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LT1228IN8

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

LT1228

TYPICAL APPLICATIONS

Video Fader

1k

VIN1

3+

gm

1

2–

5

100Ω 1k

+

LT1223

CFA

–

10k

5.1k

–5V

10k

10k

5.1k

1k

1k

VIN2

100Ω

3+

5

gm

1

2–

VS = ±5V

VOUT

LT1228 • TA12

Video DC Restore (Clamp) Circuit

NOT NECESSARY IF THE SOURCE RESISTANCE IS LESS THAN 50Ω

1000pF

200Ω

V+

3+

7

gm

2–

5

4 10k

V–

5V

1+

0.01µF

CFA

8–

RF

RG

3k

VIDEO

INPUT

LOGIC

INPUT

RESTORE

2N3906 3k

6

VOUT

LT1228 • TA13

The video fader uses the transconductance amplifiers

from two LT1228s in the feedback loop of another cur-

rent 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.

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 ISET is zero. With ISET

equal 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 RF and RG. When

the logic signal is low, the PNP turns on and ISET goes to

about 1mA. Then the transconductance amplifier charges

the capacitor 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 RG to the

video input instead of to ground.

1228fd

15

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