I
SET
= 100µA
I
SET
= 1mA
100
2.0
1.5
1.0
0.5
V
–
–50
V
–
= –2V TO –15V
0.1
I
SET
= 100µA
I
SET
= 1mA
0.01
1
10
100
1000
LT1228 • TPC04
10
10
100
1k
FREQUENCY (Hz)
10k
100k
LT1228 • TPC05
–25
0
25
50
75
100
125
INPUT VOLTAGE (mV
P–P
)
TEMPERATURE (°C)
LT1228 • TPC06
Small-Signal Control Path
Bandwidth vs Set Current
100
V
S
= ±2V TO ±15V
V
IN
= 200mV
(PIN 2 TO 3)
1.0
0.9
CONTROL PATH GAIN (µA/µA)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1
10
100
SET CURRENT (µA)
LT1228 • TPC07
Small-Signal Control Path
Gain vs Input Voltage
V
+
OUTPUT SATURATION VOLTAGE (V)
–0.5
–1.0
Output Saturation Voltage
vs Temperature
–3dB BANDWIDTH (MHz)
10
∆I
OUT
∆I
SET
∆I
OUT
∆I
SET
±2V ≤ V
S
≤ ±15V
R1 =
∞
+1.0
+0.5
V
–
–50
–25
0
25
50
75
100
125
1000
0
0
40
80
120
160
200
INPUT VOLTAGE, PIN 2 TO 3, (mVDC)
LT1228 • TPC08
TEMPERATURE (°C)
LT1228 • TPC09
1228fd
5
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