5 Watt RF Power Amplifier for 900 MHz

Overview

This is a project for a simple 5 watt, 900 MHz RF power amplifier which can be used stand-alone or as the driver stage for the Motorola STF2520A 150 watt amplifier project.

Construction of this amplifier will be very easy, as we'll be using the RF power amplifier module from an old Radio Shack CT-1050 800 MHz (analog) cellular phone.  These old cellular phones, and other models using the same RF power amplifier module, show up from time-to-time at various ham radio swapfests.  The Radio Shack CT-1050 is ideal for this project as they use a Hitachi PF0030 RF power module mounted on its own little heatsink.

The Hitachi PF0030 RF power module is designed for 824 - 849 MHz and has a maximum RF input of 20 mW (+13 dBm).  It has over 30 dB of gain in that frequency range and the compression point is around 6 watts (+38 dBm) at +12 VDC.  The PF0030 is non-linear biased, so it's only useful for constant envelope modulations - like FM, FSK, or CW.  The PF0030's frequency range is fairly wideband, with its gain dropping off 10 dB at around 780 MHz on the low side and 915 MHz on the high side.  There is over 3 dB of gain difference over the entire 902 - 928 MHz ham band, so keep this in mind.  That isn't a big deal in most ham radio applications since the convenience and low cost of the Hitachi PF0030 RF power module makes up for any downfalls.

The Hitachi PF0030 RF power module itself is also fairly rugged.  You can run them at +15 or +18 VDC and increase the RF input power for a little bit of extra gain.  The PF0030 does get fairly hot, and this will require proper heatsinking.  This is often overlooked when working with these RF power modules as the mounting flange is also the ground.  Adding heatsink thermal compound to the flange often causes the RF power module to break into oscillation, in certain instances.  The key to prevent oscillation is to have a common ground connection between the RF power module's flange and its RF input/output and DC power connections, and this can't be done with thermal compound under the flange.  You'll then need to mount this first heatsink onto another one for additional heatsinking.

An optional transistor circuit will be added to this amplifier to toggle the RF power control voltage.  This is useful for powering down the amplifier during a receive cycle.  Since the current draw of the PF0030's control pin is minimal, we can just use a regular 2N3906.

Hitachi PF0030 Amplifier Specifications


RF           RF        RF
Frequency    Input     Output
(MHz)        (dBm)     (dBm) (Watts)
830-850      -3        +38 (6.3)
780          -3        +28 (0.6)
880          -3        +36 (4.0)
902          -3        +33 (2.0)
915          -3        +28 (0.6)




Pictures & Construction Notes

Stock Radio Shack CT-1050 (Model No. 17-1021) analog cellular phone with its internal circuit board removed.  This phone's FCC ID is: GML-C250

Note how the Hitachi PF0030 RF power module is mounted to its own little heatsink.

Alternate view of the Hitachi PF0030 RF power module and its heatsink.

We'll be making our own circuit board to connect up to the PF0030.

The PF0030 requires only five connections:

Making a new faceplate for the front of the amplifier's case.

It's a piece of 2-inch wide aluminum stock with a few 1/2-inch diameter holes drilled in it for the TNC RF input/output connectors and DC power banana jacks.

A new circuit board was also made for the PF0030 RF power module.  This will hold the coaxial cable on the RF input/output, the power control transistor, and the voltage conditioning components.


Cleaning up the PF0030's heatsink and making the PC board.

I removed all the old thermal compound and sandblasted the heatsink to help clean it up a bit.

The PC board is double-sided and should have a large ground plane which is common to the heatsink.


Completed Hitachi PF0030-based amplifier.

RF input is on the left, RF output is on the right via the TNC jacks.

There is NO heatsink thermal compound between the PF0030 and the aluminum mounting block.  This keeps a strong common ground between the PF0030 and the circuit board holding the other components.

The +12 VDC for the Hitachi PF0030 is provided via the RED and BLACK wires which connect to the banana jacks on the amplifier's front panel.

Be sure not to over tighen the mounting screws for the PF0030 or the stress can crack the internal circuit board.

Amplifier internal view.

The PF0030 is mounted to the aluminum base without using any heatsink compound under its flange.

Then the aluminum base holding the PF0030 is mounted inside the case using a very thin amount of heatsink compound.

Finished 5 watt, 900 MHz amplifier.

RF input of around 1 mW (0 dBm) is via the TNC connector on the left.

RF output of around 2 watts (+33 dBm) is via the TNC connector on the right.

The banana jacks are for the +12 VDC power.  RED is positive and BLACK is negative.  Current draw is under 3 amps.

The optional 1,000 pF feed-thru capacitor is used for the "ground for transmit" line.