Op-amp booster circuit. Critique/advice sought

[Rixen]

Move the diode to the other side of the cap to prevent damage to the capacitor in event of supply voltage reversal..

Use a Schottky diode for low Vf..

[Rixen]

..what Antonis said.. :-)

[suncrush]

Suncrush-
You might read up on low pass filters & how to calculate the frequency:

http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php

Ah.  Very helpful.

[antonis]

Oversizing R3 & R4 (say to 1M - 2M2) results in omitting R8 but also to high noise level...!!!
With current values there is a total voltage drop on R1 about 50mV (no big deal..)

IMHO, R5 & R6 should be lowered by a factor of 10..

[suncrush]

Oversizing R3 & R4 (say to 1M - 2M2) results in omitting R8 but also to high noise level...!!!
With current values there is a total voltage drop on R1 about 50mV (no big deal..)

IMHO, R5 & R6 should be lowered by a factor of 10..

Ok, I think I understand MOST of what's going on now.  A few things I don't get.

Do C1 and R8 make a filter?  I don't understand why C1 wouldn't just be 47 nF.

Why is C2 there?  We have C3 to clean out any AC noise from the wall wart, and the unlabelled 47uF cap is functioning in the voltage splitter.

Why is R8 there?  I thought the idea was to drop the DC voltage on the input to half of the input voltage.  R8 will cause a voltage drop, and lower the voltage at the + input even more.

R7 is there because a previous poster said driving a capacitive load was a problem.  Are you suggesting it shouldn't be there at all?

C5 was a copy-paste error, pretty sure.

You said you would drop R5 and R6 by a factor of 10?  May I ask why?  For gain calculation purposes, it seems it would make no difference.




Thanks again for the patience and all the help.  I appreciate it.  I'm learning a lot, but this is pretty complicated.

[samhay]

^ (to antonis' post)
R7 can be 10-20 x smaller.
I would add a 1-10k series resistor to the input.
I would also add a pulldown resistor to the output side of C5.
R2 can be bigger.
+1 to smaller R5 and R6.

Edit:

>do C1 and R8 make a filter?
Yes.

> I don't understand why C1 wouldn't just be 47 nF.
Decide on the value of R8, then determine a value of C1 that gives a sensible frequency cutoff. Unless you make R8 quite small, 47n aught to be fine.

>Why is C2 there? 
It keeps the op-amp happy. Key to this is placing it very close to the V+ pin.

> Why is R8 there? 
Look up 'noiseless biasing' on geofex.

>R7 is there because a previous poster said driving a capacitive load was a problem.  Are you suggesting it shouldn't be there at all?
Good practice says leave something there.

>C5 was a copy-paste error, pretty sure.
It's a good choice if you want to drive headphones (you would have to remove R7 though).

[induction]

Why is C2 there?  We have C3 to clean out any AC noise from the wall wart, and the unlabelled 47uF cap is functioning in the voltage splitter.

The single value of capacitance quoted for a given capacitor is just an approximation. Capacitance is actually frequency-dependent, and it turns out that the capacitance of electrolytics is notably lower at higher frequencies. So for filtering noise out of DC, it's a good idea to put a smaller film cap in parallel with a larger electro to get good capacitance at a wider range of frequencies.

Why is R8 there?  I thought the idea was to drop the DC voltage on the input to half of the input voltage.  R8 will cause a voltage drop, and lower the voltage at the + input even more.

Remember, it's not absolute resistance that drops voltage, it's resistance within a voltage divider that's important. The input impedance of a TL07x is quoted in the datasheet at about 10¹² ohms. Assuming R8 = 1M, the voltage divider equation gives Vout = Vin * R2/(R1+R2) = Vin*0.999999

Edit to add: The idea of R8 is to maintain a high input impedance (R1 || R8 || op-amp input Z). You could do this with higher value resistors in the bias network, but without R8, the Vref filter cap will dump high frequencies in the input signal to ground, so without R8 you can't filter Vref. It's better to have lower value resistors in the bias network to reduce thermal noise, but because of the very high input Z of the op-amp, almost no current runs through R8, and thermal noise isn't much of an issue so the resistor can be very large. This is the basis of the 'noiseless biasing' (or more realistically, 'low-noise biasing') a previous poster referred to.

You said you would drop R5 and R6 by a factor of 10?  May I ask why?  For gain calculation purposes, it seems it would make no difference.

Read this.

As you have probably noticed, a lot of practical electronics involves recognizing how the behavior of a circuit varies when ideal components are replaced by real components.

[suncrush]

>Why is C2 there? 
It keeps the op-amp happy. Key to this is placing it very close to the V+ pin.

Could omitting that cap cause squeal?  I'm working on an op-amp fuzz where I got the vero design from someone.  It doesn't include that cap, and it squeals like a pig, no matter what I try.

[duck_arse]

because I've missed the bulk of the thread ......

Roger.
Ok, the only thing I think I need to figure out before I go put this on vero is how big a resistor to use to connect the output of U2 and the + input of U1.

For standard electric guitar and bass , 1M should be fine.
What is the purpose of the R and C to ground from the output of U2?

The MIT notes include the cap to ground in the voltage follower schematic.  duck_arse suggested the R to avoid issues of the op-amp driving a capacitive load.

I was suggesting removing that cap (C6) entirely. I would like to find the thread where one of the bosses says "don't", but try searching for opamp, or cap, see where that gets you.

carry on.

[samhay]

>Why is C2 there? 
It keeps the op-amp happy. Key to this is placing it very close to the V+ pin.

Could omitting that cap cause squeal?  I'm working on an op-amp fuzz where I got the vero design from someone.  It doesn't include that cap, and it squeals like a pig, no matter what I try.

It might, but in the case of the Muff Fuzz and similar, I would try a cap across the op-amp feedback loop first.

[suncrush]

>Why is C2 there? 
It keeps the op-amp happy. Key to this is placing it very close to the V+ pin.

Could omitting that cap cause squeal?  I'm working on an op-amp fuzz where I got the vero design from someone.  It doesn't include that cap, and it squeals like a pig, no matter what I try.

It might, but in the case of the Muff Fuzz and similar, I would try a cap across the op-amp feedback loop first.

Yeah, it's a Muff Fuzz.  I'll see if that helps solve the issue.

[antonis]

Could omitting that cap cause squeal?  I'm working on an op-amp fuzz where I got the vero design from someone.  It doesn't include that cap, and it squeals like a pig, no matter what I try.

An Op-Amp with a maximum gain of 10 (20dB) ISN'T any kind of Fuzz...

I don't think that "squealing like a pig" is ought to small Vref variations (due to missing cap) so try first the cap on NFB loop..

What's the value of R7..??

edit 1: Sam is faster as always..!!
edit 2: I thought you refer to current circuit...

[suncrush]

An Op-Amp with a maximum gain of 10 (20dB) ISN'T any kind of Fuzz...

I don't think that "squealing like a pig" is ought to small Vref variations (due to missing cap) so try first the cap on NFB loop..

What's the value of R7..??

edit 1: Sam is faster as always..!!
edit 2: I thought you refer to current circuit...

Ah, no.  Different circuit.  The one in this thread is a clean boost.

[suncrush]

In case anyone is lurking and learning, I found this pdf on designing op-amp circuits from Texas Instruments.

http://www.ti.com/lit/ml/sloa076/sloa076.pdf

[boppy100]

Thanks for the link, I had forgotten about it.
This has been an enjoyable and enlightening thread for me.