1
LT1228
100MHz Current Feedback
Amplifier with DC Gain Control
U
S
A
O
PPLICATI
s
Very Fast Transconductance Amplifier
Bandwidth: 75MHz
g
m
= 10
I
SET
Low THD: 0.2% at 30mV
RMS
Input
Wide I
SET
Range: 1
A to 1mA
s
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
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
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.
S
FEATURE
D
U
ESCRIPTIO
s
Video DC Restore (Clamp) Circuits
s
Video Differential Input Amplifiers
s
Video Keyer/Fader Amplifiers
s
AGC Amplifiers
s
Tunable Filters
s
Oscillators
Frequency Response
U
A
O
PPLICATI
TYPICAL
FREQUENCY (Hz)
100k
24
GAIN (dB)
15
3
3
6
1M
10M
100M
LT1228 TA02
0
6
9
12
18
21
I
SET
= 100
A
V
S
= 15V
R
L
= 100
I
SET
= 1mA
I
SET
= 300
A
LT1228 TA01
+
+
+
+
R3A
10k
R2A
10k
R3
100
R2
100
4.7
F
R4
1.24k
R6
6.19
R5
10k
I
SET
R1
270
R
G
10
R
F
470
4.7
F
15V
g
m
CFA
V
OUT
1
8
6
3
2
7
5
4
15V
+
V
IN
HIGH INPUT RESISTANCE
EVEN WHEN POWER IS OFF
18dB
<
GAIN
<
2dB
V
IN
3V
RMS
Differential Input Variable Gain Amp
LT1228
2
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
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
W
U
U
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
LT1228MJ8
LT1228CJ8
LT1228CN8
LT1228CS8
S8 PART MARKING
1228
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
OS
Input Offset Voltage
T
A
= 25
C
3
10
mV
q
15
mV
Input Offset Voltage Drift
q
10
V/
C
I
IN
+
Noninverting Input Current
T
A
= 25
C
0.3
3
A
q
10
A
I
IN
Inverting Input Current
T
A
= 25
C
10
65
A
q
100
A
e
n
Input Noise Voltage Density
f = 1kHz, R
F
= 1k, R
G
= 10
, R
S
= 0
6
nV/
Hz
i
n
Input Noise Current Density
f = 1kHz, R
F
= 1k, R
G
= 10
, R
S
= 10k
1.4
pV/
Hz
R
IN
Input Resistance
V
IN
=
13V, V
S
=
15V
q
2
25
M
V
IN
=
3V, V
S
=
5V
q
2
25
M
C
IN
Input Capacitance (Note 2)
V
S
=
5V
6
pF
Input Voltage Range
V
S
=
15V, T
A
= 25
C
13
13.5
V
q
12
V
V
S
=
5V, T
A
= 25
C
3
3.5
V
q
2
V
CMRR
Common-Mode Rejection Ratio
V
S
=
15V, V
CM
=
13V, T
A
= 25
C
55
69
dB
V
S
=
15V, V
CM
=
12V
q
55
dB
V
S
=
5V, V
CM
=
3V, T
A
= 25
C
55
69
dB
V
S
=
5V, V
CM
=
2V
q
55
dB
Inverting Input Current
V
S
=
15V, V
CM
=
13V, T
A
= 25
C
2.5
10
A/V
Common-Mode Rejection
V
S
=
15V, V
CM
=
12V
q
10
A/V
V
S
=
5V, V
CM
=
3V, T
A
= 25
C
2.5
10
A/V
V
S
=
5V, V
CM
=
2V
q
10
A/V
PSRR
Power Supply Rejection Ratio
V
S
=
2V to
15V, T
A
= 25
C
60
80
dB
V
S
=
3V to
15V
q
60
dB
Noninverting Input Current
V
S
=
2V to
15V, T
A
= 25
C
10
50
nA/V
Power Supply Rejection
V
S
=
3V to
15V
q
50
nA/V
Inverting Input Current
V
S
=
2V to
15V, T
A
= 25
C
0.1
5
A/V
Power Supply Rejection
V
S
=
3V to
15V
q
5
A/V
ELECTRICAL C
C
HARA TERISTICS
Current Feedback Amplifier, Pins 1, 6, 8.
5V
V
S
15V, I
SET
= 0
A, V
CM
= 0V unless otherwise noted.
