4

520 - 38 - 3

DETAILED DESCRIPTION

The GT4123 and GT4123A are the first dedicated single

device, two input video mixer ICs available to the professional

video and multimedia markets.

The internal topology of the devices is shown in Figure 1.

Fig. 1

Functional Block Diagram of the GT4123 and GT4123A

Each input signal is applied to a conventional differential

amplifier (AMP A and AMP B). From the amplifiers, the signals

are applied 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

.

These voltages are derived from a unity gain differential

amplifier (AMP C) whose outputs (true and invert) are the

difference between an externally applied CONTROL voltage

(V

internal DC offset of 0.5 volts is applied to the controlling

voltage summing circuits

Σ2 and Σ3.

Therefore,

V

CA

= 0.5V + (VC - 0.5V) and

V

CB

= 0.5V - (VC - 0.5V)

When the CONTROL input (V

C

) equals 0.5 volts, V

CA

and

V

CB

= 0.5 volts and exactly 50% of each input signal passes

to the output of the multiplier stages.

When V

C

is less than 0.5 volts, V

CA

reduces and V

CB

increases

in proportion so that less of the Channel A signal and more of

the Channel B signal is transferred. Similarly, when V

C

is

greater than 0.5 volts, the opposite occurs.

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

voltage of 0 volts completely cuts off Channel A and fully turns

on Channel B. Similarly, a CONTROL voltage of 1 volt will fully

turn on Channel A and completely turn Channel B off.

Figure 10 shows the CONTROL transfer characteristics of the

GT4123 and GT4123A.

There is a small ‘dead band’ at either extreme of the CONTROL

input. The amount of ‘dead band’ is about 100 mV and is

shown in Figure 10. The CONTROL input can be preceded by

an operational amplifier so biased as to overcome this ‘dead

band’ as well as level shift the control signal so that other than

0 to 1 volt ranges can be used. The bandwidth of the

CONTROL input is sufficient to allow very fast keying and is in

the order of 20 MHz at -0.1 dB.

The linear portion of the transfer characteristic has a linearity

of 1% or better.

The outputs from the multipliers are applied to an analog

summing circuit (

Σ1) whose output feeds a wideband amplifier

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

The inverting inputs of each input amplifier 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 in order to

function, they lend themselves to high density, multi-functional

PC board layouts. Several devices can be used in parallel

applications such as R-G-B mixers and four-layer keyers

where close control law tracking is essential.

The only difference between the GT4123 and the GT4123A is

the fact that the latter device can operate with non-equal

power supplies. The negative supply can be as low as -3 volts

unlike the GT4123 which can only operate down to

±9 volt

supplies.

COMP

+

OUT

AMP B

IN A

IN B

CONTROL

+

+

+

+

+

-

+

AMP D

AMP C

-

+

XB

XA

-

+

AMP A

0.5V

REF

Σ1

Σ2

Σ3