I need Ground, -ve and +ve for my circuit, but my power supp

[David Bridgen]

Hi Len.

To call something "earth" when it is 't connected to the planet can be downright dangerous apart from being confusing.

As far as "ground" is concerned I think it imperative that it be clarified whether a connection is made, say, to chassis which can be touched and whether or not it is connected to earth. Some authors are careful to say either "chassis ground" or "earth ground." But most people use the word because they see or hear other people (mis)using it while not really understanding it.

I find the use of the word unfortunate and in most cases totally inapropriate.

Why call one side of the battery supply in a hearing aid "ground"? Yet people do do daft things like that.

 

[ljcox]

I agree David, I prefer 0V, but I find many people confused by this term.

Len

 

[audioguru]

Your non inverting single supply has 3 resistors and one capacitor connected to the + input. This could be done with only 2 resistors.

Hi Len,
My opamp circuits work under all conditions *. Your simplification attenuates supply noise and ripple by 2 but then the opamp circuit amplifies it. My circuit has a well-filtered reference voltage.

My inverting dual supply circuit can also be simplified sometimes. If you don't care about the offset voltage caused by opamp input current or are using a Fet-input opamp, its resistor and cap aren't needed at the + input, just a wire to ground.
I know, some circuits don't need its cap at the + input.

* I had a battery powered single-supply audio amp that used my circuit with its well-filtered reference voltage. It clipped symmetrically with a continuous sine-wave input.
But with music, the positive-going peaks always clipped. When the battery voltage sagged by the massive current during loud peaks, the filtered reference voltage stayed up where it is too high for the reduced supply voltage during peaks. I fixed it by replacing the voltage divider with a fixed voltage reference IC with its voltage chosen for symmertrical clipping during loud music peaks.
Then it produced a higher output during peaks because it was better balanced and produced much lower distortion at low frequencies. :lol:

 

[ljcox]

Your non inverting single supply has 3 resistors and one capacitor connected to the + input. This could be done with only 2 resistors.

Hi Len,
My opamp circuits work under all conditions *. Your simplification attenuates supply noise and ripple by 2 but then the opamp circuit amplifies it. My circuit has a well-filtered reference voltage.

Yes, I thought that may have been the reason, but I wanted to make the point that you don't necessarily need 3 resistors - given Thevenin's theorm.

Len

 

[Sebi]

If You waste some time, can modify the wallwart supply cct:

 

[eblc1388]

Yes, I thought that may have been the reason, but I wanted to make the point that you don't necessarily need 3 resistors - given Thevenin's theorm.

What Thevenin's theorm has anything to do here? Do you mean that the input resistor to the opamp is not needed?

 

[audioguru]

Hi Eblc,
Len was talking about these two equal circuits:

 

[Nigel Goodwin]

However, the two resistor version has two resistors generating noise into the input of the opamp, the three resistors version only has a single resistor contributing noise - assuming you decouple the junction of the potential divider, which is a good idea anyway.

This might, or might not, be a consideration, but it's a slight limitation on the two resistor version.

 

[eblc1388]

Hi Eblc,
Len was talking about these two equal circuits:

Yes, I know. The question is I just wonder what Thevenin's theorm has anything to do with either the two or three resistors circuit. Obviously one cannot simplify the three resistors with just two resistors in these circuit because the resistors are doing different functions.

Or simple put, one can choose to use either the three resistors circuit or the two resistors circuit, but there is NO two-resistor equivalent of the three resistor circuit.

 

[audioguru]

Hi L. Chung,
In both circuits the function of the resistors is the same (+6V reference voltage) and the opamps see exactly the same resistance of 2R. :lol:

 

[eblc1388]

Hi L. Chung,
In both circuits the function of the resistors is the same (+6V reference voltage) and the opamps see exactly the same resistance of 2R. :lol:

I see the where the problem is now.

My original post is referring to the the original circuit you have posted, where there is a capacitor at the +6V resistor junction to 0V. You have removed that capacitor in the subsequent post. You are right, without the capacitor, you can replace three resistors with just two.

What happens if you replace the capacitor?

 

[audioguru]

You can't use the filter cap with the two resistors. It would short the input signal to ground.