11

The RF MOSFET Line

15W, to 400MHz, 28V

M/A-COM Products

Released - Rev. 05202009

MRF136

• North America Tel: 800.366.2266 / Fax: 978.366.2266

• Europe Tel: 44.1908.574.200 / Fax: 44.1908.574.300

• Asia/Pacific Tel: 81.44.844.8296 / Fax: 81.44.844.8298

Visit www.macomtech.com for additional data sheets and product information.

M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make

changes to the product(s) or information contained herein without notice.

ADVANCED: Data Sheets contain information regarding a product M/A-COM Technology Solutions

is considering for development. Performance is based on target specifications, simulated results,

and/or prototype measurements. Commitment to develop is not guaranteed.

PRELIMINARY: Data Sheets contain information regarding a product M/A-COM Technology

Solutions has under development. Performance is based on engineering tests. Specifications are

typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available.

Commitment to produce in volume is not guaranteed.

RF POWER MOSFET CONSIDERATIONS

DESIGN CONSIDERATIONS

The MRF137 is a RF power N–Channel enhancement-

mode field–effect transistor (FET) designed especially for

VHF power amplifier applications. M/A-COM RF MOS

FETs feature a vertical structure with a planar design, thus

avoiding the processing difficulties associated with V–

groove vertical power FETs.

M/A-COM 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

mismatched loads without suffering damage. Power output

can be varied over a wide range with a low power dc con-

trol signal, thus facilitating manual gain control, ALC and

modulation.

DC BIAS

The MRF137 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.

See Figure 10 for a typical plot of drain current versus gate

voltage. RF power FETs require forward bias for optimum

performance.

formany applications. The MRF137 was characterized at

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 generally be just a simple

resistive divider network. Some special applications may

require a more elaborate bias system.

GAIN CONTROL

Power output of the MRF137 may be controlled from

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

dc gate voltage. This feature facilitates the design of man-

ual gain control, AGC/ALC and modulation systems. (See

Figure 9.)

AMPLIFIER DESIGN

Impedance matching networks similar to those used

with bipolar VHF transistors are suitable for MRF137. See

M/A-COM Application Note AN721, Impedance Matching

Networks Applied to RF Power Transistors. The higher

input impedance of RF MOS FETs helps ease the task of

broadband network design. Both small signal scattering

parameters and large signal impedances are provided.

While the s–parameters will not produce an exact design

solution for high power operation, they do yield a good first

approximation. This is an additional advantage of RF MOS

power FETs.

RF power FETs are triode devices and, therefore, not

unilateral. This, coupled with the very high gain of the

MRF137, yields a device capable of self oscillation. Stabil-

ity may be achieved by techniques such as drain loading,

input shunt resistive loading, or output to input feedback.

Two port parameter stability analysis with the MRF137 s–

parameters provides a useful tool for selection of loading or

feedback circuitry to assure stable operation. See M/A-

COM Application Note AN215A for a discussion of two port

network theory and stability.

LOW NOISE OPERATION

Input resistive loading will degrade noise performance, and

noise figure may vary significantly with gate driving imped-

ance. A low loss input matching network with its gate im-

pedance optimized for lowest noise is recommended.