LT1228 Linear Technology, LT1228 Datasheet - Page 15

• Manufacturer Part Number: LT1228
• Description: 100MHz Current Feedback Amplifier with DC Gain Control
• Manufacturer: Linear Technology

Video Fader
The video fader uses the transconductance amplifiers
from two LT1228s in the feedback loop of another current
feedback amplifier, the LT1223. The amount of signal
from each input at the output is set by the ratio of the
set currents of the two LT1228s, not by their absolute
value. The bandwidth of the current feedback amplifier
is inversely proportional to the set current in this
configuration. Therefore, the set currents remain high
over most of the pot’s range, keeping the bandwidth over
15MHz even when the signal is attenuated 20dB. The pot
is set up to completely turn off one LT1228 at each end of
the rotation.
TYPICAL
V
V
IN1
IN2
100
100
1k
1k
–5V
1k
1k
A
PPLICATI
3
2
3
2
+
–
+
–
10k
10k
g
g
m
m
5
5
10k
5.1k
5.1k
1
1
O
U
V
S
S
= ±5V
+
–
LT1223
CFA
LT1228 • TA12
V
OUT
Video DC Restore (Clamp) Circuit
The video restore (clamp) circuit restores the black level of
the composite video to zero volts at the beginning of every
line. This is necessary because AC coupled video changes
DC level as a function of the average brightness of the
picture. DC restoration also rejects low frequency noise
such as hum.
The circuit has two inputs: composite video and a logic
signal. The logic signal is high except during the back
porch time right after the horizontal sync pulse. While the
logic is high, the PNP is off and I
to zero the feedback to pin 2 has no affect. The video input
drives the noninverting input of the current feedback
amplifier whose gain is set by R
signal is low, the PNP turns on and I
Then the transconductance amplifier charges the capaci-
tor to force the output to match the voltage at pin 3, in this
case zero volts.
This circuit can be modified so that the video is DC coupled
by operating the amplifier in an inverting configuration.
Just ground the video input shown and connect R
video input instead of to ground.
RESTORE
LOGIC
INPUT
NOT NECESSARY IF THE SOURCE RESISTANCE IS LESS THAN 50
1000pF
2N3906
200
3
2
+
–
4
5V
V
–
3k
3k
g
m
10k
V
+
7
5
VIDEO
INPUT
SET
0.01 F
F
and R
SET
is zero. With I
R
1
8
G
+
–
goes to about 1mA.
G
CFA
. When the logic
R
F
LT1228
SET
6
LT1228 • TA13
15
G
to the
equal
V
OUT