MRF157_15 Datasheet, PDF (7/9 Pages) M/A-COM Technology Solutions, Inc. – Linear RF Power MOSFET

MRF157
Linear RF Power MOSFET
600W, to 80MHz
M/A-COM Products
Released - Rev. 07.07
RF POWER MOSFET CONSIDERATIONS
MOSFET CAPACITANCES
The physical structure of a MOSFET results in capacitors
between the terminals. The metal oxide gate structure de-
termines the capacitors from gate–to–drain (Cgd), and
gate–to–source (Cgs). The PN junction formed during the
fabrication of the RF MOSFET results in a junction capaci-
tance from drain–to–source (Cds).
These capacitances are characterized as input (Ciss),
output (Coss) and reverse transfer (Crss) capacitances on
data sheets. The relationships between the inter–terminal
capacitances and those given on data sheets are shown
below. The
Ciss can be specified in two ways:
1. Drain shorted to source and positive voltage at the
gate.
2. Positive voltage of the drain in respect to source and
zero volts at the gate. In the latter case the numbers
are lower. However, neither method represents the
actual operating conditions in RF applications.
GATE CHARACTERISTICS
The gate of the RF MOSFET is a polysilicon material, and
is electrically isolated from the source by a layer of oxide.
The input resistance is very high — on the order of 109
ohms
— resulting in a leakage current of a few nanoamperes.
Gate control is achieved by applying a positive voltage
slightly in excess of the gate–to–source threshold voltage,
VGS(th).
Gate Voltage Rating — Never exceed the gate voltage
rating. Exceeding the rated VGS can result in permanent
damage to the oxide layer in the gate region.
Gate Termination — The gates of these devices are es-
sentially capacitors. Circuits that leave the gate open–
circuited or floating should be avoided. These conditions
can result in turn–on of the devices due to voltage build–up
on the input capacitor due to leakage currents or pickup.
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.
LINEARITY AND GAIN CHARACTERISTICS
In addition to the typical IMD and power gain data pre-
sented, Figure 5 may give the designer additional informa-
tion on the capabilities of this device. The graph represents
the small signal unity current gain frequency at a given
drain current level. This is equivalent to fT for bipolar tran-
sistors. Since this test is performed at a fast sweep speed,
heating of the device does not occur. Thus, in normal use,
the higher temperatures may degrade these characteristics
to some extent.
DRAIN CHARACTERISTICS
One figure of merit for a FET is its static resistance in the
full–on condition. This on–resistance, VDS(on), occurs in
the linear region of the output characteristic and is specified
under specific test conditions for gate–source voltage and
drain current. For MOSFETs, VDS(on) has a positive tem-
perature coefficient and constitutes an important design
consideration at high temperatures, because it contributes
to the power dissipation within the device.
IMPEDANCE CHARACTERISTICS
Device input and output impedances are normally ob-
tained by measuring their conjugates in an optimized nar-
row band test circuit. These test circuits are designed and
constructed for a number of frequency points depending on
the frequency coverage of characterization. For low fre-
quencies the circuits consist of standard LC matching net-
works including variable capacitors for peak tuning. At in-
creasing power levels the output impedance decreases,
resulting in higher RF currents in the matching network.
This makes the practicality of output impedance measure-
ments in the manner described questionable at power lev-
els higher than 200–300 W for devices operated at 50 V
and 150–200 W for devices operated at 28 V. The physical
sizes and values required for the components to withstand
the RF currents increase to a point where physical con-
struction of the output matching network gets difficult if not
impossible. For this reason the output impedances are not
given for high power devices such as the MRF154 and
MRF157.
However, formulas like
for a single ended
design
or for a push–pull design can be
used to obtain reasonably close approximations to actual
values.
7
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