Con el circuito presentado es posible utilizar micrófonos de baja impedancia e incluso pequeños altavoces como micrófonos en equipos con pequeña sensibilidad de entrada o aún alta impedancia. Se trata de un circuito casador de impedancias que también funciona con otros transductores.

   Lo que describimos es el montaje de un simple preamplificador con baja impedancia de entrada y alta impedancia de salida.

   Utilizando un transistor en la configuración de base común, tenemos una excelente ganancia y bajo consumo.

   Con ello, el circuito puede ser alimentado por una batería de 9 V, con excelente autonomía.

   La impedancia de entrada va de 2 a 200 ohms y la impedancia de salida es de aproximadamente 100 k ohms.

   En la figura 1 tenemos el diagrama completo del aparato.

 

   Figura 1 - Diagrama completo del preamplificador
   Figura 1 - Diagrama completo del preamplificador | Haga click en la imagen para ampliar |

 

   El mejor montaje es el que hace uso de una pequeña placa de circuito impreso con el patrón mostrado en la figura 2.

 

   Figura 2 - Placa para el montaje
   Figura 2 - Placa para el montaje | Haga click en la imagen para ampliar |

 

    Un montaje descompromisado puede efectuarse sobre la base de un puente de terminales, tal como se muestra en la figura 3.

 

   Figura 3 - Montaje en puente de terminales
   Figura 3 - Montaje en puente de terminales | Haga click en la imagen para ampliar |

 

   

En ambos casos, el montaje debe utilizar la caja metálica para servir de blindaje y el cable de salida debe ser blindado.

   Los resistores son de 1/8 W y el transistor también puede ser el BC549 para un nivel de ruido menor.

 

Q1 - BC548 - transistores NPN de uso general

MIC - pequeño altavoz o micrófono dinámico de baja impedancia

S1 - Interruptor simple

B1 - 9 V - batería

P1 - 100 k ohms - pote

R1 - 22 k ohms x 1/8 W - resistor - rojo, rojo, naranja

R2 - 220 k x 1/8 W - resistor - rojo, rojo, amarillo

R3 - 330 k ohms x 1/8 W - resistor - naranja, naranja, amarillo

C1 - 470 nF - capacitor de cerámica o poliéster

C2, C3 - 100 uF x 12 V - capacitor electrolítico

 

Varios:

Placa de circuito impreso o puente de terminales, conector de batería, caja para montaje, jack de salida, hilos, soldadura, etc.

 

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