I'm trying to put together a comparator using a 741 op amp and the
output goes high as it should, but only goes to about 2volts when it
should be low. I'm only using a 5v single ended supply for power. The +
input to the op amp is set at 2.5v and the - input is the input value to
be compared. There is no feedback resistor and I'm wondering if that
might be the problem. I'm open to any ideas anyone has.
Thanks, Mike Lawson

"mlawson" wrote in message
...
I'm trying to put together a comparator using a 741 op amp and the
output goes high as it should, but only goes to about 2volts when it
should be low. I'm only using a 5v single ended supply for power. The
+ input to the op amp is set at 2.5v and the - input is the input
value to be compared. There is no feedback resistor and I'm wondering
if that might be the problem. I'm open to any ideas anyone has.
Thanks, Mike Lawson

Unless you capacitive coupled the 741 it will not work properly in a
single supply configuration.
Add a -5v supply.


Cheers

"mlawson" wrote in message
...
I'm trying to put together a comparator using a 741 op amp and the output goes
high as it should, but only goes to about 2volts when it should be low. I'm
only using a 5v single ended supply for power. The + input to the op amp is
set at 2.5v and the - input is the input value to be compared. There is no
feedback resistor and I'm wondering if that might be the problem. I'm open to
any ideas anyone has.
Thanks, Mike Lawson


As you've found, an old opamp like this is ill-suited to be a comparator. Yes,
it will perform the switching that you want, but the output swing is highly
dependent on the output circuitry in the opamp. Look at the datasheet, and
you'll see that the 741 isn't really meant to be run from a single voltage power
supply.
Another thing to look at, and will more directly answer your question about the
2 volt minimum output, is the output voltage swing figures. Notice that even
with +/- 15 volt supplies, the output will only swing to within 2-3 volts of the
supply voltages.

What this all means is that you really need to select a comparator IC (not an
opamp), that is designed to be operated from a single supply. The output of a
comparator is usually open collector, which means that the output high level
will go to whatever the voltage source supplying the collector current is, and
the low level can be drawn down to ground, minus the collector-emitter voltage
of the output transistor in the chip.
The collector-emitter voltage of the output transistor is dependent upon the
current that is sinking, so be careful not to overload it with too low a
resistance to the positive voltage source. Some comparators can sink more
current than others, so be sure to check the datasheet for those values.
Assuming that speed isn't a consideration for you, a suggestion for a single
supply comparator would be the LM339 or LM2901. It's a quad comparator, and can
run from a single supply (2-36 volts)or dual supplies (+/-1v - +/-18v).
The outputs are open collector, and can take the output down to within 1/4 volt
of ground. That's good enough for almost any logic circuit to handle.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster
it goes.

On Sun, 01 Mar 2009 22:12:27 -0500, the renowned mlawson
wrote:

I'm trying to put together a comparator using a 741 op amp and the
output goes high as it should, but only goes to about 2volts when it
should be low. I'm only using a 5v single ended supply for power. The +
input to the op amp is set at 2.5v and the - input is the input value to
be compared. There is no feedback resistor and I'm wondering if that
might be the problem. I'm open to any ideas anyone has.
Thanks, Mike Lawson

The 741 won't swing to either supply. Half an LM358 or 1/4 an LM324
will work much better on a single 5V supply (of course op-amps make
slow comparators, and some op-amps will malfunction or fail under this
service).


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Mon, 02 Mar 2009 19:55:28 -0500, mlawson
wrote:

DaveM wrote:
[snip]
Assuming that speed isn't a consideration for you, a suggestion for a single
supply comparator would be the LM339 or LM2901. It's a quad comparator, and can
run from a single supply (2-36 volts)or dual supplies (+/-1v - +/-18v).
The outputs are open collector, and can take the output down to within 1/4 volt
of ground. That's good enough for almost any logic circuit to handle.

Sounds like a plan. I'll redo it with an LM339 instead. Thanks again, Mike

Actually, unless heavily loaded, an LM339 will go to about 50mV from
ground... not very fast, but it will ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

All Hail King Obama, The Resurrected Messiah :-(

DaveM wrote:
"mlawson" wrote in message
...
I'm trying to put together a comparator using a 741 op amp and the output goes
high as it should, but only goes to about 2volts when it should be low. I'm
only using a 5v single ended supply for power. The + input to the op amp is
set at 2.5v and the - input is the input value to be compared. There is no
feedback resistor and I'm wondering if that might be the problem. I'm open to
any ideas anyone has.
Thanks, Mike Lawson


As you've found, an old opamp like this is ill-suited to be a comparator. Yes,
it will perform the switching that you want, but the output swing is highly
dependent on the output circuitry in the opamp. Look at the datasheet, and
you'll see that the 741 isn't really meant to be run from a single voltage power
supply.
Another thing to look at, and will more directly answer your question about the
2 volt minimum output, is the output voltage swing figures. Notice that even
with +/- 15 volt supplies, the output will only swing to within 2-3 volts of the
supply voltages.

What this all means is that you really need to select a comparator IC (not an
opamp), that is designed to be operated from a single supply. The output of a
comparator is usually open collector, which means that the output high level
will go to whatever the voltage source supplying the collector current is, and
the low level can be drawn down to ground, minus the collector-emitter voltage
of the output transistor in the chip.
The collector-emitter voltage of the output transistor is dependent upon the
current that is sinking, so be careful not to overload it with too low a
resistance to the positive voltage source. Some comparators can sink more
current than others, so be sure to check the datasheet for those values.
Assuming that speed isn't a consideration for you, a suggestion for a single
supply comparator would be the LM339 or LM2901. It's a quad comparator, and can
run from a single supply (2-36 volts)or dual supplies (+/-1v - +/-18v).
The outputs are open collector, and can take the output down to within 1/4 volt
of ground. That's good enough for almost any logic circuit to handle.

Sounds like a plan. I'll redo it with an LM339 instead. Thanks again, Mike