7

MRF175LU MRF175LV

MOTOROLA RF DEVICE DATA

Gate Protection — These devices do not have an internal

monolithic zener diode from gate–to–source. If gate protec-

tion is required, an external zener diode is recommended.

Using a resistor to keep the gate–to–source impedance

low also helps damp transients and serves another important

function. Voltage transients on the drain can be coupled to

the gate through the parasitic gate–drain capacitance. If the

gate–to–source impedance and the rate of voltage change

on the drain are both high, then the signal coupled to the gate

may be large enough to exceed the gate–threshold voltage

and turn the device on.

HANDLING CONSIDERATIONS

When shipping, the devices should be transported only in

antistatic bags or conductive foam. Upon removal from the

packaging, careful handling procedures should be adhered

to. Those handling the devices should wear grounding straps

and devices not in the antistatic packaging should be kept in

metal tote bins. MOSFETs should be handled by the case

and not by the leads, and when testing the device, all leads

should make good electrical contact before voltage is ap-

plied. As a final note, when placing the FET into the system it

is designed for, soldering should be done with a grounded

iron.

DESIGN CONSIDERATIONS

The MRF175L is a RF power N–channel enhancement

mode field–effect transistor (FETs) designed for HF, VHF and

UHF power amplifier applications. Motorola FETs feature a

vertical structure with a planar design.

Motorola Application Note AN211A, FETs in Theory and

Practice, is suggested reading for those not familiar with the

construction and characteristics of FETs.

The major advantages of RF power FETs include high

gain, low noise, simple bias systems, relative immunity from

thermal runaway, and the ability to withstand severely mis-

matched loads without suffering damage. Power output can

be varied over a wide range with a low power dc control sig-

nal.

DC BIAS

The MRF175L is an enhancement mode FET and, there-

fore, does not conduct when drain voltage is applied. Drain

current flows when a positive voltage is applied to the gate.

RF power FETs require forward bias for optimum perfor-

cal for many applications. The MRF175L was characterized

parameters.

The gate is a dc open circuit and draws no current. There-

fore, the gate bias circuit may be just a simple resistive divid-

er network. Some applications may require a more elaborate

bias sytem.

GAIN CONTROL

Power output of the MRF175L may be controlled from its

rated value down to zero (negative gain) by varying the dc

gate voltage. This feature facilitates the design of manual

gain control, AGC/ALC and modulation systems.