domingo, 7 de febrero de 2010

Basic Op Amps

The operational amplifier (Op Amp) is a staple item in electronic circuits and

is a building block that often is one of the main components in linear audio and
video circuitry. The op amp is basically a high gain amplifier that is used in
conjunction with feedback networks to make up a circuit whose properties are
determined by linear passive components, such as resistors, capacitors, inductors, as
well as nonlinear components (diodes, varistors, thermistors, etc). The term
“operational amplifier” comes from the use of these devices in analog computers
that were used decades ago to perform mathematical operations (addition,
multiplication, differentiation, integration, summation, etc) on input quantities. The
term has stuck and is still used, even though analog computers have largely
departed the scene, having been replaced by digital computers long ago. The
operational amplifier of today is a sophisticated device, being composed of many
transistors, diodes, and resistors, all in a chip, and packaged in various
configurations. There are thousands of types of op amps available, from flea
powered microwatt units to units capable of handling a few hundred watts of power,
from a few cents to many dollars in cost. As you may imagine, the specs and
performance requirements, as well as reliability, temperature range, and packaging,
all affect cost. Op amps that can do many ordinary jobs very well are available for
under 50 cents, owing to low cost plastic packages and large scale integration, and
high volume production. Technologies commonly used are bipolar, FET, CMOS and
combinations. Some large or high power op amps are made using monolithic
fabrication methods.

From a circuit viewpoint, for the purposes of explanation, an ideal amplifier
is used to represent an op amp. An ideal amplifier has the following properties:
Infinite forward gain, bandwidth and input impedance, with zero output
impedance, noise voltage, DC offset, bias currents, and reverse gain.  In
practice, all op amps have some bias current that flows in the inputs, this being
almost negligible for JFET and CMOS types, but more significant in bipolar types.
This current must be considered in high impedance circuits, and in DC and
instrumentation amplifiers, and in circuits that must operate over a wide
temperature range. In addition, even if you were to short the op amp inputs together
you may not get zero output voltage, but some random DC level. This DC voltage
can be considered as an equivalent DC input offset voltage present at the input. DC
offset can also be produced from equal input bias currents flowing through unequal
resistances in the inverting and noninverting input circuits. This will produce a DC
input voltage differential at the input. Some op amps have external pins to which a
potentiometer can be connected to balance out or otherwise cancel this voltage,
bringing the DC output to zero under zero signal input conditions. These are widely
used in instrumentation amplifiers and related applications where nulling or zero
adjustments are required. All amplifiers generate some noise, which is due to
thermal and semiconductor junction effects, and can be considered as an equivalent
input noise voltage. Amplifiers are available with low noise characteristics for those
applications where noise must be kept to a minimum.
 A real world op amp has a lot
of gain (>1000X voltage gain) and a fairly high input impedance (>100K). Generally
there are two inputs shown, an inverting and a non inverting input, and one output
referenced to ground (but not always, differential outputs are sometimes used in
certain applications). One of the inputs may be grounded in many common
applications where a single ended signal source is present. This is a common
situation. There are limitations on the DC levels allowable on the inputs, and
limitations on the available output voltage swing. Op amps are available that allow a
full output voltage swing between the positive (Vcc) supply and negative (Vdd)
supply. These are sometimes referrred to as “rail to rail” capable. In addition, if the
exact same voltage is present on the inverting and non-inverting inputs, ideally the
output voltage should be zero. This is not always so, and the degree of imperfection
is called the common mode rejection ratio. This is usually 60 dB or better, with 70-
80 dB as a minimum. Note that this may vary with input voltage levels to some
degree. Also, variations of power supply voltage may show up as equivalent input
signals. The degree to which the op amp rejects this is called the supply voltage
rejection ratio. It is usually better than 60 dB and typically 70 to 80 dB or better.
After all, nothing is perfect in life.
Op amp power supply connections are sometimes shown in diagrams, especially if
decoupling capacitors and resistors are necessary, but more often shown elsewhere
in the schematic, as they play no part in the primary circuit function other than to
power the amplifier. Many general-purpose op amp chips have two or four separate
operational amplifiers in one package, with common power supply connections. In
practice the ideal amplifier criteria requirements are met only approximately, but
as will be shown, close enough for most purposes. Practically, an op amp will have a
gain of 10,000 or more, an input impedance of megohms, and a 3 dB bandwidth of
several tens of hertz or more. If an amplifier has a 3 dB bandwidth of 40 Hz and a
gain of 100,000 times, this is a gain bandwidth product of 4 million hertz, or 4 MHz.
(40 x 100,000). It is advantageous in many feedback applications to have the gain
falling at 6 dB per octave or 20 dB per decade at frequencies beyond the corner
frequency (that frequency at which the amplifier gain has fallen 3 dB or 70.7
percent of its DC value). Since the op amp is used in mainly in feedback circuits
having much lower closed loop gain, these performance figures are good enough in
many cases. In fact, even a single high gain (100X) common emitter transistor
amplifier stage can be treated as an op amp if feedback is employed, with
surprisingly little error. In many cases a single transistor will work almost as well as
a more expensive op amp device. One example is a simple audio amplifier stage
from which a moderate gain (5-20X) is required. This will be shown in an example

One of the most popular op amps of all time is the venerable LM741, its dual
version LM747 and their many descendents. The JFET input TLO8X series is also
very popular, coming in single (TLO81), double (TLO82) and quadruple (TLO84)
units. The TLO81 and TLO82 come in 8 pin DIP packages, while the TLO84 comes
in a 14-pin DIP package. These op amps operate well from 5 to 12 volt experimenter
supplies, and require both a plus and a minus supply. These are also cheap and
widely available. Other general purpose types are the LM324 and LM1458 (bipolar)
and LM3900, and all their variations and flavors. There are many others, but these
types mentioned are easily obtained by the hobbyist wishing to experiment with
them, and are cheap and in plentiful supply. Many manufacturers make them, so
obsolescence should not be a problem for a long time. We will use the TLO8X series
for circuit examples, as they are general purpose JFET types, allowing the use of
higher resistance values and therefore smaller capacitor values, which is often more
convienient from a design standpoint. The TLO8X series have an open loop (no
feedback used, the full gain the amp can deliver) voltage gain of over 10000 and
having JFET inputs, an input impedance of a million megohms. The gain bandwidth
product (obtained by measuring frequency where gain falls to unity) is rated at at 4
MHz for the TLO8X series. Op amps are available with gain bandwidth products to
several hundred MHz and even higher, and these are used in video and RF



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