A study of current feedback

[digi01]

this is a headamp design by I and my friend kknd.
anyone have build the same configuration current feedback headamp or power amp?

the attached is a sch file.

[diy_audio_fo]

Originally posted by digi01
this is a headamp design by I and my friend kknd.
anyone have build the same configuration current feedback headamp or power amp?

Dear digi01,

I never built such amplifier but I have few quick notes:

1) The Q1 collector - Q5 base (and the simmetrical Q2 collector -
Q6 base) connection is same form of positive feedback that recalls me the White follower (see http://www.dself.dsl.pipex.com/ampins/discrete/ef.htm at the end of the page)

It seems a neat topology although not so often used

2) The voltage bias between Q3 - Q4 base is defined by 3 means:

a) D3, D4, D5 string
b) the Vbe multiplier Q7 and R3, R4
c) Q1 Vbe + Q2 Vbe + D6 + D7

As the 3 voltages are paralleled the lowest "wins" (perhaps the D3, D4, D5 string) and Q1, Q2 are underbiased

3) Are you shure that the configuration is a curent feedback one?

A few questions:

1) Are you able to simulate the circuit?

2) Which are you references (Web and books) about current feedback?

Regards
Marco

[peranders]

Is it just me but I can't see the picture, it's totally white????

Zang, if you want me to take a peek email the picture. As you know I have designed a couple of CFA amps by now. Those who haven't seen them, check QRV-03, 06, 07 and 08 at my homepage.

[diy_audio_fo]

Originally posted by peranders
Is it just me but I can't see the picture, it's totally white????

Dear peranders,

if you login and then read the digi01 post you can see a tumbnail picture.

Right Click on it and the Save_as: you will get a readable schematic

Regards
Marco

[peranders]

Done so but the image is white....

[diy_audio_fo]

Originally posted by peranders
Done so but the image is white....

Dear peranders,
you are (partially) right.

Try with several viewer or programs

On my PC Windows XP I get white image with IE6 but all is OK with Paint and Nero PhotoSnap Viewer

Regards
Marco

[bscally]

for me... I clicked on the image
it opened a white picture.
I ctrl-A to select it.
IE highlited it.. there was the circuit

[digi01]

sorry the file must be broken when upload.
try again.
thanks for the feedback.

ZANG

[digi01]

Originally posted by diy_audio_fo

A few questions:

1) Are you able to simulate the circuit?

2) Which are you references (Web and books) about current feedback?

Regards
Marco

the attached is a simulating by kknd.

the input ic that I used is AD811.
http://www.analog.com/zh/prod/0,2877,AD811,00.html

this ic is a wideband current-feedback opamp.

[peranders]

Zang, I have my doubts about using a CFA opamp + a rather slow output stage (oscillations) but what are the possibly good idea here? Pay also attention to the feedback. The value determines also the speed and if you have too high resistance you will get stabilty problems.

Notice also that 100k at the input and much less at the feedback will create offset. The bias current are around 2 uA, rather much!