Peak power output indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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Peak output power indicator with amplifier load resistance equal to 4 ohms provides indication of output power in the range of 1,5...100 W. The indicator consists of identical cells, the number of which depends on the selected step of the displayed power value.
Each of the cells is a threshold device made on transistors of different structures. The output signal of the LF amplifier is fed to the bases of transistors IV1 - NV1 of all cells through resistors 1R1 - NR1. The cell response threshold (for example, consider cell A1) is determined by the voltage at the emitter of the transistor 1V1, which depends on the ratio of the resistances of the resistors 1R6 and 1R4. With the amplitude of the positive half-wave of the signal, greater than the voltage at the emitter (for silicon transistors - by about 0,6 V), the 1V1 transistor begins to open. With its collector current, it opens the 1V2 transistor, and the voltage on the collector of the latter (with respect to the common wire) increases. Through the 1R2 feedback resistor, it enters the base of the 1V1 transistor, and it opens even more. The process proceeds like an avalanche until the transistor 1V2 is fully opened, as a result of which the 1V3 LED lights up. As the amplitude of the output signal of the amplifier increases, the LEDs 2V3 - NV3 are sequentially lit.
The indicator is calibrated in units of output power P at the nominal load resistance Rn of the amplifier.
With a feedback depth of 10% (determined by the ratio of the resistances of the resistors 1R2 and 1R1), the minimum recorded output voltage level is approximately 0,1Upit. Thus, the sensitivity of the device can be increased by lowering the supply voltage. In this case, it is necessary to reduce the resistance of the resistors in the LED circuit, otherwise they will glow weakly. The resistances of these resistors (in kiloohms) are calculated by the formula R5 = 0,1 (Upit - 2,5).
The indicator can use any silicon transistors of the appropriate structure with a static current transfer coefficient h21e>100 and an allowable voltage between the emitter and the collector of at least Upit. It is desirable to power the device from a stabilized source (this guarantees the preservation of the calibration when the mains voltage changes).
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