10 watt 2304 MHz linear amp by W3KM

The following amplifier design was done by W3KM several years ago. This is a great amplifier to follow a 1-2 watt 2304 transverter. It uses a 28 volt device so that may be an issue for some. Check on availability of the device before you go about making parts for it. The device Dave used is no longer manufactured and I am not aware of any substitute though there may be one.

Dave's description:

This is not a Heathkit. The following are general hints for building a 10-Watt common base linear amplifier for 2304 MHz. It is assumed that the builder has previous experience with microstrip power amplifiers and their mechanical construction.

The amplifier is built on a brass or copper plate that is attached to a suitable heat sink. The heat sink for this power level should be in the order of 4" X 6" or larger, with ¾" to 1" high fins. A sidewall is used to mount the feed thru caps and power supply connections. Alternately, a milled out aluminum or brass enclosure could be used, the brass or copper amplifier pallet being fastened inside the enclosure.

A slot is milled in the pallet to accommodate the power transistor. The width of the slot is kept to a minimum so the leads of the transistor will be as short as possible. At the same time, the pc board is positioned flush with the edge of the milled slot for the same reason, otherwise lower gain and lower power output will result. The depth of the slot is such that the device leads will flush with the pcb. Slight bending of the leads is OK as long as the bend is not forced. The device needs to be installed with essentially no lead length, but at the same time it is important not to force the device against the pcb which may shear off the device leads. The leads of the device can be trimmed with a scissors, as the full length of the leads are not required. If you trim the leads, make sure you cut the collector lead on an angle to maintain the lead identification.

As far as the exact length of the pc boards, they were made to fit into a particular size amplifier housing. If you want to make the output board shorter, this is OK. The output line is 50-ohms, so it can be shortened. The dc blocking cap can also be moved a little closer to the device to make the board length shorter.

An important note which pertains to any amplifier construction: Don't mount the dc blocking caps against the SMA center pin, as this will cause chip cap breakage if the pin moves or the board is flexed.

You will need a -5V supply and a 24-26V supply both capable of several amperes. A variable 18 to 26V supply is required for tune up. The -5V P.S. feeding the TO-3 regulator is keyed on during TX only. The regulator must be mounted on a suitable heat sink and a socket is recommended for ease of installation.

The common base bias supply must be checked and set to minimum voltage before the connections are made to the microwave power transistor. Make sure the voltage goes up with the CW rotation of the pot, not down. The purpose of the voltage divider on the output of the bias source is two-fold. First, the power transistor's collector current must also flow thru a low impedance in the emitter path. The shunt resistor is therefore mounted on the amplifier housing. Second, voltage regulators have a minimum output voltage, -1.2V in the case of the LM337. A voltage divider is used to halve this to -0.6V, which is where the bias supply is initially set. The 10-turn pot provides a smooth bias adjustment.

Once the bias supply is working properly, connect it to the amplifier. Connect an RF load and power meter to the output. With a current meter connected in the collector supply, set 18-20V on the collector, with no RF drive. Turn on the -5V supply to the bias source and adjust the bias pot for 10-20mA idling current. Due to the leakage of the device, it may already draw a few mA before you adjust the bias pot. Bias adjustment is easily done with a digital current meter capable of handling a few amperes.

Apply input RF slowly while watching the output power meter. While tuning the output capacitor, adjust the drive up to the 1watt level. Increase the collector voltage to 24-26V. Check the idling current (10-20mA). Some tuning of the microstrip stub capacitors will probably need to be done to achieve the gain and power output. Depending on the devices, I have seen gains between 9 and 11dB and power levels of 10-15 watts saturated power. Power input can be up to 1.5 watts.