15W FM Linear Amplifier


The 15 watt FM linear amplifier is based on the PD55015-E MOSFET and operates with a 12-15V supply voltage. It can be built very easily on a 1.6mm thick FR4 epoxy board. It is capable to deliver up to 15W on a 50 ohm load when driven from about 250-300mW. So, it is really very useful for upgrading any low power FM PLL exciter.

The printed circuit board (PCB) used in the prototype is a double sided one. All the components are soldered in one side and the other side serves as a ground plane. The circuit board is attached to a heatsink. There are many plated holes on the PCB. These plated holes serve as vias to connect one side of the board to the other. If you are not able to manufacture such a circuit board with plated holes, you still have the option to make the vias by using wire pins. However, regarding the vias on the pad located below the mosfet, some special care is essential because this specific pad ensures proper heat sink.

electronic schematic
Fig 1. The electronic circuit of the 15W FM Linear Amplifier

The PD55015-E is a surface mount type component and it needs to be in direct contact with a proper heatsink via a pad or by some other means. For more information on how to ensure proper heatsink for this type of mosfet you may refer to the mounting recommendations in the application note AN1294, from ST.

For full scale output, the input signal should be slightly greater than 250 milliwatts in the frequency range of 88-108MHz. C1, C2, C3 and L1 serve as an input impedance matching network to match any 50 ohm input source to about 4 ohms, which is the characteristic input impedance of the PD5015 in the specified frequency range. L3, C12 and C13 form an output impedance matching network to mach the output impedance of the MOSFET, which is about 6-8 ohms to a 50 ohm standard output. L4, together with the C14-C15 capacitors, form a low-pass filter which suppresses any spectral content above 130 MHz.

How to assemple the amplifier
Fig 2. How to assemble the FM Linear amplifier

To make the MOSFET work as an RF amplifier it is necessary to bias its Gate with a fixed DC voltage of about 2.7 volts.This voltage is provided from the R2-R3 resitive voltage divider which is powered from a classic 78L05 voltage regulator (IC1). The main power from 12-15V is applied to the mosfet’s drain through an appropriate filtering network of capacitors and rf blocking inductors (rf chokes).

Making the inductors
Fig 3. Making the inductors on a 7mm drill bit

The 22μH inductors are common high frequency RF chokes, able to handle up to 5Α. In the prototype, we use the old standard type VK200 RF choke which is no more in production. The L1, L2, L3 and L4 inductors are made of 3, 5, 3 and 6 turns, respectively, in 7mm diameter, from 1mm diameter (SWG 19) wire (see figure 3).

Attaching the heat sink
Fig 4. Attaching the heat sink to the ground plane side


Printed Circuit Board Artwork for the 15W FM Linear Amplifier on pdf 

Note: The prototype's printed circuit board is made on a common FR4 board of 1.6mm thickness.


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