Материал: mrf134rev6

± j50

Figure 16. S11, Input Reflection Coefficient

versus Frequency

Figure 18. S21, Forward Transmission Coefficient versus Frequency

VDS = 28 V ID = 100 mA

Figure 17. S12, Reverse Transmission Coefficient versus Frequency

VDS = 28 V ID = 100 mA

± j50

Figure 19. S22, Output Reflection Coefficient versus Frequency

VDS = 28 V ID = 100 mA

DESIGN CONSIDERATIONS

The MRF134 is a RF power N±Channel enhancement mode field±effect transistor (FET) designed especially for VHF power amplifier and oscillator applications. Motorola RF MOS FETs feature a vertical structure with a planar design, thus avoiding the processing difficulties associated with V±groove vertical power FETs.

Motorola Application Note AN±211A, 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 control signal, thus facilitating manual gain control, ALC and modulation.

DC BIAS

The MRF134 is an enhancement mode FET and, therefore, 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 MRF134 was characterized at IDQ = 50 mA, which is the suggested minimum value of IDQ. For special applications such as linear amplification, IDQ may have to be selected to optimize the critical parameters.

The gate is a dc open circuit and draws no current. Therefore, 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 MRF134 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 MRF134. See Motorola 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 MRF134, yields a device capable of self oscillation. Stability may be achieved by techniques such as drain loading, input shunt resistive loading, or output to input feedback. The MRF134 was characterized with a 68±ohm input shunt loading resistor. Two port parameter stability analysis with the MRF134 s±parameters provides a useful±tool for selection of loading or feedback circuitry to assure stable operation. See Motorola Application Note AN215A for a discussion of two port network theory and stability.

Input resistive loading is not feasible in low noise applications. The MRF134 noise figure data was generated in a circuit with drain loading and a low loss input network.

2

S K