Voltage regulator vs. voltage divider

[dano12]

Let's say I have a circuit that wants 5 volts DC and I have a 9 volt battery. Why not just build simple voltage divider with two resistors?

In other words, what does a voltage regulator (i.e. 7805)  do for me that a simple voltage divider doesn't?

[Sir H C]

PSRR, the 5 volts will be there if the supply goes to 7 or 11.  This can actually be a negative in some circuits.
Can use less current.
Lower output impedance, might not matter if you are not pulling much from this node.
And don't forget the cap for the divider, otherwise you have crappy AC response.

There are more I would think, but I am not in a thinking mood right now.

[frequencycentral]

The voltage divider will fluctuate - buffer it with an opamp unity gain buffer.

[dano12]

The voltage divider will fluctuate - buffer it with an opamp unity gain buffer.

The voltage divider will fluctuate--based on what? The supply voltage, the current draw, or both?

and thanks for the opamp idea.

[frequencycentral]

I think stuff such as LFOs and such will draw more current at different times in their cycle...

For distortions etc it might not matter.

I cribbed the opamp idea from synth circuits where you never see a vref voltage divider without an opamp buffer.

[ayayay!]

So thereby it's behaving as a Voltage Regulator now frequencycentral, if you add the unity gain buffer?    

Let's take the LPB for example.  The voltage divider on that can be 430k/43k or 470k/47k or I've seen 560k/56k.  The common theme in all those is that it gives you "9/10ths" of the 9 volts.  Or 9volts X .9 = 8.1Volts.  

If we added that unity gain buffer would it now for all intents and purposes be considered a 8.1 volt regulator?  Y'know, by a poor mans method?  

[slacker]

The voltage from the divider will vary if the supply varies, if you're using a battery as the voltage drops the divider won't give you 5 volts. Likewise if you use different wallwarts with differing voltages you won't get 5 volts. Using a 7805 regulator any input voltage between 35 and about 7 volts will give you 5 volts out.
If the current draw exceeds what the divider can supply at 5 volts then the voltage will drop, R.G. has a good page about this http://www.geofex.com/circuits/Biasnet.htm.
Using the opamp method lets the divider provide more current and is much more stable, but the output will still change if the supply voltage changes.

[slacker]

If we added that unity gain buffer would it now for all intents and purposes be considered a 8.1 volt regulator?  Y'know, by a poor mans method?  

No it's not a regulator if your supply drops to 8 volts then the output of the opamp would be 8 x 0.9 = 7.2volts

[Sir H C]

Now if you replaced the lower resistor with a zener diode, that would be an unrolled regulator.

[ayayay!]

No it's not a regulator if your supply drops to 8 volts then the output of the opamp would be 8 x 0.9 = 7.2volts.

Ah so I'm comparing apples to oranges.  I see the difference.  Thanks guys!