200mW GaAsFET AMPLIFIER FOR 24 GHz

Michael Kuhne, DB6NT

200mW GaAsFET Amplifier for 24 GHz

VHF Communications 3/1995

Many stations have been constructed for the 24-GHz band in recent years which can generate transmit power levels ranging from a few 100 mW to a maximum of 100 mW. These values could be obtained using low-cost transistors (selected MGF 1303 parallel). The power gain thus obtained for a 100 mW PA with MGF 1303 was approximately 13 dB.

1. INTRODUCTION

In this article, a 200 mW amplifier will be described with an amplification of 20 dB. FLR016FH and FLR026FH K-Band Power FET's are used here, from the Fujitsu company. Building 24-GHz amplifiers calls for great experience in the SHF range and for a lot of patience. You have to work for every dB or mW when you're calibrating, using "little flags" and putting MOS foam and copper strips into the housing!

2. THE AMPLIFIER CIRCUIT

Fig.1 shows the 24-GHz amplifier circuit. The amplifier is built on a 0.25 mm. thick Teflon printed circuit board made from RT/duroid 5870. Stages T1 and T2 are connected to one another through 50 Ohm striplines and coupling capacitors, and are capacitively tuned by means of soldered-on "little flags".

The parallel high-level stages with T3 and T4 are connected to one another through four l/4 couplers. This circuit option makes it possible to obtain good decoupling without feedback between the two end transistors, and makes a considerable contribution to the stability of the circuit. Should any asymmetry arise in the power stages, the radio frequency output this generates is absorbed in the resistor connected to port-4. These 47 - 50 Ohm resistors on the couplers are small SHF types - 1.4 x 1.4 mm. format. Di-Cap capacitors are used as coupling capacitors between the stages.

The negative voltage for the FET's is generated through an MKU 55 hybrid module. This is laterally fastened to the internal wall of the tinplate housing. At the output, the circuit has a directional coupler with a BAT 15-03W SMD Schottky diode from Siemens. The power delivered can be monitored at any time at this test output using a moving coil instrument.

The LL 101 diodes are SMD Schottky diodes and help to delay a breakdown of the FET's in the event of any short-circuiting of the coupling capacitors. SMA 3.5 mm. microstrip connectors should be used as coaxial jacks.

3. ASSEMBLY

The amplifier can be assembled in either a wave guide or a coaxial version. Different housings and printed circuit boards are used in each case.

First the 0.25 mm. thick Teflon printed circuit board (Fig.2a, coaxial format) is cut to dimension, drilled and through-hole plated. 0.1mm. copper foil should be used for through-hole plating.

The printed circuit board is now inserted into the aluminium housing as a drilling jig and the appropriate bores are drilled in the housing. The components shown in the layout diagram (Fig.2b) are now placed on the track side (top) of the printed circuit board. The BAT 15-03W diode is soldered "overhead" to obtain lower housing inductivity levels.

Before being put into the housing, the printed board assembly should be washed down with spirit and then aligned straight to guarantee that it is installed flat. The underside of the board is coated with heat conducting paste in the centre to improve the heat dissipation from the FET's. The coupling and decoupling side of the printed circuit board is coated with a second adhesive from below. Optimum mounting is obtained by pressing on the printed circuit board while the adhesive dries. This operating procedure must be quick and precise, as the adhesive dries very quickly and no changes can be made to the printed circuit board's position thereafter. The printed circuit board is now fastened using 5 x M2 screws. As an example, Fig.3 shows a milled two-part aluminium housing.

4. CALIBRATION

A power supply with fine control current limit should be used for calibration. A 24-GHz signal source with an adjustable output and a suitable power meter are required for calibration. The 1k trimmer is used to set UG1, UG2, UG3 and UG4 individually in such a way that the drain currents given in the circuit diagram flow.

An initial output of 200 mW was reliably obtained from all the rigs tested. Fig.4 shows the amplification curve of a prototype.

5. PARTS LIST

(not included here)

Printed circuit boards for coaxial or semi-conductor assemblies, together with MKU55 hybrids for generating negative voltage, can be obtained from the author.

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