11

The RF MOSFET Line

80W, 175MHz, 28V

M/A-COM Products

Released - Rev. 07.07

MRF173

• 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.

DESIGN CONSIDERATIONS

The MRF173 is a RF MOSFET power N–channel en-

hancement mode field–effect transistor (FET) designed for

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

feature a vertical structure with a planar design, thus avoid-

ing the processing difficulties associated with V–groove

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 mis-

matched loads without suffering damage. Power output can

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

signal, thus facilitating manual gain control, ALC and modu-

lation.

DC BIAS

The MRF173 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 9 for a typical plot of drain current versus gate

voltage. RF power FETs require forward bias for optimum

performance. The value of quiescent drain current (IDQ) is

not critical for many applications. The MRF173 was charac-

terized at IDQ = 50 mA, 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 generally be just a simple re-

sistive divider network. Some special applications may re-

quire a more elaborate bias system.

GAIN CONTROL

Power output of the MRF173 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.

(see Figure 8.)

AMPLIFIER DESIGN

Impedance matching networks similar to those used with

bipolar VHF transistors are suitable for MRF173. See M/A-

COM Application Note AN721`, Impedance Matching Net-

works Applied to RF Power Transistors. The higher input

impedance of RF MOSFETs helps ease the task of broad-

band network design. Both small–signal scattering parame-

ters 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.