Buff <-> Boost Query

[antonis]

Just an obvious (and silly) question.. 

If we replace the feedback resistor of an inverting amplifier with a pot (capable to reach 0 ohms at it's zero position) can we say that we've got a Buffer & Booster circuit..??


P.S.
Only <YES> answers all wellcome...

[Mark Hammer]

As I understand it, non-inverting op-amps have a higher input impedance than inverting ones.

Just as important, if the feedback resistance is greater than the input resistance, then the gain is <1.

So, it would not be much of a buffer, although it could be a booster.

My guess is that you meant to say "non-inverting".

EDIT:  Oh man, it's contagious!    I meant to say "When the feedback resistance is LESS than the input resistance.  This flip-flop disease is like ebola!

[antonis]

My guess is that you meant to say "non-inverting".

Your guess is absolutely right, Mark..
(allways had a problem with (+) and (-) cause I wrongly match the (-) with "non")

I just want to make an adjustable gain Booster which will serve as a unity gain Buffer too..
(something like this...)

[R.G.]

Yes, we can say that.

Of course, this is the internet, and people can - and do! - say anything that occurs to them, so that's not much help.

"Can we say" has the flavor of "does this fit within the meanings of the words", so let's talk about the words.

"Booster" means, in general, "makes it bigger/taller/faster/more.  It doesn't mean much without a qualifying adjective in front of it, like "voltage booster", "current booster", "velocity booster", "mileage booster", and so on. In advertising speech, "booster" may mean simply "increases my bank balance". A piece of lead *might* be called a booster, when what is meant is "weight booster".

In the micro world of pedals, "booster" all by itself usually means something like "signal voltage booster". But you can have current boosters, or current *and* voltage boosters. It doesn't mean much without the qualifier to say what is boosted.

"Buffer" is another word with variable meaning. It's something like "keeps outside stuff from dragging down or adversely affecting something". In chemistry, a buffer is a chemical that keeps (usually) the pH of a solution from varying much as other chemicals are added, the actual description being "a pH buffer". In pedals, it's almost always used to means "keeps heavy loads from dragging down a signal voltage", and as such tends to mean  a current booster.

But since a buffer doesn't necessarily have a gain of one, you can have a buffer and a booster in the same circuit. Buffers are most commonly unity gain, about, but not necessarily. In electronics practice, buffers may also be inverting or non-inverting as well. There's usually an adjective used to make the meaning clear, although noninverting is more common.

So - aside from Mark's comments about how it works - the same circuit might be a buffer and a booster at the same time.

And yes, when the feedback resistor becomes zero. the voltage gain becomes unity, as the 100K/10uF becomes simply another load on the output.

[PRR]

> the same circuit might be a buffer and a booster at the same time.

YES! It buffs! It boosts! It polishes your shoes and re-fills your beer! Call in the next 3 minutes and we will throw-in a Shamm-Wowie dog-towel!

We can put together any combination of features. With extra complexity.

But you seem to be looking for a single opamp trick.

> feedback resistor of an inverting amplifier

Inverters are usually lousy buffers. The input impedance is just Rin. If Rin is small (10K), it loads-down the source (doesn't Buff). If Rin is large (1Meg), it hisses (hisses a LOT if you want to Boost).

The non-inverting is the better buffer, and can boost as much as needed. It also has an Rin but in-use its hiss is bypassed by the source. This is the lowest-hiss configuration.

The non-inverting can boost but it does not cut (without further tricks). When Rfb is zero it is unity-gain.

There is anothe variant, only slightly trickier, which is non-inverting and can boost or cut (to zero) with a single pot. One of the guitar-amp makers uses it a LOT, though it can also be found in 1940s radios.

> adjustable gain Booster which will serve as a unity gain Buffer too.. (something like this...)

With those specs, your plan works.

Your proposed plan puts half the power-supply crap on the input. You should review "noiseless biasing" at www.geofex.com

The 100K in the NFB loop adds non-negligible hiss to guitar-type signals (Noise-Figure 2dB to 6dB).

Also 100K against 10uFd gives sub-sub-sonic response (not needed) and an e-cap here may leak enough to matter. The opamp can drive 2K load. Make your bottom resistor something like 2K (1.5K, 2.7K, whatever you grab first).

You show a maximum gain of 3.5. This is probably wise. Much music depends on un-wise gain choices. Since it is variable, I would top-out at higher gain, 5 or 10. Proposed values give gain from 1 to 7 (0dB to 17dB) with mild 2.25 gain (7dB) in the middle.

[knutolai]

Quick question. Does the size  of the 10uF cap between the 100k resistor and ground in any way affect the voltage gain?

Also check out ValveWizards glassblower booster:
http://www.valvewizard.co.uk/glassblower.html
P1, R5 and R6 illustrates a way to get a more balanced response across the pots rotation.

[antonis]

Thanks very much Knutolai...!!!
(but I don't need this kind of "killer" - just a plain booster with a volume pot for a small trans amp..)

[samhay]

>There is another variant, only slightly trickier, which is non-inverting and can boost or cut (to zero) with a single pot. One of the guitar-amp makers uses it a LOT, though it can also be found in 1940s radios.

I'll bite. Care to elaborate?

