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Power Amplifier Mosfet OCL 70W HIFI Stereo with IRF640,IRF96

2015-01-05 10:48  
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This circuit is from David White’s article in Electronics World (August 2001). It uses inexpensive HexFetâ„¢ Power MOSFETs as output devices. Since their transconductance is low, the rest of the design is geared towards producing maximum open-loop gain. Overall Negative Feedback is used to set the gain to 27dB and to reduce the distortion.The input devices used (2SA970) are low-noise, high gain PNP devices. They have poor Vce(max), so their supply voltage is reduced using a simple Zener diode regulator built using R3, ZD1 and C3. T1 is used as a current source. The author of the original EW article mentioned using a plain gate-source connected JFET, which is selected to produce 1mA current. At Rs. 25 a pop, I was not about to do this with BF245s. Besides, the current for my G-S connected BF245s is around 7mA. Insert a resistance in series with the source to reduce the current. Since the source-drain current is dependent on the manufacturing process, you should select the resistor for each BF245 in a multiple-amp. system. The value should be selected for 1mA current. If required, use a 1k pot here.IRF640