150W Broadband FM Amplifier

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This is a 150W broadband power amplifier for FM Broadcast. No tune is required for this linear amplifier. Although normal output is at 150W, its maximum power is about 170W. The initial design is based on the MRF9180 MOSFET. The amplifier also works very well with other similar types of MOSFET. The circuit has also been tested with the MRF186. By using an MRF186 instead of the MRF9180, the normal output is at 100W. 

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 impedance.

The required Input power for 150W output is about 3.5W. The typical efficiency of the broadband amplifier is about 70% and its power gain is almost 17db. The on-board LPF (low pass filter) provides good suppression for harmonics. When this amplifier is driven from a good exciter, output harmonics are at least 40db below the career (-40dbc). The on board directional coupler provides two outputs for simultaneously voltage readings of Forward (FW) and Reverse (RW) output power. Coupler’s outputs are exponential depended on the power. This exponential dependency is due to the nature of the diodes used as the power detectors. However, regarding power in db (dbm or dbW), voltage readings are actually linear.

The electronic schematic of the 150W broadband Fm Amplifier
Figure 1. The electronic schematic of the 150W broadband Fm Amplifier

For normal operation, the amplifier requires power supply at 26V DC. For 150W output, the supply current is about 7.5A. To bring the amplifier to its limits and get almost 200W, you may approach up to 9A but do not exceed this limit!

The amplifier requires a large heat sink. A typical heat sink for this amplifier must be at least about 200x120x80mm. Air fan should also apply on this heat sink.

How to build the amplifier

The critical aspects regarding this amplifier are the T1 balun and the output matching network. T1 is a balun which serves as an input matching network and it must be build with care, according to the instructions provided below. Failing to appropriate construct this balun will result in a total failure for the whole design.

The MRF9180 was mainly designed for operation at industrial applications at frequencies from 865 to 895MHz. It is a broadband device with no internal matching so it is possible to make it work at 87-108MHz. The device contains a pair of matched MOSFETs and each MOSFET gate has an RF input impedance of about 10ohms. When fed together will result in the matching transformer looking at 5 ohms impedance and so T1 is actually a 9 to 1 step down transformer or of a turns ratio of 3 to 1.

The 150W Broadband FM Amplifier
Photo 1. The 150W Broadband FM Amplifier

The output network is quite straightforward. It consists of the CB1, CB2 and CB3 coaxial cables.  Since the linear amplifier is a no – tune design, its other aspects regarding the construction are relatively simple. You will need the printed circuit board, the MRF9180, 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 0.062inch (1.5mm). 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.

How to assemble the 150W Broadband amplifier
Figure 2. How to assemble the circuit 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). The preferred option for C16-19 and C20-25 is the ATC type ceramics.

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 R2. These two resistors should be of 2512 SMD case type, or of regular through – hole 1/4W type.

L1 and L2 and L3 are of 3 turns of a AWG 18 enameled wire (1mm), of 6mm in diameter (internal diameter of each turn). RFC2 is a high frequency rf choke, made from 4 turns of a AWG 18 wire. The 4 turns are wound on a TF9X8X5 Ferrite toroidal from Ferrocore. The specific core is a typical Ni-Zn ferrite core for TVs, FM and AM tuners. The inductance of RFC2 is about 22 to 33uH and it is not very critical, so you may build this choke on any other high frequency ferrite.For RFC1, 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 (about 22uH).

RFC2 details
Photo 2. RFC2 is made from 4 turns

For the RF amplifier to work properly, only one setting is required. You should set the R5 potentiometer to get about 2.7 to 3V on MF1’s gate.

How to build the T1 balun

T1 is actually a 9 to 1 step down transformer. This results to a turns - ratio of 3 to 1. Its primary winding is made from 3 turns of an AWG30 (0.254mm) enameled wire. Its secondary is made from 2 metal tubes and two small pieces of circuit board. These small circuit board pieces are part of the PCB Artwork which is provided below.

The T1 is made on a Binocular ferrite core from Amidon. In our design, we used the Amidon BN43-202 core.

How to assemble the T1 balun
How to assemble the T1 balun

The two tubes must be pushed through the holes of the binocular core and the two small pieces of the circuit board must be positioned at each end and carefully soldered to the two metal tubes (see photographs for details). Essentially, when the assembled transformer core is soldered to the pcb, these two tubes form a one turn loop, which is actually the secondary winding of the transformer. Finally, the three turns of the primary winding of the T1 must pass inside the tubes (see photo 3).

T1 construction details
Photo 3. The primary winding passes inside the tubes

Each metallic tube is tin plated and about 17.5mm in length, and is external diameter is 3.8mm (the same as the hole diameter of the core). It is actually somehow hard to find tubes at exactly this size. So, you may build your own from a thin tin- platted foil sheet or from a copper foil sheet. An alternative solution could be the use of the semi-rigid shield of an RG142 coaxial wire – the same wire that can be used for CB3.

Output Matching Network

The output matching network consists of the CB1, CB2 and CB3 coaxial cables. CB1 and CB2 are made from two pieces of a 25 Ohm semi rigid coaxial cable, and CB3 is made from a piece of a 50 Ohm semi rigid coaxial cable. All three cables have the same length. Their lenght is 17cm. We used Teflon based semi rigid cables. The outer diameter of the cables is about 2.1 and 4.5mm, for CB1-CB2 and CB3 cables, respectively.  

Attachments

Printed Circuit Board Artwork for the Broadband 150W FM Broadband Amplifier (paid-download)

 

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