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TDA7284D 데이터시트(PDF) 8 Page - STMicroelectronics |
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TDA7284D 데이터시트(HTML) 8 Page - 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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