FET es el acrónimo de Field Effect Transistor o Transistor de Efecto de Campo.

FET es un dispositivo que se puede utilizar en las mismas aplicaciones que los transistores bipolares, es decir, como un amplificador, tecla u oscilador.

El FET básico está formado por una pieza de material semiconductor N en la que se forman dos regiones P dejando entre ellas un canal, como se muestra en la figura 1.

 


 

 

 

La corriente a través del (figura 1) canal de material N se puede controlar mediante una tensión aplicada al material N.

Pequeñas variaciones de la tensión aplicada al terminal de compuerta (gate o G) causan mayores variaciones de corriente entre el drenaje (drain = d) y la fuente (source = s).

El FET se puede encontrar en dos tipos básicos: canales N y P dependiendo del material utilizado. Este tipo básico de transistor de efecto de campo donde hay una juntura entre los materiales P y N también llamado de FET de juntura o JFET.

El FET se puede utilizar en las mismas configuraciones básicas donde se conectan transistores de juntura, como se muestra en la figura 2.

 


 

 

 

Advertencia: Los FETs son dispositivos sensibles. Una carga estática puede dañarlos. Nunca los sostenga junto a los terminales.

 

La figura 3 muestra los símbolos utilizados para representar los FETs, así como los aspectos más comunes.

 


 

 

 

Tenga en cuenta que no hay diferencia en la apariencia externa de los transistores bipolares comunes y de los FETs.

Saber cuál es uno o cuál es otro simplemente conocer el tipo o tener el diagrama del aparato donde se utilizan.

Las especificaciones eléctricas de los FETs son importantes para saber cuál es el sustituto en una aplicación o para poder probar una que tenemos a mano.

Las principales especificaciones son:

 

a) Tensión máxima entre el drenaje y la fuente (Vds)

Es la tensión máxima que el transistor puede manejar sin quemar.

Para los tipos comunes está entre 20 y 60 V. Esta especificación también se puede dar como Vds(máx.).

b) Máxima corriente de drenaje (Id)

Es la corriente máxima que puede pasar a través del componente cuando está en funcionamiento.

c) Transconductancia

Esta es la medida equivalente al beneficio de transistores bipolares. La transconductancia se mide en Siemens (S). En algunas publicaciones y diagramas antiguos encontramos la antigua unidad mho (ohm escrito al revés o el símbolo omega "al revés").

d) Potencia de disipación (Pd)

Es la misma especificación de transistores bipolares, que se miden en watts.

Los JFETs se encuentran en circuitos donde las señales deben ser generadas, controladas o amplificadas. Se pueden utilizar en modo lineal, como amplificadores o como teclas. Al utilizar o reemplazar un FET es importante conocer el tipo, y si se utiliza un sustituto diferente, necesita saber cuál es la disposición de sus terminales de compuerta, drenaje y fuente.

 

Los tipos más comunes de FETs que se encuentran en los proveedores locales son MPF102 y BF245. Otros tipos ya son más raros y se pueden obtener en consulta.

Del libro Curso de Electrónica - Electrónica Básica de Newton C. Braga

 

 

 

 

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