520 - 25 - 4

3

Referring to the block diagram, the input signals are applied

to conventional differential amplifiers (AMP1 and AMP2).

Each amplifier has provisions for individually adjusting the

DC offset (OFFSET).

Following each input amplifier, the signals are applied to

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

product of the incoming signals and controlling voltages (V

or (V

0.5V source (V

up of the sum of the nominal voltage V

V

between an externally applied reference voltage (V

an externally applied CONTROL voltage (V

and -V

gain is A

R

according to the following formula:

Note that R

and R

Each of the voltages (+V

circuits (

Σ2 and Σ3) whose second inputs are DC voltage

sources that can also be slightly varied. The nominal value of

these voltage sources is 0.5 volts.

When they are exactly 0.5V and when V

each signal channel of the mixer is 0.5 (50%).

By connecting the ends of an external potentiometer

(CONTROL OFFSET) between the offset pins COS1 and

COS2, the voltage sources can be altered differentially. If a

second potentiometer (50% GAIN) is connected between the

wiper of the CONTROL OFFSET potentiometer and the supply

voltage, the voltage sources can be varied in a common

mode fashion. In this way not only can the control range of

the mixer be varied but also the point at which 50% of each

input signal appears at the output.

The outputs from the multiplier circuits (XA and XB) are then

applied to a summing circuit (

Σ1) whose output feeds a

wideband amplifier (AMP4) and presents the mixed signals to

the outside world.

Although there are two separate differential inputs, the usual

operational amplifier gain-setting methods can be applied to

determine the closed loop gain of the mixer. Usually the

mixer will be configured for unity gain by connecting both

inverting inputs (-IN A , -IN B) to the common output (OUT). In

this case, the general transfer function is:

V

V

(Unity gain configuration)

Where V

and +IN B respectively, and V

Note that V

0.45V < V

DETAILED DESCRIPTION

0.85 • R

EXT

A

—————

[1k

Ω < R

EXT

< 3k

Ω]

R

SPAN

+

-

GND

BIAS

AMP 3

S1

S 2

COMP

OUT

AMP 1

AMP 2

+

-

+

-

XA

XB

+

+

AMP 4

-

+

0.5V

Σ2

Σ3

+

-

+

-

+

+

+

Σ1

The GT4122 is a broadcast quality monolithic inte-

grated circuit specifically designed to linearly mix two

video signals under the control of a third signal.

FUNCTIONAL BLOCK DIAGRAM