MRF151 Datasheet, PDF (6/8 Pages) Motorola, Inc – N-CHANNEL BROADBAND RF POWER MOSFET

Gate Protection — This device does 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 MRF151 is an RF Power, MOS, N–channel enhance-
ment mode field–effect transistor (FET) designed for HF and
VHF power amplifier applications.
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 MOSFETs include
high gain, low noise, simple bias systems, relative immunity
from thermal runaway, and the ability to withstand severely
mismatched loads without suffering damage. Power output
can be varied over a wide range with a low power dc control
signal.
DC BIAS
The MRF151 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-
mance. The value of quiescent drain current (IDQ) is not criti-
cal for many applications. The MRF151 was characterized at
IDQ = 250 mA, each side, which is the suggested minimum
value of IDQ. For special applications such as linear amplifi-
cation, IDQ may have to be selected to optimize the critical
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 MRF151 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.
MRF151
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MOTOROLA RF DEVICE DATA