300w Rf Amplifier Using Low Priced Mosfet

By :  Ir.Eliza Sukiman.MM.MA

I have read Mosfet Rf Amplifier  diagram using mosfets on the ARRL Handbook . It might be of further interest to describe a higher power version of a linear amplifier which I had reason to design, using two IRFP 150. The Circuit can deliver a power off 250w at 1.8 MHz, 300w at 3.5 MHz and 200w at 7 MHz with 36 v . The IRF150 transistors used were the International Rectifier (IR) .

The two push pull amplifier and combiner was biased to operate in a nominal class AB mode with a standing drain current of 150 mA per transistor and a current swing up to 5 amp per transistor at full power. The power supply used was nominally 36 volts.

However, the power MOSFET has very high input capacitance; in the case of the IRFP 150, around 2200 to2800 pF. For a broadband amplifier, this capacitance must be loaded down with shunt resistance so that the impedance presented to the driving amplifier is reasonably constant over the required frequency range.

A small amount of drive power is therefore consumed in this resistance load. The other alternative is to have a tuning system at the input which is used to resonate the circuit at whatever specific frequency is in used.
Gate to source biasing is developed from a 5V supply through a LM 7805 network.

Each transistor gate is loaded down with a 470 resistor selected to mask the input capacitive reactance within the MF range. At amateur band frequencies, input impedance is essentially the input capacitive reactance of the MOSFET transistors and lower values of load resistance could have been considered. R 10 ohm is parasitic suppressors which should be mounted directly on the transistor gate pins. These were included as a precaution rather than because of any problem experienced.

At low frequencies, the value of Vp approaches the voltage of the supply rail giving an output power calculation of over 300w. As frequency is increased, a limiting factor on the value of Vp called slew rate comes into effect. If we consider an output signal waveform, the maximum rate of change in voltage of the waveform, or the maximum slope of the waveform, increases with both frequency and amplitude of the waveform . For a given frequency, maximum possible output voltage is achieved when the slope equals the amplifier slew rate. When frequency is increased, the maximum voltage swing is reduced and so also is the maximum power output. This explains why we can only get 250w at 1.8 MHz,300w at 3,5 Mhz and an even lower value of 200w at 7 MHz.
see . . .   schematic

If toroidal cores are used to construct the inductors, iron dust cores and not ferrite types are recommended. The ferrite cored inductors have been found to change their value of inductance when high power is pumped through them, resulting in de-tuning of the filter. The iron dust cored inductors are more stable in this respect.

IRF MOSFET can be used to provide moderately high power amplification on the lower frequency amateur bands. Maximum power output decreases as frequency is increased but quite reasonable performance can be achieved at 1.8 and 3.5 MHz using an amplifier such as the one described. The power reduced at 7 Mhz but the Amplifier is still usable. . . .

See Youtube 300w ...  Click Here     

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See Youtube  800w ... Click Here

 

Test Report Mosfet Linear RF Amplifier

Freq

3.80 Mhz

7.00 Mhz

Input

Output

 

Watt

Watt

 

1

20

9

2

40

18

3

60

25

4

85

40

5

110

50

6

125

60

8

160

70

10

220

90

15

350

120

20

 

185

25

 

200


 
References
1 International Rectifier HEXFET Databook - Power MOSFET Application & Product Data.
2 Motorola Handbook - Power MOSFET Transistor Data.
3 Amateur Radio Hand book 2007 - ARRL
4 Lloyd Butler - Tank Circuits & Output Coupling - Amateur Radio May 1988, plus corrections July 1988.

New power-MosFets coming out every week or so, look for ones with a low Rds-on and high power dissipation. The main drawback is the high gate capacitance, several thousand pF, which makes them hard to drive above 7MHz. Check out IRF , IXYS and APT ( FET manufacturers) .

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