Question of rail splitter

[huangyong 0]

 

Thinking of making the rail splitter above. but the current output capability seems not good (below 100mA). Any idea how to improve the current output?

put a few opamps in series? parallel? or use a high power opamp.

 

Thanks.

[huangyong 0]

or am i wrong?? this is only virtual ground and has nothing to do with current output?

[huangyong 0]

came across this, really confusing.

[bpribadi 0]

Thanks for those informative pictures!

 

For your first drawing, since the virtual ground is the ground for the load, it must be able to pass sufficient current, so the higher current capacity the better.

 

Use OPA552, it has 200 mA output current, 24 v/us slew rate, with max rail-to-rail voltage of 60 volts.... Yes, 60 volts

 

It is a single op-amp in DIP 8, cost around SGD 8 in Farnell.

 

Actually I plan to make high power CMoy with it, but no time at the moment... I haven't even populate jason PCB he gave me too busy lah...

 

For higher current (also more expensive), around 2A, you can use OPA544 with your first drawing. It cost around S$ 23 in Farnell.

 

Using discrete transistor is cheaper, but Op-Amp is simpler and smaller.

 

Actually if you see the TLE2426 schematic, it is the same as your first drawing.

 

I use resistor voltage divider with BUF634 for my Black Amp, and it works fine. But I think Op-Amp can give better stability than buffer (no negative feedback).

Edited October 26, 2004 by bpribadi

[huangyong 0]

can i use power opamp for this application?

opamp such as LM3875.

[bpribadi 0]

Yes, why not. But that will be overkill isn't it And you must check if the chip is stable for unity gain

 

And FYI, the 'regulator type' circuit on the top right of the picture, I don't recommend it. Because, it doesn't give you a good virtual ground with high current capacity.

 

For the buffer type, remember, buffer output is not 100% of the input, but only around 99.5%, so you will need a small pot on the divider to adjust the output voltage of the buffer to get exactly 50% of the rail-to-rail voltage.

Edited October 27, 2004 by bpribadi

[heady 0]

But I think Op-Amp can give better stability than buffer (no negative feedback).

 

bpribadi, I don't understand this statement, especially the feedback part. Can explain? Thanks.

 

[bpribadi 0]

If you see the buffered type drawing, see picture A and B. While A is using buffer like the BUF634, and B is using Op-Amp which can be use for unity gain (buffer).

 

The Op-Amp one has feedback from output to the negative input. This is what we call negative feedback (NFB). With NFB, the op-amp will control the output to match the negative input voltage (which is the same as the positive input). So if there is any changes at the output (like current changes), the op-amp can adjust itself to stabilize the output. But Op-Amp in general has limited output current, while buffer is designed to handle larger output current. So for high output, buffer is better, while for maintaining stable output voltage theoritically op-amp is better (within it's output current capacity).

 

Last time I haven't found the cheap high power Op-Amp such as OPA552, so I use the BUF634. But to replace TLE2426, combination of resistor divider and OPA552 is a good choice, giving you 200 mA of ground current.

 

Or using power op-amp such as OPA544 (2A), but more costly. In this case discrete approach is more economical. With darlington transistors should give stable performance.

[heady 0]

bpribadi - how much is a "cheap" OPA553? Is it possible to roll a BUF634 inside the feedback loop of the OPA for the virtual ground similar to the Signal ground of the PPA/Pimeta?

 

[bpribadi 0]

Check my first reply to Huangyong

It is around S$ 8.- in Farnel... OPA552 not 553.

 

Yes, possible (change circuit though), but bear in mind, combination OPA627 and BUF634 has higher slew rate than OPA552.

Edited October 27, 2004 by bpribadi

[huangyong 0]

Bripadi. i decided to use LM1875, a small brother of LM3875. whcih can output 20W of power. I think this is enough for the application. I checked the price in Farnell, it's just around S$5. not bad.

Thanks anyway.

 

1 more Q. is LM1875 unity gain stable??

Edited October 27, 2004 by huangyong

[bpribadi 0]

I think you cannot use it, since it is only stable for gain 10 or above

Edited October 27, 2004 by bpribadi

[heady 0]

Check my first reply to Huangyong wink.gif

 

bpribadi,

Ooops sorry, sudah orang tua.

 

Are you going to use a TLE for each channel in your PPA? Or some other arrangements?

 

[bpribadi 0]

My PPA will be AC powered, so no need rail splitter. The ground will be from the transformer. But I will use Op-Amp to isolate the source ground, but in this case, no need for high current capacity.

 

Op-Amp will sounds better with high supply voltage. So I will use as high voltage as possible for the supply. To me, it's kind of inapropriate to use battery with PPA.

[heady 0]

My PPA will be AC powered, so no need rail splitter. The ground will be from the transformer

bpribadi, ok, I can understand that. If the transformer you will use is small enough to fit into my casing I won't mind going that way too. Have you decided which transformer you will use? Unfortunately I know nothing about the rectification diodes etc.

 

Thanks.