This is a no-tune 40W FM transmitter amplifier for 88-108MHz band. The amplifier has two stages. The first stage uses the 2N6080 transistor to drive the final stage of a 2N6084. The input power required to get 40-45W at the output, is about 0.8 Watt. Below, we present the circuit schematic and the PCB layout of the broadband 40W FM Linear Amplifier.
Some micro-strip lines are used at the input and output matching networks. They are radio frequency transmission lines of 50 and 35.5 ohm impedance having exact widths of 2.9 and 4.5mm respectively. These widths hold true for common epoxy fibre-glass circuit board of relative dielectric constant er=4.5 (@100MHz) and dielectric thickness of 1/16inch (1.59mm). The lengths of those lines have also been carefully chosen for achieving appropriate delay times (phase relations) for the rf signal. Thus, it is important to notice that the proposed pcb-layout design will work only for the specified FR4 board.
The broadband RF power amplifier is a class-C type amplifier. Class-C concept is preferred because of its high efficiency advantage. However, it is well known that class-C amplifiers produce a lot of harmonics. In order to reduce the harmonics at an acceptable level we use a 5-pole Chebyshev low pass filter (LPF) at the output stage. The specific Chebyshev filter has a small amplitude ripple of about 1db within the 88-108 MHz pass-band and it is designed for 3rd harmonic suppression of about 50db.
The broadband linear amplifier operates normally when powered from a 13.8V – 15V power supply unit. The current consumption, for 45W output, is about 5.5A. The total power gain within the 88-108MHz pass-band (FM Band/ Europe) is about 17db, and the efficiency approaches 60%.
The circuit schematic is quite simple. However, take some extra care for providing good grounding and adequate heat sinking. 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 transistors bodies and 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 voltage ceramics and the resistors are of standard 5% tolerance, 1/2W, and carbon type. The 22μH inductors are common high frequency rf chokes, able to handle up to 5Α. In the prototype, we use the famous VK200 rf choke which is no more in production. However, you can use any other type of high frequency rf choke having the same specifications. L1 is about 130μH and it is made of 6 turns in 7.5 mm diameter from a 14.1cm segment of a 1mm diameter (SWG 19) wire. L2 and L3 are of the same type. Their value is about 100μH and they are made of 5 turns in 7.5 mm diameter from a 11.7cm segment of a 1mm diameter (SWG 19) wire.
Printed Circuit Board Artwork for the no-tune 40W FM linear amplifier (paid-download)