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DETAILED DESCRIPTION

The GT4123B is a low power two quadrant video multiplier for

use in a wide range of applications including broadcast and

multimedia.

The internal topology of the device is shown in Figure 1 below.

Fig. 1

Functional Block Diagram of the GT4123B

Each input is applied to a differential amplifier (AMP A and AMP

B). From the amplifiers, the signals are passed to analog

multiplier circuits (XA and XB) whose outputs are the product

of the input signals and internally generated controlling voltages

V

CA

and V

CB

.

The internal control voltages are derived from a unity gain

differential amplifier (AMP C) whose outputs (true and invert)

are the difference between an internal 0.5 V reference voltage,

and the externally applied CONTROL voltage. In addition, the

internal DC offset of 0.5 V is applied to the controlling voltage

summing circuits

Therefore,

V

CA

= 0.5 V + (V

C

- 0.5 V)

and

V

CB

= 0.5 V - (V

C

- 0.5 V)

When the control input V

C

equals 0.5 volts, V

CA

= 0.5 V and

V

CB

= 0.5 V, and 50% of each input signal passes to the output

of the multiplier stages.

When V

C

is less than 0.5 V, V

CA

reduces and V

CB

increases in

proportion so that less of Channel A and more of Channel B

signal is transferred. Similarly, when V

C

is greater than 0.5 V,

the opposite occurs.

The SPAN or control range is internally set so that a CONTROL

voltage of 0 V completely cuts off Channel A and fully turns on

Channel B. Similarly, a CONTROL voltage of 1 V will fully turn

on Channel A and completely turn off Channel B.

There is a small dead band at either end of the CONTROL

input.

The amount of dead band is about 100 mV.

The

CONTROL input can be preceded by an operational amplifier

to overcome the dead band and level shift the control signal

so that other than 0 to 1 V range can be used. The bandwidth

of the CONTROL input is in the order of 20 MHz at ±0.1 dB to

allow for very fast KEY signals.

The linear portion of the transfer characteristic has a linearity

of better than 1.5%.

The outputs from the multipliers are applied to an analog

summing circuit (

(Amp D) and presents the mixed signals to the outside world.

The inverting nodes of each input are directly connected to the

output.

In this manner, the closed loop gain is nearly unity providing

wideband, stable operation. Because the devices have only 8

pins and require virtually no external parts, the GT4123B lends

itself to high density, multi-function PC board layouts in devices

such as RGB mixers and four layer keyers where close control

law tracking is essential.

COMP

+

OUT

AMP B

IN A

IN B

CONTROL

+

+

+

+

+

-

+

AMP D

AMP C

-

+

XB

XA

-

+

AMP A

0.5V

REF

Σ1

Σ2

Σ3