Help me understand opamp biasing.

[chumbox]

Hi All

Been trying to get my head around opamps and how they function in a circuit and I'm mostly there but I don't quite understand the role of opamp biasing. I see the opamp eg TL072 receives 9v power through pin 8 but why is there generally a 4.5v bias attached? I see this 4.5v is created by two equal resistors of any value but why is there sometimes a capacitor or an extra resistor attached to these two, does it change the sound? My guess is the 9v powers the whole opamp but the 4.5v is like a bias resistor on a single transistor but I'm not sure.

I have done a search and read geofx tech of a TS but I just can't quite find the answer. Free free to direct me to a good thread.
Thanks

[ashcat_lt]

The opamp can only put out voltages that fall between the extremes of it's power supply.  If the "bottom" of the supply is 0V, and you send it a signal that swings around 0V, then the output will (at best) be only the positive half of the input signal, so you have to bias it up far enough that both sides of the swing will get through.  In order to be as safe as possible, we usually choose half of the power supply so that we can get as much swing as possible before either side hits the rail.  Course, some opamps will allow you to get closer to one rail than the other, and sometimes we'd prefer an asymmetrical clipping, but...  Yeah, it's pretty much the same reason we do it with transistors.

Any capacitor attached there is to let AC noise from the power supply, bias resistors, or other things hitting this node to short out rather than riding the DC bias into the amplifier and turning into audible noise on the output.  You can use just two resistors with the input of the opamp between them, but the impedance will be the parallel combination of the two.  To make that big enough to connect directly to a guitar, they will be big enough to add significant noise, and you can't put a cap there, because it will also roll off treble from the input signal.  So instead we run the big resistor from the middle of the (cap bypassed) divider to carry that bias voltage and less noise.

AMZ has a great article on this, but I'm not going to search for it right now.  Maybe google "noise free bias" or something...

[Transmogrifox]

This one might help:
http://www.muzique.com/lab/buffers.htm

It's about buffers, but throughout you can see the common thread between setting the input level at V/2 between both transistors and amplifiers.  From that perhaps you can imagine why this is still important when you configure an op amp for gain.

[R.G.]

Opamps amplify the difference between the + and - inputs by several tens or hundreds of thousand times.

In practice, they're almost always used with overall DC feedback from the output to the inverting (-) input. If there is no DC path from the (-) input to anywhere else, the DC gain of this setup is unity, and the output follows the (+) input.

This is why using a +4.5V reference on a +9V supply is so useful that it's almost universal. It puts the output smack in the middle of the power supply so that the output has the widest possible undistorted swing.

There are vast elaborations on this very simplistic look at the issue. If you're really trying to get your head around opamp biasing, you would be best served reading something like "Opamps for everyone" from the TI site.

[chumbox]

Thanks so much everyone and for such detailed replies. Jacks page helps a lot too. As usual super helpful and I actually now finally understand. Now to go tinker and see what I can do.

[teemuk]

Different bias arrangements discussed:
http://www.analog.com/library/analogdialogue/archives/35-02/avoiding/index.html

(..or http://www.analog.com/library/analogDialogue/archives/35-02/avoiding/avoiding.pdf )

[GibsonGM]

Follow this guy's lead - funny, entertaining, and a good way to learn this stuff!  Less tech mumb-jumbo than many places, but you still get the 'real deal' on what is happening...

https://www.youtube.com/watch?v=7FYHt5XviKc

I wonder if Duck Arse knows him??

[bluebunny]

Australians wouldn't give a Castlemaine XXXX for any other EE tutor.  And I'll bet he eats Vegemite too.

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Oops!  Wrong thread!  

[GibsonGM]

Australians wouldn't give a Castlemaine XXXX for any other EE tutor.  And I'll bet he eats Vegemite too.

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Oops!  Wrong thread!   

+1  Love the guy!  And I now use his little expression, "...and it'll be done in half a bee's (fill in the blank)"

Good teacher, gets points across in a way you'll retain and which are easy to understand.