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DPRG: op amp question

Subject: DPRG: op amp question
From: MIKE MCCARTY -- 93789 JMCCARTY at DEVEL.USA.ALCATEL.COM
Date: Tue Aug 3 09:31:53 CDT 1999

)Maybe this is a dumb question, I don't know. (Are there any op-amp tutorials
)on the Internet? Something like the Encoder basics articles would be nice.
):) Is there any reason for V+ and V- supplies on an op amp? Can I connect
)ground to V-? I have a RadShack TL082 "dual bifet op amp" which doesn't need
)a ground connection, only V+ and V-...  For other op amps that have gnd, why
)can't I use a voltage divider to create a level between V+ and the real
)ground?

There are no dumb questions, only unasked ones.

There are several reasons for having a "split" supply. One is that the
input transistors can't operate with 0 V bias. They need some positive
bias for proper operation. This is related to what is called "common
mode input range". Both inputs must be within the common mode input
range, which means typically a few tenths of a volt above V- up to a
few tenths of a volt below V+. If V- is the same as "ground", then 0V
is not a legal input voltage.

Another is that the outputs typically cannot swing all the way to V-
and V+. If V- and "ground" are the same, then 0V is not an available
output voltage.

So, even the TL082 needs a split supply. A "pseudo-ground" may be
created with a voltage divider. For various reasons, this is
undesirable. First, this "ground" is not very "stiff", and is usually
incapable of absorbing or sourcing much if any current without changing
significantly. The use of resistors also couples noise and voltage
variations from the power supply to the output, which is undesirable.
They also usually need fairly heavy bypass. Some use a Zener diode, but
they tend to be noisy. One compromise I've seen is the use of a
regulated power supply, a bypassed voltage divider feeding an op-amp
used as a voltage follower (to give low output impedance), with the
output of the voltage follower being the pseudo-ground.

What is your aversion to a split supply?

Mike
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