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Old 16th June 2009, 08:29 PM   #81
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In my opinion, Bipolar transistors should only be used Class A.
If you go AB, D or any of them other funky letters, best abuse
some other type of device with less carrier storage weirdness.

Or drive yourself crazy making them only approximate class B,
never quite turning fully on or off. Very tricky business that....

Recovery from saturation and/or cutoff usually involves some
sort of tail spike, overshooting its way blindly out of the dark
and back into the land of analog control.
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Old 23rd June 2009, 01:24 AM   #82
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I'm going on a Rush Rampage.

Here a version of my circuit using the Rush cascode. Distortion at 1KHz is actually better than with the LTP, but is marginally worse at 20KHz. The spectrum looks a lot better though!

The offset compensation is abused to adjust for the offset produced by the base current of the input, but the value of R3 should really be around 1.1k.

Here is the schematic.

Click the image to open in full size.

And here is the FFT at 20KHz. This is exactly the same as what it looks like at 1KHz, just magnified by the affects of Cdom.

Click the image to open in full size.

Overall, it seems the Rush Cascode has the effects of lower distortion as well as dominant 2nd harmonic, but the affect of Cdom is worse at high frequencies. If second harmonics are as pleasant as they're supposed to be, then this should not be a problem.

- keantoken
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Old 23rd June 2009, 01:50 AM   #83
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Try returning Cdom to between the emitters of Q1/Q2. You will get improved PSRR that way because it's no longer referenced to the supply rail.
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Old 23rd June 2009, 02:47 AM   #84
Bigun is offline Bigun  Canada
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Keantoken,

This is a fascinating circuit, it might even tempt me into ClassA one day
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Old 23rd June 2009, 03:18 AM   #85
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Thank you, Bigun. Personally, I got into class A not because of low distortion, but because it is easier. I recommend starting with class A in order to familiarize yourself with the theory without unintentional issues.

I theorize that a class-A output stage is the best to reveal the Rush Cascode's true nature...

Evil, I simulated your suggestion. I had to increase Cdom to keep it from oscillating. Seems like a good idea, but it increased THD at 20KHz. Oh, the humanity! I will have to somehow weight the benefits with the disadvantages.

- keantoken
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Old 23rd June 2009, 03:57 AM   #86
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I don't see what keeps your DC center from drifting?
With temperature and whatnot...

Is 2SJ108 gate forward biased? If so, BJT mode now...
Nothing that would actually stop it from working....
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Old 23rd June 2009, 01:14 PM   #87
Bigun is offline Bigun  Canada
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Keantoken,

Hope you don't mind, but I borrowed your spice simulation commands from post 82. They work really nicely. I'm learning everyday
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Old 23rd June 2009, 01:52 PM   #88
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by keantoken
...Evil, I simulated your suggestion. I had to increase Cdom to keep it from oscillating. Seems like a good idea, but it increased THD at 20KHz. Oh, the humanity! I will have to somehow weight the benefits with the disadvantages...
Well, ultra-low distortion looks nice on paper, but it's not necessary, especially at 20kHz where all the harmonics are outside the range of human hearing anyway.

Another idea: You're not taking advantage of half the gain available - put the other output from the input stage to use somewhere by putting a current mirror on the collector of Q1.
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Old 23rd June 2009, 06:05 PM   #89
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Quote:
Originally posted by kenpeter
I don't see what keeps your DC center from drifting?
With temperature and whatnot...

Is 2SJ108 gate forward biased? If so, BJT mode now...
Nothing that would actually stop it from working....
When the temperature increases, Q4's Vbe decreases, decreasing the CCS output current. Since R2=2*R3, there is 2*Vbe(Q4) across this resistor, which compensates for the voltage drop of the two Rush transistors, which also vary with temperature. You must choose Q4 to have a tempco similar to the Rush transistors in order to have good temperature compensation.

The benefit of the Jfet is that the base current doesn't skew the output. At low currents I consider this necessary. You must know that the CCS is deliberately misadjusted to compensate for the offset created by the base current of Q2.

Quote:
Originally posted by Bigun
Keantoken,

Hope you don't mind, but I borrowed your spice simulation commands from post 82. They work really nicely. I'm learning everyday
I want to soon make some posts on simulation that will include the full command sets, so absolutely no problem there.

Quote:
Originally posted by Mr Evil

Well, ultra-low distortion looks nice on paper, but it's not necessary, especially at 20kHz where all the harmonics are outside the range of human hearing anyway.

Another idea: You're not taking advantage of half the gain available - put the other output from the input stage to use somewhere by putting a current mirror on the collector of Q1.
I have done this before in simulations. My modification is attached. At first I had stability problems, but the capacitors shown keep stability in check nicely it seems, with minimal rise in 20KHz distortion. Because of the voltage drop of the diode in the VAS, we have reduced VAS gain, which increases distortion to more than it was without the modifications however. I think this method is more suitable for driving a Jfet as a VAS (!).

I don't know if I remember correctly, but I think Q17 increases stability (it's probably not necessary).

Also, the feedback cap is replaced by a voltage source to keep the wandering offset from skewing the FFT.

- keantoken
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Old 25th June 2009, 12:37 AM   #90
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Hi keantoken,

I am following your design with some interest!

I happened to notice that your slew rate was only 20v/usec?
Wondering if there was a way to get that up to about 75v/usec?
Or, what is slowing it down...?

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