T
J MAX
=
175
C,
JA
=
100
C/W (J)
T
J MAX
=
150
C,
JA
=
100
C/W (N)
T
J MAX
=
150
C,
JA
=
150
C/W (S)
8
7
6
5
4
3
2
1
I
OUT
IN
+IN
V
I
SET
V
OUT
V
+
GAIN
TOP VIEW
N8 PACKAGE
8-LEAD PLASTIC DIP
S8 PACKAGE
8-LEAD PLASTIC SOIC
LT1228 POI01
J8 PACKAGE
8-LEAD CERAMIC DIP
+
g
m
Consult Factory for Industrial grade parts.
3
LT1228
ELECTRICAL C
C
HARA TERISTICS
Current Feedback Amplifier, Pins 1, 6, 8.
5V
V
S
15V, I
SET
= 0
A, V
CM
= 0V unless otherwise noted.
Transconductance Amplifier, Pins 1, 2, 3, 5.
5V
V
S
15V, I
SET
= 100
A, V
CM
= 0V unless otherwise noted.
ELECTRICAL C
C
HARA TERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
OS
Input Offset Voltage
I
SET
= 1mA, T
A
= 25
C
0.5
5
mV
q
10
mV
Input Offset Voltage Drift
q
10
V/
C
I
OS
Input Offset Current
T
A
= 25
C
40
200
nA
q
500
nA
I
B
Input Bias Current
T
A
= 25
C
0.4
1
A
q
5
A
e
n
Input Noise Voltage Density
f = 1kHz
20
nV/
Hz
R
IN
Input Resistance-Differential Mode
V
IN
30mV
q
30
200
k
Input Resistance-Common Mode
V
S
=
15V, V
CM
=
12V
q
50
1000
M
V
S
=
5V, V
CM
=
2V
q
50
1000
M
C
IN
Input Capacitance
3
pF
Input Voltage Range
V
S
=
15V, T
A
= 25
C
13
14
V
V
S
=
15V
q
12
V
V
S
=
5V, T
A
= 25
C
3
4
V
V
S
=
5V
q
2
V
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
A
V
Large-Signal Voltage Gain
V
S
=
15V, V
OUT
=
10V, R
LOAD
= 1k
q
55
65
dB
V
S
=
5V, V
OUT
=
2V, R
LOAD
= 150
q
55
65
dB
R
OL
Transresistance,
V
OUT
/
I
IN
V
S
=
15V, V
OUT
=
10V, R
LOAD
= 1k
q
100
200
k
V
S
=
5V, V
OUT
=
2V, R
LOAD
= 150
q
100
200
k
V
OUT
Maximum Output Voltage Swing
V
S
=
15V, R
LOAD
= 400
, T
A
= 25
C
12
13.5
V
q
10
V
V
S
=
5V, R
LOAD
= 150
, T
A
= 25
C
3
3.7
V
q
2.5
V
I
OUT
Maximum Output Current
R
LOAD
= 0
, T
A
= 25
C
30
65
125
mA
q
25
125
mA
I
s
Supply Current
V
OUT
= 0V, I
SET
= 0V
q
6
11
mA
SR
Slew Rate (Notes 3 and 5)
T
A
= 25
C
300
500
V/
s
SR
Slew Rate
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 400
3500
V/
s
t
r
Rise Time (Notes 4 and 5)
T
A
= 25
C
10
20
ns
BW
Small-Signal Bandwidth
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 100
100
MHz
t
r
Small-Signal Rise Time
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 100
3.5
ns
Propagation Delay
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 100
3.5
ns
Small-Signal Overshoot
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 100
15
%
t
s
Settling Time
0.1%, V
OUT
= 10V, R
F
=1k, R
G
= 1k, R
L
=1k
45
ns
Differential Gain (Note 6)
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 1k
0.01
%
Differential Phase (Note 6)
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 1k
0.01
DEG
Differential Gain (Note 6)
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 150
0.04
%
Differential Phase (Note 6)
V
S
=
15V, R
F
= 750
, R
G
= 750
, R
L
= 150
0.1
DEG
LT1228
4
ELECTRICAL C
C
HARA TERISTICS
Transconductance Amplifier, Pins 1, 2, 3, 5.
5V
V
S
15V, I
SET
= 100
A, V
CM
= 0V unless otherwise noted.
