Q&A
with TJ Byers
Electronics Q&A
In this column, I answer
questions about all aspects
of electronics, including
computer hardware,
software, circuits, electronic
theory, troubleshooting, and
anything else of interest to
the hobbyist.
Feel free to participate
with your questions, as
well as comments and
suggestions.
You can reach me at:
TJBYERS@aol.com
Operational
Amplifier
Q.I recently had to make a circuit
to subtract two DC voltages. I
first considered using a differential
op-amp with one voltage feeding
the positive input and the other
connected to the negative input. After
doing the math to balance out the
four resistors for the gain I wanted, it
struck me that a summing amp has
an advantage: only one feedback
resistor has to be changed to alter the
gain!
I used an inverting op-amp for
one input and fed it to a summing
amplifier and ended up with the right
number (Figure 1). What are the
ramifications between the differential
amp and my solution to produce the
difference of two voltages?
What's Up:
John
via Internet
Got batteries? I’ve got
answers for monitoring
gel-cells. Have a transistor
you can’t replace? Got
that covered, too.
A. Operational amplifiers (op-amps)
are so named because they were
originally used to model the basic
mathematical operations of add,
subtract, multiply, integrate, etc. in
electronic analog computers. Figure
2 shows how the op-amp is configured to “calculate” a few popular
math functions.
In the Add circuit, each voltage is
weighed equally and there is no limit
(well, almost) to the number of inputs
you can deal with. Notice that only
one resistor (Rf) is needed to define
the gain of the circuit and the output
is inverted. The Subtract circuit
measures the difference between V1
and V2 by using the inverting and
non-inverting inputs of the op-amp.
It’s limited to just two voltages. With
the values shown, both of these
circuits provide a gain of 10x.
The Weighed and Average circuits
are special cases of the Add circuit.
The gain of the Weighed circuit is
17. 5 for the values shown and the
Average circuit has unity gain. In the
Inverting configuration, both resistors
have to be the same value.
What are the ramifications?
Obviously, your inverting design
allows you to mix both addition and
subtraction with multiple inputs,
something a single op-amp can’t do.
Also, a single resistor determines the
gain of the stage. On the other hand,
each op-amp in the chain adds its
share of noise to the output, which
could be a factor in very low voltage
measurements.
Ding Dong
Q. I am trying desperately to build
a doorbell intercom. I want it to
sound like a ding dong or ding ding
ding version — something pleasant to
that extent. I have close to 100 LM555
timers in my possession and I am
willing to use as many as it takes.
Alex Belenkiy
via Internet
And just for fun, I threw
E
l
e
c
t
r
o
n
i
c
s
NUTS & VOLTS
F
o
r
in a couple of one-night
projects — Geiger
E
v
e
r
y
t
h
i
n
g
meter, anyone?
10k
Figure 1
10k
V2
10k
10k
10k
Subtraction V1
Circuit
-Vout
14
A. With the doorbell chime circuit
in Figure 3, you’ll
have close to 98 left
over for other projects.
Yep, it only takes two
555 chips to make a
reasonable facsimile
JANUARY 2005
