From: "Tom Faloon"
Subject: Re: Return current path and magnitude??
Date: Fri, 20 Sep 2002 12:10:13 +0100
NNTP-Posting-Date: Fri, 20 Sep 2002 11:07:57 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
You are correct in most of what you say.
BUT most op amps have only two power terminals.
V+ and V- They do not usually HAVE a ground terminal.
If an op amp is connected to (say) +15 V and -15V, then current drawn to
power the OP amp's internal circuitry flows from V+ to V- . It does not know
that ground exists !
Current supplied by the OP amp output, to external loads, flows through the
load, to the load's reference, which is often ground, but can be other
points, such as V+ or V- .
As to your question of how much current can be drawn from ground to V-.
It is limited by one of two things.
1 The maximum current rating of the V- supply.
2 Does the 'GND' line have a limited current capability. (Sometimes, when
only a single supply rail is available, a resistive divider, or an active
device is used to provide a mid rail voltage as a 'GND' reference for an OP
amp. This often has a very low current carrying capability.
Hope this helps.
Håkan wrote in message
> This I think is quite obvius:
> An op-amp is connected to V+ and GND (0-Volt), then current flows from V+
> GND. Making GND current return path. The current flowing in GND can be
> large. Therefore, it is important in critial op-amp applications to assure
> that this current in GND traces is separated from low current GND traces.
> Q: Now suppose, an op-amp connected to V+, V- and GND. Is GND still the
> current return path? Then this would imply that the op-amp draw current
> V+ to GND, which is as above. Wouldn't this also imply that current is
> from GND to V-, thus a negative current from V- to GND (or positive
> from GND to V- ;-) ). Thus resulting in an alternating current flowing
> through GND. Correct ???
> If it is so, how much current can be drawn from GND to V-? I have never
> considered GND to be current source but rather a "black hole" that can
> "swollow" what ever current comming from real current sources, until
> the fuse.