non-inverting opamp gain

[onboard]

This is the correct configuration for a non-inverting gain stage using single supply, right?



I tried a LM741 I *thought* was good, and got nothing. Hooked it up to be inverting, perfect. Go back to the non-inverting arrangement, dead. Then I grabbed some others and had the same thing, inverting works and non-inverting doesn't.

Are my opamps bad, or am I missing something? I'm 100% positive that it was hooked up correctly....oh, R1 was 100k, R2 was 470k.

[brett]

Hi.  Try connecting the "+" input to Vcc/2 via a 1M resistor.  
Also, it is common to run R1 to AC ground.  ie connecting it to ground with a cap (for a 100k resistor, a 0.1uF cap will do).
cheers

[toneman]

at first look, your schemo of the non-inv amp looks OK.
your gain equasion looks OK 2..  1+ R2/R1.
pretty sure R1 goes to 1/2 Vcc, or gnd if bi-polar.
did U simulate it ??  U say works as inverting ,but, not like this(?)
mayB someone else???

[onboard]

One of my drawings from way back showed (+) input tied to Vref through a 1M and R1 to ground.

But a quick glance at a TI Single Supply design sheet is the opposite, (+)input fed with signal only and R1 tied to Vref...

No sims, just breadboarding. Inverting has both inputs and out idle at V/2. What about non-inverting? If the bias (?) resistor feeding the (-) input is tied to V/2, then it should sit at V/2 minus the voltage drop across the R, but I'm not sure about the (+) input or output pins.

I've only used inverting stages. This is going one step forward, three steps back. I'll try looking it up again.

Oh, and I was using a DC blocking cap at the output, forgot that in the drawing.  :?

[seanm]

EDIT: Oops, I overwrote the original image. The image below is not the image some of the replies are for.

Try this. You can replace the pot with a single resistor.



This will have more gain than your example.

[onboard]

That puts V/2 at both inputs -  through 1M for the (+) input, and across the (-) input bias resistor?  Hmm. OK, I'll try it.

It would be just as easy to use an inverting buffer before or after an inverting gain stage, but I'ld like to eliminate the extra step. Thanks for the help!

[The Tone God]

Instead of connecting the inverting side to 1/2 V+ put a cap in connected to ground. You should connect 1/2 V+ after the decoupling cap.

See Figure 9 in Opamperation

Andrew

[seanm]

Instead of connecting the inverting side to 1/2 V+ put a cap in connected to ground. You should connect 1/2 V+ after the decoupling cap.

See Figure 9 in Opamperation

Andrew

I assume you mean this? Which I think is the same thing.....

[The Tone God]

Not quite. Thats just a bias/reference network (which you should remove the cap if attaching to the non-inverting input). Figure 9 is what I'm talking about.

Andrew

[DiyFreaque]

Nice page, BTW, Andrew.

Cheerio,
Scott

[The Tone God]

Thanks.

Andrew

[seanm]

Not quite. Thats just a bias/reference network (which you should remove the cap if attaching to the non-inverting input). Figure 9 is what I'm talking about.

Andrew

Would the following work? And what value would you recommend for C2? It looks like I have been biasing wrong  :oops:

[onboard]

Hey, I'm glad I brought this up - maybe other folks have wanted to work with non-inverting stages but weren't sure how to go about it with a single supply.

Thanks for the link Andrew!! Kind of "everything you wanted to know but were afraid to ask..."

I'll try the config in figure 9.

seanm - I'm thinking +10uf for the DC decoupling cap you're asking about?

[seanm]

seanm - I'm thinking +10uf for the DC decoupling cap you're asking about?

That would be my first guess to.

[The Tone God]

Would the following work? And what value would you recommend for C2? It looks like I have been biasing wrong  :oops:

Yep. That should work. C2 can be just about anything. Try something from say a 0.01uF to 1uF depending on what your looking for in a sound. Starting to notice that it looks similar to a Dist+/250 ?

Thanks for the link Andrew!! Kind of "everything you wanted to know but were afraid to ask..."

No problem. Thats why I write those articles.

Andrew

[brett]

Hi.  As a rule of thumb, for non-inverting designs the cap and resistor tying the feedback loop to ground need to have a resistor value (in k) multiplied by the cap value (in uF) greater than 10.  This allows for low frequencies to pass ok.  

For the maths-minded, when RC>10mohmfarads(milliohmfarads :shock: ) fc<16Hz, according to fc=1/(2.pi.R.C)

cheers

[onboard]

Excellent addition - it's good to be able to "do the math"!

Does the inverting arrangement typically get used out of simplicities' sake? Or is there an opamp related answer...it seems to me I read something along the lines of inverting gain configurations being more stable?