TDA7284D datasheet(8/14 Pages) STMICROELECTRONICS

8 / 14 page

CIRCUIT DESCRIPTION

OPERATIONAL AMPLIFIER

The operational amplifier consists essentially of a

very low noise input stage decoupled from the

single-ended output stage by means of an emitter

follower (fig. 15 ).

The compensations provided in order to have

high gain bandwith product allowing the use for

double speed recording application.

AUTOMATIC LEVEL CONTROL SYSTEM (ALC)

This system maintains the level of the signal to be

recorded at a value which prevents saturation of

the tape and which optimizes the signal to noise

ratio even there are notable variations in the input

signal.

Before presenting the ALC circuit of TDA7284 it is

worth describing the operation of the automatic

level control as a system.A diagram showing the

basis of operation is given in fig.16.

This consists of an amplifier (op-amp) having con-

stant gain (GV = 1+R4/R3),which in feedback

transforms output signal level information (usually

by means of a peak-to-peak detector) into a con-

tinuous voltage which drives the networks indi-

cated by T and Rd.

The element T transforms the continuous voltage

level into a signal capable of modifying the circuit

conditions symbolized by variable resistor Rd.

The value assumed by the resistor Rd is a func-

tion of the output signal level Vo and is such that

the voltage Vc at the input of the op-amp is con-

stant,even variations of Vi are present.Obviously

if Vo is less than a certain value the system is not

controlled.

In this case :

VI =VC=VO /GV

(GV is the gain of the op-amp)

For the TDA7284 the value of Vo below which the

system is not controlled is around 1 Vrms.

Let us now consider the speed of response of the

system (when controlled) to positive and negative

changes of the input signal i.e. the limiting

time,the time for return to nominal level (1 Vrms)

and the recovery time.

Limiting time, and time for return to nominal

level.

Let us suppose that at certain moment To, the in-

put signal increases by +

∆Vi as shown in fig. 17.

Figure 15

Figure 16: Basic Diagram of the ALC stage

TDA7284

8/14

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