Most op amps will stop a few volts shy of rail to rail. Does anyone have ideas how to extend this limitation on output voltage swing? Normal negative feedback will compensate for a multitude of sins. Even going to the + rail would be useful. I have an application that could use this trick.

 

Are you asking in a linear and/or digital mode?
SG

 

Op amps are linear, comparators are digital.

Making a Comparator Rail to Rail

 

Op amps are linear, comparators are digital.

Just asking.
Then what do you mean by cheap? Why not just get Rail to Rail opamps?
SG

 

741 is as cheap as they come,so is the 1548 (dual741)
Wendy's Index
High Speed Op Amp Query

My index is a personal index of things I find useful, as well as linking to my personal articles. Hey as hobbies go it is both cheap and harmless.

Care to suggest your favs?

I am also a fan of quick and dirty. If there is a way to do this I want to know how.

 

Rail-to-rail output can be achieved by adding two external transistors, one in series with each power pin. Here is the general idea, but you break the connection between pin 7 and the output, and add a resistor from pin 7 to GND. Stability is tricky. I think there is an older version in an National Semi app note.

ak

 

Care to suggest your favs?

As far as RR chips I use the MCP6002 often.
They are less then 50 cents but only have a max supply of 5.5 volts.
SG

 

http://www.ti.com/lit/an/sboa009/sboa009.pdf

 

Using digikey cheapest OpAmp, 4558, was a little over 7 cents, cheapest RRO
was ~13 cents, all in some volume. Makes one wonder if you add up costs of
externals to get to RRO, insertion costs, procurement and test costs, if its worth-
while.

Then one has to consider load conditions, and whats "good" enough for swing
at output.

Regards, Dana.

 

And then there is this trick:

The op-amp is an LM358. A Fairchild KSP10 (made in 2014) and a National Semiconductor NT3906 (made in the mid-1970's) connected as avalanche photocurrent sources are used as an output stage with and output swing that includes voltages higher than the positive power supply and below ground.

The output going more than 300 mv below ground:


Because of the low currents available from the current sources this trick cannot be used with fast signals.

 

The last R-R opamp I bought was TLV272IP, 97¢ each from Mouser in single quantities, before freight. (Now they're $1.33) I don't recall why I chose that particular one. The LM358 opamps I bought off eBay were 27.5¢ each delivered in a lot of 20.

I'm not willing to spend much time and effort avoiding the difference between 27.5¢ and 97¢.

 

I frequently return back to a subject I am interested in. I had some thoughts on this subject I will post when they jell a bit more. For now I've bookmarked it on my Index.