HexFet Amp Based on Philips AH578 and LMK

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#41
NTC placement

@Minek. I think this is the easiest and perhaps most effective placement of NTC. What do you think about this?
 

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#42
I think this leg will be much colder than the transistor die.
In your example the good thing is that thermistor is soldered to the pcb, no wires needed.
I'm planning to glue it directly to the metal heatsink (metal tab) of the transistor, on its side.
I think this is the spot that's 'closest' to the die, and therefore it's temperature is also closest to the die temperature.
 

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#44
egra said:
Not bad, but not as good as HEXFET version.
Click to expand...


Just a question, to check my own understanding:


Does the flow go:


1. Input to opamp non inverting
2. Output of opamp goes to common base voltage amp
3. Output of common base voltage amp goes to another common base voltage amp
4. This follows into normal driver into output device


Is this about right? More wondering if I'm right about the common base into common base stuff. Thanks.
 

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#46
drinkingcube said:
Just a question, to check my own understanding:


Does the flow go:


1. Input to opamp non inverting
2. Output of opamp goes to common base voltage amp
3. Output of common base voltage amp goes to another common base voltage amp
4. This follows into normal driver into output device


Is this about right? More wondering if I'm right about the common base into common base stuff. Thanks.
Click to expand...

@drinkingcube The grounded base topology is very fast, Cob being put out of the running, Cbe is much smaller, and does not suffer from the Miller effect. Its current gain less than 1. It offers high isolation between the input and output results in little destructive feedback and high stability. So it's a great I/V converter. The only drawback is that it has low input impedance that's why we need opamp as a buffer at input. Sounds very clean, smooth and tube-like.

@minek I think it would be necessary to make an explanatory diagram to illustrate how this unique topology works. I would mark the HiZ and LoZ nodes and current and voltage conditions on it. I might ask for your help making this.
 
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#47
>explanatory diagram to illustrate how this unique topology works
Well, that would be great...

Some help could be provided by original articles describing these amps:
1) Original Wiederhold's article + crude translation
2) Original article (Lomakin and Parshin + crude translation

All relevant articles zipped and attached here.
And of course lengthy discussions in this thread.
That's where it all started 🙂

Maybe the best starting point would be to translate these articles correctly, and understand them?
My Russian is as good as my German 🙂 Some basics from 30 years ago...
 

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#49
egra said:
@drinkingcube The grounded base topology is very fast, Cob being put out of the running, Cbe is much smaller, and does not suffer from the Miller effect. Its current gain less than 1. It offers high isolation between the input and output results in little destructive feedback and high stability. So it's a great I/V converter. The only drawback is that it has low input impedance that's why we need opamp as a buffer at input. Sounds very clean, smooth and tube-like.

@minek I think it would be necessary to make an explanatory diagram to illustrate how this unique topology works. I would mark the HiZ and LoZ nodes and current and voltage conditions on it. I might ask for your help making this.
Click to expand...
Wow that is very interesting. Zin for each common base is re = 1/gm? EDIT: how is gain set? I see on wiki that the gain is gmRc, is Rc the input impedance to next common base in parallel with the 75 ohm resistor?
 
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