STA550 datasheet(14/16 Pages) STMICROELECTRONICS

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STA550

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APPLICATION HINTS

PREAMPLIFIER AND COMPRESSOR

In the application circuit showed in figure 7, R1/R3 (or R2/R4) ratio fix the gain of the preamplifier.

If the input signal is very low, is possible to increase the gain fixing the product Vin

∗G = cost.

In that case is possible to increase G decreasing R1,2 from 10KΩ until 2KΩ without relevant effetcs on the cir-

cuitbehavior and remaining in the operating range Iin_max = Vin_max/R1(2),<1mA.

So it is possible to increase the preamplifier gain until 25.

If no compression is present (equivalnt compressor Gm=0), the effects are:

– The output voltage offset increase

– The SNR decrease

The following table shows these variations:

R3(4) = 50KΩ and all the other external components are the same

Attenuation = 0 dB

If the compression is active the circuit behaviour is the same.

It”s also possible to eliminate the compressor. In this case the ATT_REL (1,2) pin must be connected to gnd.

STBY-MUTE CIRCUIT

In the suggested application circuit (figure 7), the resistor for Standby/Mute function (R13) is connected between

the Standby/Mute switches and 5V Supply.

It is possible to connect the resistor to another Supply Voltage level VL, but in that case also the resistor value

(R13,14) must be changed according to the following formula (fixing VSTBY/MUTE = 2.5V and R15 = 10KΩ):

HEADROOM

In the suggested application circuit the supply voltage to obtain 75W (Power Output) on 8

Ω (Rload)

is:

It is also possible to increase the system’s efficiency forcing the headroom to follow the output signal (variable

drop insteadof a constant drop).

In that case:

R1,2

VIN MAX

VOFFSET

10K

Ω

15mV

µV

Ω

30mV

µV

Ω

1.6V

75mV

µV

–

⋅

()KΩ

⋅

()KΩ

suppl y

∆VI

LM AX

DS on

⋅

sup ply

∆VI

L V

() R

DS on

⋅

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