Maker Pro
Maker Pro

Op amp supplies

N

Nir Goren

Jan 1, 1970
0
I'd like to generate a stable negative voltage for my Op amp. The best I
could come up with is using a “Low Noise, Switched Capacitor-Regulated
Voltage Inverters”, but as far as I can see this low noise is pretty noisy…



Is there a better way of doing things?



Nir Goren

VLSI center

BGU University

Tel : 08-6477154

Fax : 08-6477620

Email : [email protected]
 
R

Robert Baer

Jan 1, 1970
0
Nir said:
I'd like to generate a stable negative voltage for my Op amp. The best I
could come up with is using a “Low Noise, Switched Capacitor-Regulated
Voltage Inverters”, but as far as I can see this low noise is pretty noisy…

Is there a better way of doing things?

Nir Goren

VLSI center

BGU University

Tel : 08-6477154

Fax : 08-6477620

Email : [email protected]

One could use either the switched capacitor scheme, or a switched
inductor, or a dual voltage supply - if a negative voltage is absolutely
needed.
If you do not like the noise at the output of the switched supplies,
then use a filter and/or a second regulator.
One could also use a single supply and either "tap" it with resistors,
or use a device that TI announced years ago that does the same thing
(only a lot better).
If the signal in and out of the op-amp will never be negative, then
use a rail-to-rail device and no negative supply.
 
R

Rich Grise

Jan 1, 1970
0
Nir Goren said:
I'd like to generate a stable negative voltage for my Op amp. The best I
could come up with is using a ?Low Noise, Switched Capacitor-Regulated
Voltage Inverters?, but as far as I can see this low noise is pretty noisy?
What's the current demand? Would a 9V battery work?

Good Luck,
Rich
 
T

Thomas P. Gootee

Jan 1, 1970
0
The type of regulator you use is dependent on the application.
However, a switched regulator is not as good as a linear regulator.
The advantage of a switched regulator is there when the load current
has a large change to it.

Yes. Either way, though, the original poster wanted to be able to
produce a negative-voltage supply, for an opamp.

I assume that he already HAS a positive supply, or a single-output DC
power supply. National's website has an appnote that has a design for
a nice "power supply SPLITTER", which takes a single voltage and
splits it into positive and negative voltages of half the original
voltage each. (And the former ground becomes the neg supply rail, with
a new ground reference provided by the circuit.)

One nice thing about that one is that it can push equal maximum
currents through both sides. And it can do 1 to 1.5 amps through both
sides when running at + and - 15 volts (or 2 to 3 amps through one
side at a time). (And it can go up to +/- 30 or 40 volts, IIRC.)

It's in a National Semi appnote that refers to either the LM675T or
LM1875T audio amplifier IC (which it uses). I think the appnote had
it using the LM675T. But I built one with an LM1875T, which is about
the same (except no free samples). And it works great!

I added the splitter to a variable-output boost-type switching power
supply that I had designed (using a 78S40 IC and an external TIP41C
power transistor), which takes 12V in and produces anywhere from about
8 to about 70 volts DC out, controlled by a pot. With an added LC
filter on each output (and a small "snubber" circuit around the
TIP41C), the switching noise on the outputs, when running under full
loads (resistive) at +/-15V (1.5A each), is about 10 mV peak-to-peak.
And the whole thing fits on about 2x4 inches of PCB. (But the
splitter, alone, should work with any power supply or battery that has
a voltage within its maximum range.) I also added a 7805 regulator,
fed by a two-resistor voltage divider across the main positive output,
so I could have a +5V (but relatively low-current, in mine) output
available, too, to power CMOS ICs, or whatever. I use this power
supply board in my Curve Tracer product. (See
http://www.fullnet.com/u/tomg/gooteect.htm . It's cool!)

The exact same kind of power supply splitter could be made using most
opamps, I think, changing mainly the maximum total output current (and
the maximum input voltage), depending on the opamp.

Cheers,

Tom Gootee

tomg AT fullnet.com

http://www.fullnet.com/u/tomg
 
Top