Resistors are much cheaper to dissipate power in than transistors. If your signal swing is less you can use less voltage across the transistor and more across the resistors. If you have, for instance, a +/-90V swing, you really need to make the idle voltage across the transistor be 1/2 the 200V power supply. You can choose which part has how much voltage, but you need to leave enough voltage across the two resistors so the biggest signal swing you want out can be accommodated. The same current will flow in them, so the power will be the current times the fraction of the 200V across each part. You have to choose the amount of the 200V supply to be dropped across the transistor versus R23+R24. It would be wise to derate that by half to 350mW. The *power* dissipated in the transistor and resistors will be a problem. The output impedance will be R23 paralleled by R24 to a close approximation. If you want a 4:1 reduction in signal level, R23 will be 1/3 of R24. They're determined by power and output impedance considerations. For R23, I assume the value depends on the biasing resistors and the output impedance I wish, but what value can I choose? I added a R24 resistor cause I feel I'll have to attenuates the signal by a factor of 4:1Actually, no, the result does not depend on the biasing resistors. recommend anything from 100R to 1M100R to 1K, anywhere in there. The higher the resistance, the bigger the noise this creates from thermal noise. If a guitar drives it directly, use 1M or 2.2M. You want the parallel combination of R21 and R22 to be greater than 10x whatever the source impedance of the signal source is. Quote-For R21 and R22 biasing, I saw values from 1M to 10M on the internet, is there a particular reason to go as high as 10M ?It depends on what is driving the input. Put a 1M or so from the outside of C16 to ground. Since you may not know that, make it as big as you can reasonably afford to. The value of C16 depends on the input impedance of whatever you'll drive. Big values of R21 and R22 make for smaller values of C15. Quote-What value can I choose for C15 and C16?The value of C15 at the input will be C = 1/(2*pi*R21||R22 * F) where F is the lowest frequency you have to pass at -6db down from the midband. The cathode of the zener goes directly to the junction if the gate/R25. So if I understand correctly, it should be something like this :
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