The various specs of op-amps
|These books describe how
marketers try to create a good image for their products and bad
image for their competitors' products. The analogy Trout and
Reis use is one of "ladders" in the mind of the consumer. The
job is to move your product up the ladder or, if possible,
create a new ladder with your product at the top of that
This is perhaps possible for rather generic products like dish soap and even automobiles. For complex analog electronic devices the approach can be problematic. One factor that can be disconcerting is that manufacturers try to simplify their job by trying to create the image that the companies are what's on the ladder and not individual products. This is an approach doomed to failure.
Analog Devices and people who order things alphabetically would have you believe that the ladder is something like Figure 1. The marketing people try to take a short cut by convincing you that if you buy from their company you will get the best op amp (or other analog product). After all, believing in a company is a lot easier then sifting through all those data sheets with their tedious and oftentimes incomprehensible specs.
Why on earth does it take 5 or 10 or 30 pages to describe a little blob of plastic with 8 little metal tabs on it? What a wonderful world it would be if life were this simple. In reality the 30 page data sheet doesn't even begin to tell you every thing you need to know about the part. The ladder is not some simple affair with 5 or 10 companies on it. In reality there is not one ladder at all but rather a dozen or more ladders all set up and pointing to that lofty goal: the perfect op amp.
The specification ladders
In fact, companies are not on the ladder at all. Certainly
companies matter for things like availability and trust to deliver on time. But let purchasing worry about that. We have
to give something to do to those poor wretches who did not pursue a technical career. The ladders that
we are concerned with are specification ladders. As I have
said there are many of them. Figure 2 shows how the
perfect op amp would sit at the top of these specification ladders. You needn't feel daunted by the specs. Even if you are
a digital, software or systems engineer the concepts are easy enough to grasp qualitatively.
I have yet to meet a software engineer who was not able to grasp the concepts of amplifier specs. I doubt that I will ever understand the concept of "coding" in C++. So don't be frustrated or intimidated by analog design. We can now go over the most important specs of op amps and build a set of ladders that will give you an intuitive feel for the range of values.
|Intuitive feel is good. It's why I can make a lot of money as a consultant. Get to know
these op amp spec ladders and you too can make a lot of money as a Silicon Valley Consultant. More importantly, your designs
Lets begin by considering the perfect op amp. We will look at op amp specs in rough order of their importance. Then we
will establish a range for these specs in real life parts and use these values as ranges for the ladders in our mind that
define the "goodness" of an op amp.
The perfect operational amplifier
The first and most important spec is price. The perfect op amp would not be free. Rather, large multinational companies
would send over brilliant, engaging people like Jim Williams and Bob Pease. They would pay you to take large Hefty® bags
full of perfect parts, but only after a pleasant afternoon discussing Jaguars and video cameras. Other specs of interest
and what they are follows:
|If the amps open loop gain is one million, the high gain will insure
an accurate output if there is negative feedback that is designed to give a closed loop gain of two or ten. A perfect gain would be infinite, but 200 or 300 db would be OK.
BandwidthThe bandwidth of an amplifier is the frequency range over which it can produce gain. An amplifier with a bandwidth of 1 megahertz will have a open loop gain of one for an input sine wave of one million cycles per second (Hertz). A gain of one is not very useful, so this value is the extreme limit of the amp. Since the gain of all op amps falls as frequency increases, the benefit of all that open loop DC gain is not available as the frequency increases.
Hence, the accuracy (lack of distortion) declines the faster the op amp is performing. This just highlights the importance of the open loop gain mentioned above. If it starts out high, there might be enough for a good circuit at high frequencies. Perfect bandwidth would be in the hundreds of Gigahertz.
NoiseThe perfect op amp would add no noise to the output. Noise specs are usually expressed as an input referred voltage or as a voltage or current per root hertz. All these are referred to the input so that you have to remember to multiply the gain of the circuit (not the open loop gain) to get the output noise.
Microvolts of input noise looks great until you need a gain of 1000. Then you have millivolts of output noise. That can turn your sixteen bit analog to digital converter into an eight bit one as far as the data is concerned. Perfect is, of course, zero noise.
Offset voltage & current, and driftThe two input pins of a perfect op amp should have zero volts across them and zero current difference between them when the open loop output is zero. Process variations make this unlikely.
Real world op amps are trimmed at the factory or have on chip compensation to reduce these offsets. Offsets ore most troublesome when trying to accurately measure or reproduce some DC value or AC wave form. Offsets can also cause non-linearity's and other problems. Drift is the change in offsets over time and temperature.
The two input pins of real world op amps have some small current that must enter or exit the pins to keep the op amp
biased (in its' linear range). Some designs use p-channel input transistors that causes current to flow out of the input pins.
Some have currents that flow in. Some are corrected at the factory so the current is less, but the downside is that it can
flow in either direction in either pin and will be different for every part in the same lot. There are CMOS input amps that
have essentially zero bias current. At last, perfection realized.
Output impedance, power, and voltage
The output of an op amp has impedance (think of it as varying
resistance over frequency). Not good. There are power
op amps that have low impedance to drive motors and speakers.
Even driving a 50 ohm cable can be difficult with the wrong
amp. Related concepts are the output power. Low output impedance
is of little use if the op amp cant get the heat out
of the die. The output voltage should also be able to swing the
full range from the top power supply rail to the bottom (or
|A similar concept is
settling time. This is an expression that accounts for both the slew rate and the
"ringing" or small oscillations that occur as the amp settles down to a new value after a large change. Once again, this is
critical for sampled data systems with large input swings. Our perfect amp has infinite slew rate and zero settling
Common-mode and power supply rejection
The two input pins to an op amp should only function with respect to each other. Common mode in this context refers to
the fact that both pins might be exposed to the same voltage even though the pins have the same differential (between them)
voltage. This is usually expressed as negative decibels.
|Fortunately it doesn't vary much from part to part of a given part
it's too high it can attenuate the input signal thus requiring
more gain to compensate for. There are also phase and other
issues. Once again, perfect is zero.
Minimum gain The concept of minimum gain (Unity gain stable) is a little
elusive. In this context the gain is referring to closed loop gain in your circuit. In order to get some op amps to work (have
gain) at high bandwidths they are internally compensated with capacitors on circuit nodes.
The perfect spec summary Let's tabulate these perfect specs:
|Bandwidth||DC to Daylight|
|Output power||67 horsepower (50 KWatts)|
|Output voltage||1000 volts|
|Slew rate||1 million volts/ microsecond|
|Common mode rejection||500 Decibels|
|Power supply rejection||900 Decibels|
|Input capacitance||0 picofarads|
|Operating temperature||-170 to +300 degrees C|
|Some parts do achieve near
perfection. PMI's OP-27 started the trend to parts with so
little noise that the noise of the input resistors would far
exceed the noise of the amplifier. A TI
LMC6001 op amp has essentially unmeasurable input bias currents
(10 femtoAmps). Let's now assign reasonable values to these
The real operational amplifierA real world Operational Amplifier has specs that are tabulated as follows:
The amplifier ladders
Figure 3 shows a top view of our amplifier ladders
with some real world numbers pasted in.
Handy guidelinesLastly, this table gives some handy guidelines to get you on the right track or help land that new job.
One more thing, remember the most important tool an engineer owns. No, not a computer, not a data library, not a CAD package and certainly not a Spice package. It's a telephone. Call the manufactures' application engineers and take advantage of some of the finest minds on earth. They are patient, friendly and understanding. I've had several occasions where an apps engineer recommended a competitor's part because that is what was best for the job. They will provide you with the collective experience of an entire industry and it's free for the asking.
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