What is the law?
Don't use RF transistors in audio circuits.
RF transistors may have low capacitance, but have notoriosly low breakdown voltages. They are any number of transistors with the low capacitance and noise, and high beta associated with a small base region that have much higher breakdown voltages.
While FETs designed for audio may have roughly the same breakdown voltage as the part you came up with, they do have the most important property: namely transconductance.
Forget about noise figure measurements at audio. That is basically a measurement for RF that really does not apply here. (It is way of deciding if the stage you are working with will have enough gain and a low enough noise contribution to set the overall noise level.) Do worry about equivalent noise voltage, and the source impedance.
And don't forget the law.........bandwidth limit the input signal.
Don't use RF transistors in audio circuits.
RF transistors may have low capacitance, but have notoriosly low breakdown voltages. They are any number of transistors with the low capacitance and noise, and high beta associated with a small base region that have much higher breakdown voltages.
While FETs designed for audio may have roughly the same breakdown voltage as the part you came up with, they do have the most important property: namely transconductance.
Forget about noise figure measurements at audio. That is basically a measurement for RF that really does not apply here. (It is way of deciding if the stage you are working with will have enough gain and a low enough noise contribution to set the overall noise level.) Do worry about equivalent noise voltage, and the source impedance.
And don't forget the law.........bandwidth limit the input signal.
Q2 does not provide any voltage or current gain. Q1 provides the current gain. R1 provides the voltage gain. Q2s purpose is to minimize changes in Vds. A BJT does this job well. If Vds noise is not too important (depends on choice of Q1) you don't need Q2.
The output transistor(s) should be BJTs. FETs have too low a Gm and are relatively non-linear in source-follower configuration, especially without feedback.
For psu I suggest a simple LM317LZ close to the circuit with a 1uF polypropylene on its output to gnd.
"I feel the need...the need for speed"
I used to show off to my friends about how fast the transistors were in my latest design. They would say "but you can only hear up to 20kHz so why do you need 100MHz ft?". I told them they simply didn't understand about circuit design. In hindsight I now realize that it was me who simply didn't understand about circuit design and even worse I discovered I wasn't Tom Cruise!
They were, of course, correct. Why does the circuit need to amplify ultra-sonic frequencies? Will Fido appreciate them? Bats? Umm no. And Jocko is right - you don't want to be amplifying a lot of ultra-sonic noise because sometimes non-linearities and circuit instabilities and unplanned feedback paths conspire to turn some of this energy into audible artifacts. It is worth considering how to limit the bandwidth of the circuit and/or the input signal as Jocko mentioned.
What's the ideal FET for this circuit? Perhaps a device with very low distortion and a flat transconductance in the audio band, and low enough input capacitance (Ciss) that the source sees a fair and linear load. Does it have to be an "unlawful" RF device - probably not.
BAM
The output transistor(s) should be BJTs. FETs have too low a Gm and are relatively non-linear in source-follower configuration, especially without feedback.
For psu I suggest a simple LM317LZ close to the circuit with a 1uF polypropylene on its output to gnd.
"I feel the need...the need for speed"
I used to show off to my friends about how fast the transistors were in my latest design. They would say "but you can only hear up to 20kHz so why do you need 100MHz ft?". I told them they simply didn't understand about circuit design. In hindsight I now realize that it was me who simply didn't understand about circuit design and even worse I discovered I wasn't Tom Cruise!
They were, of course, correct. Why does the circuit need to amplify ultra-sonic frequencies? Will Fido appreciate them? Bats? Umm no. And Jocko is right - you don't want to be amplifying a lot of ultra-sonic noise because sometimes non-linearities and circuit instabilities and unplanned feedback paths conspire to turn some of this energy into audible artifacts. It is worth considering how to limit the bandwidth of the circuit and/or the input signal as Jocko mentioned.What's the ideal FET for this circuit? Perhaps a device with very low distortion and a flat transconductance in the audio band, and low enough input capacitance (Ciss) that the source sees a fair and linear load. Does it have to be an "unlawful"
RF device - probably not.BAM
Fets vs. Bipolar
Well...... I am starting to get through to some people. I still maintain that fets sound better than bipolars. Don't take my word for it, go see what the best designers are using for voltage gain stages. (Pass, Curl, Borbely, McCormac) If you will bias them hot, output stages as well. JFET followers are fine for driving down to 10K loads. I do it all the time and they sound great. I prefer fets for the upper parts of cascodes and for current sources as well. Read the reviews of the newest solid state amps, go to the designers web sites. Stop reading and go build some circuits and
listen! There are a great couple of jfet articles at:
http://www.borbelyaudio.com/
H.H.
Well...... I am starting to get through to some people. I still maintain that fets sound better than bipolars. Don't take my word for it, go see what the best designers are using for voltage gain stages. (Pass, Curl, Borbely, McCormac) If you will bias them hot, output stages as well. JFET followers are fine for driving down to 10K loads. I do it all the time and they sound great. I prefer fets for the upper parts of cascodes and for current sources as well. Read the reviews of the newest solid state amps, go to the designers web sites. Stop reading and go build some circuits and
listen! There are a great couple of jfet articles at:
http://www.borbelyaudio.com/
H.H.
As for JFET followers.........
I have been designing and building equipment with them professionally for about 20 years, so don't try to tell me they don't work.
Unless you are trying to drive a 600 ohm load.
If one doesn't have enough transconductance, then put more in parallel.
Back to bandwidth limiting the signal.........
Where is your gate damping resistor in the FET circuit? Even if you insist on running it wide open, you are likely to have some strange anomalies in the response without one. (Of course, you may need a good 'scope and square wave generator to see it, but it will be there.)
Some of the designers I have worked with swear by the fact that any circuit with funny high-frequency response will have strange sounding bass. Their claim, not mine. But if does build a circuit that does weird things outside of the audio band, it will sound strange somehow.
Hairy Holler is right........we should charge for information this good. Heaven knows the crooks we design this stuff for don't.
I have been designing and building equipment with them professionally for about 20 years, so don't try to tell me they don't work.
Unless you are trying to drive a 600 ohm load.
If one doesn't have enough transconductance, then put more in parallel.
Back to bandwidth limiting the signal.........
Where is your gate damping resistor in the FET circuit? Even if you insist on running it wide open, you are likely to have some strange anomalies in the response without one. (Of course, you may need a good 'scope and square wave generator to see it, but it will be there.)
Some of the designers I have worked with swear by the fact that any circuit with funny high-frequency response will have strange sounding bass. Their claim, not mine. But if does build a circuit that does weird things outside of the audio band, it will sound strange somehow.
Hairy Holler is right........we should charge for information this good. Heaven knows the crooks we design this stuff for don't.
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