Sunday, January 16, 2011

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IRF9530|IRF530 Simple MosFet Amplifier

This mosFet amplifier circuit was a sort of challenge: designing an audio amplifier capable of delivering a decent output power with a minimum parts count, without sacrificing quality.
The Power Amplifier section employs only three transistors and a handful of resistors and capacitors in a shunt feedback configuration but can deliver more than 18W into 8 Ohm with <0.08% THD @ 1KHz at the onset of clipping (0.04% @ 1W - 1KHz and 0.02% @ 1W - 10KHz) and up to 30W into a 4 Ohm load.

IRF9530|IRF530  Simple MosFet   Amplifier Circuit

List Component
R1 : 2K2 
R2 : 27K 
R3,R4 : 2K2 1/2W Trimmers Cermet or Carbon (or 2K)
R5 : 100R 
R6 : 1K 
R7,R8 : 330R 

C1 : 22µF/25V
C2 : 47pF 
C3,C4 : 100µF/50V  
C5 : 2200µF/ 50V
Q1 : BC550C 
Q2 : IRF530 
Q3 : IRF9530 

Setting up the Power Amplifier:
  • Connect the Power Supply Unit (previously tested separately) to the Power Amplifier but not the Preamp: the input of the Power Amplifier must be left open.
  •  Rotate the cursor of R4 fully towards Q1 Collector.
  • Set the cursor of R3 to about the middle of its travel.
  • Connect a suitable loudspeaker or a 8 Ohm 20W resistor to the amplifier output.
  • Connect a Multimeter, set to measure about 50V fsd, across the positive end of C5 and the negative ground.
  • Switch on the supply and rotate R3 very slowly in order to read about 23V on the Multimeter display.
  • Switch off the supply, disconnect the Multimeter and reconnect it, set to measure at least 1Amp fsd, in series to the positive supply (the possible use of a second Multimeter in this place will be very welcomed).
  • Switch on the supply and rotate R4 very slowly until a reading of about 120mA is displayed.
  • Check again the voltage at the positive end of C5 and readjust R3 if necessary.
  •  If R3 was readjusted, R4 will surely require some readjustment.
  • Wait about 15 minutes, watch if the current is varying and readjust if necessary.
  • Please note that R3 and R4 are very sensitive: very small movements will cause rather high voltage or current variations, so be careful.
  • Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.
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