8-11

Any combination of switch input circuits can be configured

with multiple, parallel, line-drive outputs. The video switch

amplifier circuit of Figure 8 illustrates how the CA3256 may

be configured in pairs to provide an 8-to-1 video switch

amplifier using a 3-bit address to select the input. It is also

possible to use the fifth channel input to tie signals to a

common bus line for distribution from the selected amplifier;

however, distributed capacitance loading will result in

reduced bandwidth. The 4 plus 1 combination of input-signal

switching provides for a wide assortment of video switch

circuit configurations.

While the BiMOS process does provide some compromises

for both the switch and the amplifier, the combined system is

capable of the performance needed in most high-quality,

switching applications. As an integrated system, many of the

problems in PC-board layout are simplified, and there is a

reduction in component count. In its simplest form, with

+12V and -5V supplies, the CA3256 may be DC connected

at the input and output; the LED indicators need not be

connected. Under these conditions, the circuit may be as

simple as the one in Figure 8.

Summary

While each video-switch amplifier is designed for a specific

application and, to that end, is tailored as far as performance

to a given set of specifications, the circuit-designer’s goal is

generally the same in every case: to make the best possible

switch for the lowest cost. In this respect, the CA3256 IC

switch and amplifier discussed provide an excellent choice

for a cost-effective high-performance video-switch amplifier,

by taking advantage of the complementary features of both

high-speed CMOS and bipolar integrated circuits.

A

B

C

CH

1-4

CH 5

+12V

-5V

CH

5-8

+12V

-5V

NC

V+

+

A

B

C

V-

GND

1

2

3

4

5

CA3256

V+

+

A

B

C

V-

GND

1

2

3

4

5

CA3256

-

-

FIGURE 8. AN 8-TO-1 VIDEO SWITCH AMPLIFIER USING TWO

CA3256 DEVICES

TRUTH TABLE

CH

CAB

1000

2001

3010

4011

5100

6101

7110

8111

CA3256