CMRR
Common-Mode Rejection Ratio
V
S
=
15V, V
CM
=
13V, T
A
= 25
C
60
100
dB
V
S
=
15V, V
CM
=
12V
q
60
dB
V
S
=
5V, V
CM
=
3V, T
A
= 25
C
60
100
dB
V
S
=
5V, V
CM
=
2V
q
60
dB
PSRR
Power Supply Rejection Ratio
V
S
=
2V to
15V, T
A
= 25
C
60
100
dB
V
S
=
3V to
15V
q
60
dB
g
m
Transconductance
I
SET
= 100
A, I
OUT
=
30
A, T
A
= 25
C
0.75
1.00
1.25
A/mV
Transconductance Drift
q
0.33
%/
C
I
OUT
Maximum Output Current
I
SET
= 100
A
q
70
100
130
A
I
OL
Output Leakage Current
I
SET
= 0
A (+I
IN
of CFA), T
A
= 25
C
0.3
3
A
q
10
A
V
OUT
Maximum Output Voltage Swing
V
S
=
15V , R1 =
q
13
14
V
V
S
=
5V , R1 =
q
3
4
V
R
O
Output Resistance
V
S
=
15V, V
OUT
=
13V
q
2
8
M
V
S
=
5V, V
OUT
=
3V
q
2
8
M
Output Capacitance (Note 2)
V
S
=
5V
6
pF
I
S
Supply Current, Both Amps
I
SET
= 1mA
q
9
15
mA
THD
Total Harmonic Distortion
V
IN
= 30mV
RMS
at 1kHz, R1 = 100k
0.2
%
BW
Small-Signal Bandwidth
R1 = 50
,
I
SET
= 500
A
80
MHz
t
r
Small-Signal Rise Time
R1 = 50
,
I
SET
= 500
A, 10% to 90%
5
ns
Propagation Delay
R1 = 50
,
I
SET
= 500
A, 50% to 50%
5
ns
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
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
= 110
and R
L
= 400
. 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
= 110
and
R
L
= 100
. 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.
5
LT1228
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
Transconductance Amplifier, Pins 1, 2, 3 & 5
FREQUENCY (Hz)
10
10
SPOT NOISE (pA/
Hz)
100
1000
1k
100k
LT1228 TPC05
V
S
= 2V TO 15V
T
A
= 25C
100
10k
I
SET
= 1mA
I
SET
= 100
A
Small-Signal Bandwidth vs
Small-Signal Transconductance
Small-Signal Transconductance
Set Current
and Set Current vs Bias Voltage
vs DC Input Voltage
TEMPERATURE (C)
50
V
COMMON-MODE RANGE (V)
0.5
1.0
1.5
V
+
25
0
25
125
LT1228 TPC06
50
75
100
0.5
1.0
2.0
1.5
2.0
V
= 2V TO 15V
V
+
= 2V TO 15V
INPUT VOLTAGE (mVDC)
200
0
TRANSCONDUCTANCE (
A/mV)
0.2
0.4
1.4
2.0
150 100 50
200
LT1228 TPC03
0
100 150
1.8
1.6
1.2
0.6
0.8
55C
V
S
= 2V TO 15V
I
SET
= 100
A
50
1.0
25C
125C
SET CURRENT (
A)
10
0.1
3dB BANDWIDTH (MHz)
1
10
100
100
1000
LT1228 TPC01
R1 = 100k
R1 = 10k
R1 = 1k
R1 = 100
V
S
= 15V
BIAS VOLTAGE, PIN 5 TO 4, (V)
0.01
TRANSCONDUCTANCE (
A/mV)
0.1
1
10
100
0.9
1.2
1.3
1.5
LT1228 TPC02
0.001
1.0
1.1
1.4
V
S
= 2V TO 15V
T
A
= 25C
1.0
10
100
1000
10000
0.1
SET CURRENT (
A)
Total Harmonic Distortion vs
Spot Output Noise Current vs
Input Common-Mode Limit vs
Input Voltage
Frequency
Temperature
INPUT VOLTAGE (mV
PP
)
1
0.01
OUTPUT DISTORTION (%)
0.1
1
10
10
1000
LT1228 TPC04
I
SET
= 100
A
V
S
= 15V
I
SET
= 1mA
100
Small-Signal Control Path
Small-Signal Control Path
Output Saturation Voltage vs
Bandwidth vs Set Current
Gain vs Input Voltage
Temperature
INPUT VOLTAGE, PIN 2 TO 3, (mVDC)
0
0
CONTROL PATH GAIN (
A/
A)
1.0
120
200
LT1228 TPC08
I
OUT
I
SET
40
80
160
0.2
0.4
0.6
0.8
0.9
0.7
0.5
0.3
0.1
TEMPERATURE (C)
50
V
OUTPUT SATURATION VOLTAGE (V) +0.5
+1.0
1.0
V
+
25
0
25
125
LT1228 TPC09
50
75
100
0.5
2V
V
S
15V
R1 =
SET CURRENT (
A)
10
1
3dB BANDWIDTH (MHz)
10
100
100
1000
LT1228 TPC07
V
S
= 2V TO 15V
V
IN
= 200mV
(PIN 2 TO 3)
I
OUT
I
SET
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