A D V A N C E D S E M I C O N D U C T O R, I N C.

REV. B

7525 ETHEL AVENUE

• NORTH HOLLYWOOD, CA 91605 • (818) 982-1202 • FAX (818) 765-3004

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ERROR! REFERENCE SOURCE NOT FOUND.

Specifications are subject to change without notice.

BLF278

BIASING ASI RF TRANSISTORS

ASI RF Mosfets are enhancement mode devices. In order to cause drain current to flow, positive

voltage of about 3 V needs to be applied to the Gate terminal. The higher the gate voltage applied the

higher the drain current. The Mosfet transistor drain current will change with temperature. As the

temperature goes up, so will the drain current. In order to prevent a runway situation, the gate voltage

needs to have negative temperature coefficient. The following circuit describes a means to achieve

a –2mv/ °C coefficient at the gate terminal.

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The temperature coefficient of a 6.8V zener is about + 2mv/ °C and the diode is about –2mv/ °C

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The gate of a transistor draws no current, so there is no current flow in R4. R4 is used as a RF isolation

resistor.

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R1 is adjusted to cause about 10ma of current flow into the circuit.

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R2 and R3 can be a potentiometer to adjust for the desired gate voltage. The total value of R2 and R3

should be high enough so that the current drawn in R2 and R3 is substantially less than 10ma.

Recommend around 2ma.