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| Audio ICs Square-law compression amplifiers BA6138 / BA6138F The BA6138 and BA6138F are square-law compression amplifiers designed for use as level meters in component stereos and tape decks. Two square root amplifiers with good linearity are included on the IC, as well as a muting pin for easy handling of power on / off. FApplications Level meters for tape decks, component stereos, and high-end radio cassette players FFeatures 1) Consists of two square-law compression amplifiers with good linearity. 2) Good balance between channels with minimal crosstalk. 3) Includes a muting pin for easy handling of power on / off. 4) Good stability with respect to power supply voltage fluctuations. 5) Operates on a single power supply. FAbsolute maximum ratings (Ta = 25_C) FRecommended operating conditions (Ta = 25_C) 825 Audio ICs FBlock diagram BA6138 / BA6138F FElectrical characteristics (unless otherwise noted, Ta = 25_C and VCC = 12V) 826 Audio ICs FMeasurement circuit BA6138 / BA6138F C3, R2, C4 These time constants determine the recovery time. With R1 = R2 = 330k and C3 = C4 = 4.7F as in the application example, the recovery time is approximately 1 second. C3 and C4 can also be used as smoothing capacitors. The attack time is related to the charging ability of the IC and the values of C3 and C4. In the application example, it is approximately 1 ms. Muting pin (Pin 1) When Pin 1 is high, muting takes place. The threshold voltage is approximately 1.5V (2VF). FApplication example If this pin is not used, open-circuit or ground it. A negative voltage of up to *10V can also be applied to this pin. R3, R4 These form the load resistance of the BA6138. By making this resistance extremely low, the recovery time will shorten, however, dispersion will increase. In the application example, the resistance can go to about 3.3k. If the recovery time has been fixed, R1 = R2 can be decreased and C3 = C4 can be increased to lower the load resistance. 827 Audio ICs FExplanation of application circuit example The application circuit shown in Fig. 2 is for the BA6138. The audio signal is compressed to its square root and generated to R3 and R4 as a DC output. By introducing the output voltage of R3 and R4 into the input of a linear level meter IC (12 to 16 points, LED or fluorescent drive level meter IC), a high precision level meter can be created. The relation between the input voltage and output voltage of the application example of Fig. 2 is shown in Fig. 3. BA6138 / BA6138F S Input resistance In the application circuit, the output voltage becomes saturated if the input signal rises above 3Vrms. By adding a resistor RIN in series with the input pin, the input-output voltage relation can be shifted. To maintain good linearity, the value of the resistor should be no more than 3k. FFor needle meters The BA6138 / BA6138F can also be used for needle meters. The load resistance in this case is lower, thus the recovery time should be set with R1 = R2 small and C3 = C4 large. FExternal dimensions (Units: mm) 828 |
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