I am restoring and modifying a vintage Yaesu FT75B hybrid SSB transceiver. The receiver channel is completelly solid state, but most of the crystal oscillators and mixers was replaced from BJT to JFET with a notable improved noise reduction of the audio recovered. The tx stage includes 12BY7 and 6DQ6 tubes but still remains inoperative.
The original AN214 was burnt and I decided to make my own audio amplifier including only MOSFET and JFET too but maintaining the original japanese spirit. The mini amplifier then was build and mounted in a piece of protoboard in the IC place.
It gives about 1.5W from 2 x 2SK941 MOSFET biased class AB with only 25mA for both transistors. The exciter is a dual JFET 2SK150 as phase splitter and a 2SK117 as CCS. A NFB loop reduces the audio BW to not more than necessary for clean and comfortable listening. Note that this amp isn't for hifi, too much bass and trebble aren't desired for SSB listening. The 22nF in the primary also reduces high frequency responses.
Power MOSFET 's are very "pentodic" so the NFB reduces the output impedance. Bias is about 1.6V got from an internal regulator in the tranceiver. No heat sinks are needed with 13.5VDC until well past the 100mA bias current.
Output transformer is 600+600 turns 0.15mm wire bifilar, the secondary is 120T 0.5mm. No insulation needed between them. Core area is 8 x 10mm using plastic bobbin.
The original AN214 was burnt and I decided to make my own audio amplifier including only MOSFET and JFET too but maintaining the original japanese spirit. The mini amplifier then was build and mounted in a piece of protoboard in the IC place.
It gives about 1.5W from 2 x 2SK941 MOSFET biased class AB with only 25mA for both transistors. The exciter is a dual JFET 2SK150 as phase splitter and a 2SK117 as CCS. A NFB loop reduces the audio BW to not more than necessary for clean and comfortable listening. Note that this amp isn't for hifi, too much bass and trebble aren't desired for SSB listening. The 22nF in the primary also reduces high frequency responses.
Power MOSFET 's are very "pentodic" so the NFB reduces the output impedance. Bias is about 1.6V got from an internal regulator in the tranceiver. No heat sinks are needed with 13.5VDC until well past the 100mA bias current.
Output transformer is 600+600 turns 0.15mm wire bifilar, the secondary is 120T 0.5mm. No insulation needed between them. Core area is 8 x 10mm using plastic bobbin.
Attachments
Nice simple all-FET circuit.
Did you show us your transformer ?
EI-Core ? Material ?
Any distortion measurements ?
Patrick
Did you show us your transformer ?
EI-Core ? Material ?
Any distortion measurements ?
Patrick
Distortion?. Nahhh. SSB transmissions are inherently distorted. They are like AM but carrier and one of the sidebands suppressed by a powerfull crystal filter. So all power is used only where it is wanted. Audio response is deliveratelly restricted from about 300 and 3000Hz. Human voices only. Music is prohibited for ham service.
Material of the core is unknown. It has been selected in order that it can be accomodated inside the tranceiver. Lamination is 35mm wide 0.35mm thickness.
Material of the core is unknown. It has been selected in order that it can be accomodated inside the tranceiver. Lamination is 35mm wide 0.35mm thickness.
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