DESIGN CONSIDERATIONS

The MRF136 is an RF power N–Channel enhancement

mode field–effect transistor (FET) designed especially for HF

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

FETs feature planar design for optimum manufacturability.

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 ther-

mal 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 modulation.

DC BIAS

The MRF136 is an enhancement mode FET and, therefore,

does not conduct when drain voltage is applied without gate

bias. A positive gate voltage causes drain current to flow (see

Figure 10). RF power FETs require forward bias for optimum

gain and power output. A Class AB condition with quiescent

drain current (IDQ) in the 25–100 mA range is sufficient for

many applications. For special requirements such as linear

amplification, IDQ may have to be adjusted to optimize the

critical parameters.

The MOS gate is a dc open circuit. Since the gate bias circuit

does not have to deliver any current to the FET, a simple

resistive divider arrangement may sometimes suffice for this

function. Special applications may require more elaborate

gate bias systems.

GAIN CONTROL

Power output of the MRF136 may be controlled from rated

values down to the milliwatt region (>20 dB reduction in power

output with constant input power) by varying the dc gate

voltage. This feature, not available in bipolar RF power

devices, facilitates the incorporation of manual gain control,

AGC/ALC and modulation schemes into system designs. A

full range of power output control may require dc gate voltage

excursions into the negative region.

AMPLIFIER DESIGN

Impedance matching networks similar to those used with

bipolar transistors are suitable for MRF136. See M/A-COM

Application Note AN721, Impedance Matching Networks

Applied to RF Power Transistors. Both small signal scattering

parameters and large signal impedance parameters are

provided. Large signal impedances should be used for

network designs wherever possible. While the s parameters

will not produce an exact design solution for high power

operation, they do yield a good first approximation. This is

particularly useful at frequencies outside those presented in

the large signal impedance plots.

RF power FETs are triode devices and are therefore not

unilateral. This, coupled with the very high gain, yields a

device capable of self oscillation. Stability may be achieved

using techniques such as drain loading, input shunt resistive

loading, or feedback. S parameter stability analysis can

provide useful information in the selection of loading and/or

feedback to insure stable operation. The MRF136 was

characterized with a 27 ohm input shunt loading resistor.

For further discussion of RF amplifier stability and the use

of two port parameters in RF amplifier design, see M/A-COM

Application Note AN215A.

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

impedance optimized for lowest noise is recommended.

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