LT1228(Rev_A) Просмотр технического описания (PDF) - Linear Technology

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LT1228
(Rev.:Rev_A)

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

LT1228

TYPICAL APPLICATI S

Video Fader

Video DC Restore (Clamp) Circuit

NOT NECESSARY IF THE SOURCE RESISTANCE IS LESS THAN 50Ω

1k

VIN1

3+

gm

1

2–

5

100Ω

1k

10k

5.1k

–5V

10k

10k

5.1k

1k

1k

VIN2

100Ω

3+

5

gm

1

2

–

+

LT1223

CFA

–

VOUT

VS = ±5V

LT1228 • TA12

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 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.

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 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 RG to the

video input instead of to ground.

15

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