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Another very fast CFA concept
Another very fast CFA concept
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Old 28th September 2017, 04:31 AM   #21
sajti is offline sajti  Hungary
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Quote:
Originally Posted by kct View Post
Hi Sajti,

well it seem to give better THD with higher values.
I will try with lower values again and see if this gets better yet.
I use 470/150ohms in my V4 amp. It's a compromise between the THD and the speed. And the current gain with the mirror is maximum 4-5, which is possible with discrete bjts.

Sajti
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Old 28th September 2017, 05:44 AM   #22
HI-FI PRO is offline HI-FI PRO  China
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I'd say the current mirror would work equally well with 470ohms emitter vs 4.7k ohms emitter.
The main thing you want for best operation is perfect matching.
I do not understand where that guy was comming from, where emitter resistance higher then your transistors are not a real current mirror anny more.
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Old 28th September 2017, 03:34 PM   #23
sajti is offline sajti  Hungary
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Quote:
Originally Posted by SMY14INCH View Post
I do not understand where that guy was comming from, where emitter resistance higher then your transistors are not a real current mirror anny more.
Why are you attack me, if You don't understand the main benefit of using current mirror????

Sajti
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Old 28th September 2017, 07:35 PM   #24
HI-FI PRO is offline HI-FI PRO  China
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Originally Posted by sajti View Post
Why are you attack me, if You don't understand the main benefit of using current mirror????

Sajti
You said that if you make the emitter reistance too high, then the transistor pair isn't a "true current mirror any more".

I do not see where your comming from, and I do not think this is correct

Quote:
Originally Posted by sajti View Post
Why R28-31 are so high value? That stage is not a real current mirror using such high values. One tenth of them would be better.

Sajti
If I actually have allot of time which I do not, I would make my own version of this high freq op amp as a test model.
However due to lack of time, I don't even have time to build any more productive amp schematics which I designed.

Last edited by HI-FI PRO; 28th September 2017 at 07:49 PM.
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Old 28th September 2017, 08:12 PM   #25
miro1360 is offline miro1360  Europe
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I did this small and cheap 5x5 cm test board (50W/8R) from basic schematic to test, if it is sounding good in basic concept, --- if yes, than any improvement can bring big value here ...
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Old 28th September 2017, 08:25 PM   #26
HI-FI PRO is offline HI-FI PRO  China
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You can always build a amp on a prototype board if your confident and exprienced.
It takes me normally 1 day to make a test model on a prototype board, if everything goes right first time I may finish a channel in a day if not takes longer.
I normally perfer to save money so I can buy more components. Sometimes things don't work out with pcb then the money is wasted.

Last edited by HI-FI PRO; 28th September 2017 at 08:45 PM.
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Old 28th September 2017, 08:48 PM   #27
miro1360 is offline miro1360  Europe
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fun is, that I have only small prototype board where is old arduino project you are right, I must buy some for these things ...
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Old 28th September 2017, 08:52 PM   #28
HI-FI PRO is offline HI-FI PRO  China
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Prototype board is a trade off, your circuit takes up more space, and your much more likeley to make connetion error when buiding. It is also more time consuming using prototype. Depends on personall prefrence

I normally use this one for most projects, more than enough for a channel with relay. installed.
20Pcs 9 x 15 cm DIY Prototype Paper PCB Universal Board Good quality-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group
You can also get 10x22 and 12x18 and 5x7 boards
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Old 28th September 2017, 09:10 PM   #29
miro1360 is offline miro1360  Europe
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these boards you think, I thought you mean these solderless:
Mb-102 Mb102 Breadboard 830 Tie Point Solderless PCB Bread Board for Arduino | eBay
... I look into that what you sent ...
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Old 29th September 2017, 04:47 AM   #30
sajti is offline sajti  Hungary
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Join Date: Apr 2003
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Quote:
Originally Posted by SMY14INCH View Post
You said that if you make the emitter reistance too high, then the transistor pair isn't a "true current mirror any more".
I do not see where your comming from, and I do not think this is correct
If I actually have allot of time which I do not, I would make my own version of this high freq op amp as a test model.
However due to lack of time, I don't even have time to build any more productive amp schematics which I designed.
The basic idea behind the current mirror, to keep the voltage change low (few mV only!), to drive the output bjt. This small change means high speed, because it don't have to charge/discharge the internal capacitances.
If You check the schematic provided by kct, using 5.76k/3.92k, You will see that the voltages are very high. And what happening, if You simply shorting the C-E of Q4, and Q8? The output current of Q3 and Q9 will be lower with some 5%, but the amplifier will work. What does it mean? It means, that Q3, and Q9 are simple common emitter stage, driven from 5.76k source impedance, which slowing down the whole amplifier. So Q4, Q8 does practically nothing, just add 0,6V voltage shift, and some thermal compensation. This is not a true current mirror any more.

Sajti
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