50W Broadband FM Amplifier

This is a 50W broadband power amplifier for FM Broadcast. No tune is required for this linear amplifier. It has the same performance from 87.5 to 108MHz. Although normal output is at 50W, its maximum power is about 80W. The design is based on the MRF173 MOSFET which is designed for broadband commercial and military applications up to 200 MHz frequency range.

The design incorporates micro-strip technology, a low pass filter and a directional coupler for Forward and Reverse output power voltage readings. Both input and output are optimized for 50 Ohm standard impendance.

With an input of 1.2W, output power is guaranteed to be more than 50W in any frequency within the 87.5 - 108MHz band. With input power up to 2.5W, output power may approach or exceed about 80W. The typical efficiency of the broadband amplifier is about 60% and its power gain is about 17db.

The on-board LPF (low pass filter) provides great suppression for harmonics. When this amplifier is driven from a good exciter, output harmonics are at least 50db below the career (-50dbc). The on board directional coupler provides two outputs for simultaneously voltage readings for Forward and Reverse output power. Typical voltage readings on this coupler are about 1.53V, 1.21V, 0.85V and 0.5V for 50W, 30W, 20W and 10W respectively. The output voltage of the coupler actually has an exponential depend on the power. This exponential dependency is due to the nature of the diodes current. However, regarding power in db (dbm or dbW) voltage readings are actually linear.

For normal operation, the amplifier requires power supply at 28V DC. For 50W output, the supply current is about 3.7A at 28V. To bring the amplifier to its limits and get almost 100W of output power, you may apply up to 30V, and maximum supply current may approach up to 5A but do not exceed these limits!

The amplifier requires a large heat sink. A typical heat sink for this amplifier must be at least about 150x80x40mm. Air fan should also apply on this heat sink, and the typical temperature on the heat-sink must not exceed about 50°C (122°F).

How to build the amplifier

Since this is a no – tune design, its construction is relatively simple. All you need is a suitable printed circuit board, the MRF173, and some good quality capacitors and resistors. You will also need to construct some helical coils, and solder them on the board.

The recommended printed circuit board has some micro-strip lines. These lines have exact dimensions (width and length). The width of a micro-strip line determines the characteristic impedance of the line, and its length determines appropriate delay (phase relations) for the RF signal. The actual dimensions on the proposed PCB hold true for common epoxy fibre-glass circuit substrate of relative dielectric constant er=4.5 (@100MHz) and dielectric thickness of 1/16inch (1.59mm). Thus, it is important to notice that the proposed pcb-layout design will work only for the specified FR4 substrate and will fail for any other substrate or board thickness. If you wish to use a different substrate, you may use some scaling to recalculate the appropriate dimensions for the micro strip lines on your custom board.

The bottom side of the PCB serves as a ground plane and its surface is completely covered by copper. The bottom side is directly connected on the heat sink. All the components are soldered on the top side of the PCB. Some copper-patches on the top are grounded (connected to the bottom side) threw rivet-vias.

All capacitors are low tolerance ceramics and they must withstand 100V at least. You may use SMD or through – hole but you should never use multilayer capacitor or non – ceramics. The PCB was actually designed for 1210 SMD capacitors (3.2 x 2.5mm), however you may also use SMD capacitors with 1206, 1218 or 2010 cases without any problem. All these cases will perfectly feet on the PCB. Moreover, if you don’t have any SMD components you may even use standard through – hole components. They will also fit in the board but they must be soldered as being SMD (by using surface mound).

All resistors are of standard 5% tolerance or better. The recommended case type is 1210 (3.2 x 2.5mm). Again, you may also use 1206, 1218, or 2010 cases without any problem. There are two exceptions for R1 and R5. These two resistors should be of 2512 SMD case type, or of regular through – hole 1/4W type.

All inductors, except L7 and L8 are made from a SWG 20 wire (0.914mm). L1, L2, L3 and L4 are of 4, 2, 2 and 5 turns, respectively, of 3.5mm in diameter. L5 and L6 are identical and they are made from 5 turns of 4.5mm in diameter. L7 is made from a SWG 21 wire (0.813mm) and has 18 turns of 6mm in diameter. L8 is a common high frequency rf choke, able to handle up to 5A and its inductance is about 100μΗ. In the prototype, for L8, we use a common VK200 rf choke for VHF. If you cannot find VK200 for VHF in any local shop you may use any other high frequency rf choke having the same inductance specifications.

For the RF amplifier to work properly, only one setting is required. You should set the R3 potentiometer to get about 2.8 to 3V on Q1’s gate.

Attachments

Printed Circuit Board Artwork for the Broadband 50W FM broadcast amplifier (paid-download)