[antonis]

Your proposed plan puts half the power-supply crap on the input. You should review "noiseless biasing" at www.geofex.com

So, if I reduce the biasing resistors values to 10k should I add a 470k - 510k resistor in series with the (+) input..??
(or just hope that OpAmp will not exhibit any over-current attitude..??)

The opamp can drive 2K load. Make your bottom resistor something like 2K (1.5K, 2.7K, whatever you grab first).

I thought that such low values should "overload" opamp's output..

Shouldn't be "safer" a value of 10k..??
(I'm not experienced in OpAmps uttermost behavior..)

You're right about the 10μF cap..!!!
(just two missing 0 from my calculator..)

[slacker]

I'll bite. Care to elaborate?

Dunno if this is what Paul meant but Peavey do this a lot http://elektrotanya.com/PREVIEWS/63463243/23432455/peavey/peavey_studio_pro_50_sch.pdf_1.png. Look at U1B, ignore all the saturation stuff, R12 is simultaneously the gain setting resistor of the opamp via R11 and the bottom of a voltage divider with R10 to set the output level. 

[slacker]

So, if I reduce the biasing resistors values to 10k should I add a 470k - 510k resistor in series with the (+) input..??

Not sure what you mean by in series, what you want is make your biasing resistors 10k and put a big cap from the junction of the resistors to ground. Then 470k-1Meg resistor from the junction of the resistors to the + input.

Shouldn't be "safer" a value of 10k..??

2k is fine for most opamps we use.

[knutolai]

(but I don't need this kind of "killer" - just a plain booster with a volume pot for a small trans amp..)

didnt mean you should use the whole circuit, only the part with the resistors i mentioned (look at the schematic below).

So, if I reduce the biasing resistors values to 10k should I add a 470k - 510k resistor in series with the (+) input..??

So I drew up the circuit I think you want to end up with:  


Some quick info: R1 is a pull-down resistors (look it up). Your input impendance is R1 and R2s parallel resistance (1M).
If you are using a dual opamp for the build. Wire the unused opamp as shown (output to negative, positive to ground).
C4, C5 and C6 are added to dampen any potential noise from your power supply.
I've swapped the volume pot (from picture shown earlier in thread) with R5 as I take it you down want to be able to dampen your signal with the circuit.

Also as PRR pointed out R3, R4 and P1 could be reduced to for instance 2,2k, 2,2k and 20k.

You could also add a protection diode for the powersupply (not included in my picture). Not sure how long you've been building. Sorry if you knew most of this already  

[antonis]

what you want is make your biasing resistors 10k and put a big cap from the junction of the resistors to ground. Then 470k-1Meg resistor from the junction of the resistors to the + input.

That's excactly what I meant..

[antonis]

Not sure how long you've been building. Sorry if you knew most of this already  

You're allways wellcome...!
(just realized that I've ommited the second cap from my actual board..)

BTW
I'm not  familiarized with Op Amps topology but I've seen many drawings that use low value resistors for creating the biasing voltage (propably to avoid noise interference) but they use a big value resistor for biasing..
Doesn't this resistor provoke noise also or I've to study A LOT about this matter..??

[knutolai]

I'm not  familiarized with Op Amps topology but I've seen many drawings that use low value resistors for creating the biasing voltage (propably to avoid noise interference) but they use a big value resistor for biasing..

http://www.muzique.com/lab/imp.htm

[ashcat_lt]

I'm not  familiarized with Op Amps topology but I've seen many drawings that use low value resistors for creating the biasing voltage (propably to avoid noise interference) but they use a big value resistor for biasing..

http://www.muzique.com/lab/imp.htm

There's another article about "noiseless biasing" that covers this better, I think, but I can't find it right now.  The answer is basically that there isn't enough current flowing through the biasing resistor to contribute much noise.  Almost as important is that you can use a big cap to dump the noise created in the bias divider itself without killing all the treble at the input.

[Gus]

Something I posted before.  A follower to a gain stage, min gain about 1

[samhay]

I'll bite. Care to elaborate?

Dunno if this is what Paul meant but Peavey do this a lot http://elektrotanya.com/PREVIEWS/63463243/23432455/peavey/peavey_studio_pro_50_sch.pdf_1.png. Look at U1B, ignore all the saturation stuff, R12 is simultaneously the gain setting resistor of the opamp via R11 and the bottom of a voltage divider with R10 to set the output level.  

Thanks Ian - I figured that it would look something like that. I don't think it can practically cut all the way to 0, but it's not far off.

Edit: Something like this should work:



Will need some tweaking - e.g. some op-amps (perhaps like the TL072) will struggle with the 1k load of R6.

[samhay]

So I drew up the circuit I think you want to end up with:  

I wouldn't ground the unused op-amp's inputs. Most op-amps (at least those that aren't single supply/rail-to-rail) won't like this very much, so leave the non-inverting input floating, or tie it to half supply.
Or even better, use the second op-amp to buffer your reference voltage.

[knutolai]

I wouldn't ground the unused op-amp's inputs. Most op-amps (at least those that aren't single supply/rail-to-rail) won't like this very much, so leave the non-inverting input floating, or tie it to half supply. Or even better, use the second op-amp to buffer your reference voltage.

thanks for correcting my misinformation  I've done this with tl072s and NE5532s without it affecting the operation of the other opamp in the package.