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.
Frequency Response
6
3
0
–3
1
4
5
8
I
SET
4.7µF
R4
1.24k
R6
6.19Ω
R5
10k
FEATURES
s
s
s
s
s
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
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
APPLICATI
s
s
s
s
s
s
S
Video DC Restore (Clamp) Circuits
Video Differential Input Amplifiers
Video Keyer/Fader Amplifiers
AGC Amplifiers
Tunable Filters
Oscillators
TYPICAL APPLICATI
R3A
10k
R2A
10k
Differential Input Variable Gain Amp
15V
4.7µF
+
3
g
m
2
V
IN
+
CFA
6
R
F
470Ω
HIGH INPUT RESISTANCE
EVEN WHEN POWER IS OFF
–18dB
<
GAIN
<
2dB
V
IN
≤
3V
RMS
LT1228 • TA01
GAIN (dB)
+
–
+
–
7
V
OUT
–15V
R3
100Ω
R2
100Ω
–
R1
270Ω
RG
10Ω
U
UO
+
UO
V
S
= ±15V
R
L
= 100Ω
I
SET
= 1mA
–6
–9
–12
–15
–18
–21
–24
100k
I
SET
= 100µA
I
SET
= 300µA
1M
10M
100M
LT1228 • TA02
FREQUENCY (Hz)
1
LT1228
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
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 7) ............
±15mA
Output Short Circuit Duration (Note 1) ......... Continuous
Operating Temperature Range
LT1228C ................................................ 0°C to 70°C
LT1228M ........................................ –55°C to 125°C
Storage Temperature Range ................. –65°C to 150°C
Junction Temperature
Plastic Package .............................................. 150°C
Ceramic Package ............................................ 175°C
Lead Temperature (Soldering, 10 sec).................. 300°C
N8 PACKAGE
J8 PACKAGE
8-LEAD PLASTIC DIP
8-LEAD CERAMIC DIP
S8 PACKAGE
8-LEAD PLASTIC SOIC
LT1228 • POI01
ORDER PART
NUMBER
LT1228MJ8
LT1228CJ8
LT1228CN8
LT1228CS8
S8 PART MARKING
1228
T
J MAX
= 175°C,
θ
JA
= 100°C/W (J)
T
J MAX
= 150°C,
θ
JA
= 100°C/W (N)
T
J MAX
= 150°C,
θ
JA
= 150°C/W (S)
Consult Factory for Industrial grade parts.
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
R
IN
C
IN
Noninverting Input Current
Inverting Input Current
Input Noise Voltage Density
Input Noise Current Density
Input Resistance
Input Capacitance (Note 2)
Input Voltage Range
T
A
= 25°C
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
T
A
= 25°C
q
q
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
MΩ
MΩ
pF
V
V
V
V
dB
dB
dB
dB
T
A
= 25°C
q
f = 1kHz, R
F
= 1k, R
G
= 10Ω, R
S
= 0Ω
f = 1kHz, R
F
= 1k, R
G
= 10Ω, R
S
= 10k
V
IN
=
±13V,
V
S
=
±15V
V
IN
=
±3V,
V
S
=
±5V
V
S
=
±5V
V
S
=
±15V,
T
A
= 25°C
q
q
q
6
1.4
2
2
±13
±12
±3
±2
55
55
55
55
25
25
6
±13.5
±3.5
69
69
2.5
q
V
S
=
±5V,
T
A
= 25°C
q
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
V
S
=
±2V
to
±15V,
T
A
= 25°C
V
S
=
±3V
to
±15V
q
q
Inverting Input Current
Common-Mode Rejection
2.5
q
q
q
10
10
10
10
PSRR
Power Supply Rejection Ratio
Noninverting Input Current
Power Supply Rejection
Inverting Input Current
Power Supply Rejection
60
60
80
10
0.1
50
50
5
5
q
2
U
µA/V
µA/V
µA/V
µA/V
dB
dB
nA/V
nA/V
µA/V
µA/V
W
U
U
W W
W
LT1228
Current Feedback Amplifier, Pins 1, 6, 8.
±5V ≤
V
S
≤ ±15V,
I
SET
= 0µA, V
CM
= 0V unless otherwise noted.
SYMBOL
A
V
R
OL
V
OUT
PARAMETER
Large-Signal Voltage Gain
Transresistance,
∆V
OUT
/∆I
IN–
Maximum Output Voltage Swing
CONDITIONS
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
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
55
55
100
100
±12
±10
±3
±2.5
30
25
300
TYP
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
MAX
UNITS
dB
dB
kΩ
kΩ
V
V
V
V
V
S
=
±5V,
R
LOAD
= 150Ω, T
A
= 25°C
q
I
OUT
I
s
SR
SR
t
r
BW
t
r
Maximum Output Current
Supply Current
Slew Rate (Notes 3 and 5)
Slew Rate
Rise Time (Notes 4 and 5)
Small-Signal Bandwidth
Small-Signal Rise Time
Propagation Delay
Small-Signal Overshoot
R
LOAD
= 0Ω, T
A
= 25°C
q
125
125
11
mA
mA
mA
V/µs
V/µs
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
q
20
ns
MHz
ns
ns
%
ns
%
DEG
%
DEG
t
s
Settling Time
Differential Gain (Note 6)
Differential Phase (Note 6)
Differential Gain (Note 6)
Differential Phase (Note 6)
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
I
OS
I
B
e
n
R
IN
Input Offset Current
Input Bias Current
Input Noise Voltage Density
Input Resistance-Differential Mode
Input Resistance-Common Mode
C
IN
Input Capacitance
Input Voltage Range
V
S
=
±15V,
T
A
= 25°C
V
S
=
±15V
V
S
=
±5V,
T
A
= 25°C
V
S
=
±5V
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
I
SET
= 1mA, T
A
= 25°C
q
q
MIN
TYP
±0.5
10
40
MAX
±5
±10
200
500
1
5
UNITS
mV
mV
µV/°C
nA
nA
µA
µA
nV/√Hz
kΩ
MΩ
MΩ
pF
V
V
V
V
T
A
= 25°C
q
T
A
= 25°C
q
0.4
20
q
q
q
f = 1kHz
V
IN
≈ ±30mV
V
S
=
±15V,
V
CM
=
±12V
V
S
=
±5V,
V
CM
=
±2V
30
50
50
±13
±12
±3
±2
200
1000
1000
3
±14
±4
3
LT1228
Transconductance Amplifier, Pins 1, 2, 3, 5.
