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General Discussions => Drawing Board => Topic started by: Samuel Groner on October 02, 2005, 01:02:57 PM

Title: Single rail vs. dual rail in op-amp circuits
Post by: Samuel Groner on October 02, 2005, 01:02:57 PM
A very nice thing about bipolar supplies is that almost no supply currents flow through ground - this makes ground much cleaner, especially when outputs are class B.

Samuel
Title: Single rail vs. dual rail in op-amp circuits
Post by: Scodiddly on October 02, 2005, 01:32:30 PM
You're going to need a 15vdc ground reference for a lot of things, I suspect.  Though it's a good idea to set things up so that the coupling caps are biased - definitely worth thinking about.
Title: Single rail vs. dual rail in op-amp circuits
Post by: bcarso on October 02, 2005, 05:01:56 PM
Most of the stuff I've done for consumer/multimedia products has used single supplies with a half-supply.  Samuel's comment is sensible, although in practice for relatively simple systems it's usually not been a problem.  

However, there was one early powered speaker that I used a passive tone control in, and consequently a lot of "make-up" gain.  The board was single-sided and the layout guy had some op amp "-" supply current running through a piece of light trace involving the tone control and gain control ground.  There was an oscillation problem as a result, which was dependent on the setting of the controls---a nasty surprise.

Of course star grounding will remedy the situation.  This can be difficult to enforce when the board size is constrained and especially for single-sided boards.

One thing I do with single-supply designs is try to make the load on the half-supply as light as possible.  To this end, I will use topologies where the op amp inputs are referenced to the half-supply with their non-inverting inputs if possible.  When there are significant currents flowing in filter components these get tied to ground rather than the half-supply, when it doesn't upset d.c. conditions.