±5V ≤
V
S
≤ ±15V,
I
SET
= 100µA, V
CM
= 0V unless otherwise noted.
SYMBOL
CMRR
PARAMETER
Common-Mode Rejection Ratio
CONDITIONS
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
q
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
60
60
60
60
60
60
0.75
q
q
TYP
100
100
100
1.00
–0.33
100
0.3
MAX
UNITS
dB
dB
dB
dB
dB
dB
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 2)
1.25
130
3
10
µA/mV
%/°C
µA
µA
µA
V
V
MΩ
MΩ
pF
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%
70
q
q
q
q
±13
±3
2
2
±14
±4
8
8
6
9
0.2
80
5
5
15
I
S
THD
BW
t
r
Supply Current, Both Amps
Total Harmonic Distortion
Small-Signal Bandwidth
Small-Signal Rise Time
Propagation Delay
q
mA
%
MHz
ns
ns
The
q
denotes specifications which apply over the operating temperature
range.
Note 1:
A heat sink may be required depending on the power supply
voltage.
Note 2:
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 3:
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
section.
Note 4:
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 5:
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 6:
NTSC composite video with an output level of 2V.
Note 7:
Back to back 6V Zener diodes are connected between pins 2 and
3 for ESD protection.
4
LT1228
TYPICAL PERFOR A CE CHARACTERISTICS
Transconductance Amplifier, Pins 1, 2, 3 & 5
Small-Signal Bandwidth vs
Set Current
100
V
S
= ±15V
R1 = 100Ω
TRANSCONDUCTANCE (µA/mV)
R1 = 1k
TRANSCONDUCTANCE (µA/mV)
–3dB BANDWIDTH (MHz)
10
R1 = 10k
1
R1 = 100k
0.1
10
100
SET CURRENT (µA)
LT1228 • TPC01
Total Harmonic Distortion vs
Input Voltage
10
V
S
= ±15V
COMMON-MODE RANGE (V)
OUTPUT DISTORTION (%)
SPOT NOISE (pA/√Hz)
1
I
SET
= 100µA
0.1
I
SET
= 1mA
0.01
1
10
100
1000
LT1228 • TPC04
INPUT VOLTAGE (mV
P–P
)
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
OUTPUT SATURATION VOLTAGE (V)
CONTROL PATH GAIN (µA/µA)
–3dB BANDWIDTH (MHz)
10
∆I
OUT
∆I
SET
1
10
100
SET CURRENT (µA)
LT1228 • TPC07
U W
Small-Signal Transconductance
and Set Current vs Bias Voltage
100
V
S
= ±2V TO ±15V
T
A
= 25°C
10
1000
SET CURRENT (µA)
Small-Signal Transconductance
vs DC Input Voltage
10000
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
125°C
25°C
V
S
= ±2V TO ±15V
I
SET
= 100µA
–55°C
1
100
0.1
10
0.01
1.0
1000
0.001
0.9
1.0
1.1
1.2
1.3
1.4
0.1
1.5
0
–200 –150 –100 –50
0
50
100 150 200
LT1228 • TPC03
BIAS VOLTAGE, PIN 5 TO 4, (V)
LT1228 • TPC02
INPUT VOLTAGE (mVDC)
Spot Output Noise Current vs
Frequency
1000
V
S
= ±2V TO ±15V
T
A
= 25°C
V
+
–0.5
–1.0
–1.5
–2.0
Input Common-Mode Limit vs
Temperature
V
+
= 2V TO 15V
I
SET
= 1mA
100
2.0
1.5
1.0
0.5
V
–
= –2V TO –15V
I
SET
= 100µA
10
10
100
1k
FREQUENCY (Hz)
LT1228 • TPC05
10k
100k
V
–
–50
–25
0
25
50
75
100
125
TEMPERATURE (°C)
LT1228 • TPC06
Small-Signal Control Path
Gain vs Input Voltage
V
+
–0.5
–1.0
Output Saturation Voltage vs
Temperature
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
∆I
OUT
∆I
SET
±2V
≤
V
S
≤
±15V
R1 =
∞
+1.0
+0.5
V
–
–50
1000
0
0
40
80
120
160
200
INPUT VOLTAGE, PIN 2 TO 3, (mVDC)
LT1228 • TPC08
–25
0
25
50
75
100
125
TEMPERATURE (°C)
LT1228 • TPC09
5
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