Hayes and Horowitz: Pages 262 - 272

Lawless: Chapters 30 - 35

Pasquale: Pages 71 - 76

As you may have noticed previously, working with transistors can be difficult: you have to consider the gain you want, set an appropriate bias point, and design your input stage such that it doesn't filter your signal.  Fortunately for us people have already packaged entire combinations of transistor circuits that will do most of the hard work for us in the form of operational amplifiers.

There are many, many different operational amplifiers, each tailored to slightly different situations.  To start with we'll use the LF411, which is a good all-purpose model.

Pin connections for the LF411 op-amp chip.  Pins are typically numbered starting with 1 in the top-left of the diagram and continuing counter-clockwise around a chip. Note that pins 1, 5, and 8 aren't used for this part; this is to keep a common footprint and make mass production simpler.	How to place an Integrated Circuit (IC) chip on a breadboard.  Note that the notch on the top of the diagram corresponds to a notch or circle on the physical chip, and that the chip should be placed across the channel in your breadboard.

While there are many, many subtleties to op-amp behavior, we'll focus on a model that will get us the furthest without getting bogged down in details.

You can get a long ways with these two rules of thumb, but there are some important common caveats:

• $V_{out}$ can never be higher than the positive power rail $V_{CC+}$ nor lower than the negative power rail $V_{CC-}$
  • Op-amps can manipulate voltages, but they don't create them.
• Often it is limited to being a volt or two lower, but this depends heavily on the specific device
• If there is not negative feedback, then
  • $V_{out} \rightarrow V_{CC+}$   if $V_+ > V_-$
  • $V_{out} \rightarrow V_{CC-}$   if $V_+ < V_-$

There are many more limitations on the behavior of real op-amps, but this simple model is enough to understand and design a lot of useful circuits.

When working with transistors, we usually just used the power supply for providing a positive voltage and ground. The op-amp circuits you'll be using are designed to use a bi-polar supply, meaning both a positive and negative voltage, as power. This means setting the low side of one variable supply to ground and the high side of the other variable supply to ground. To set facilitate this, we can use a metal jumper to make the connections for us. The method is shown below.

Unscrew the ground terminal's cover	Place the bar over the ground terminal, then loosen the adjacent + and - terminal covers
Rotate the bar under the adjacent terminal covers and tighten them	Replace the ground terminal cover

Now the leftmost red terminal will be positive with respect to ground and the right black terminal will be negative with respect to ground.

To start, consider the op-amp circuit shown below:

A diagram of an op-amp non-inverting amplifier circuit. The numbers next to the five different connections refer to the pin connection on the physical chip; this isn't standard but we've included it here to make the initial construction simpler.

You should use your power supply for the the $\pm 15 V$ rail connections here.  Refer to the previous labs for instructions on how to set up a negative voltage.

For the input, use a sine wave with the following parameters:

• 1 kHz frequency
• 1V pk-pk amplitude
• 0V offset

Measure the amplitudes of both signals, and include a sketch or screenshot of the two signals.
Calculate the measured gain $g = \dfrac{V_{out}}{V_{in}}$ for this circuit.  Is it close to what you'd expect?
Why is this circuit called a non-inverting amplifier?

Suppose that you swapped the position of the 22k and 10k resistors. 

Predict what the gain would be for this instance, and briefly explain

Keep the op-amp on your board, you'll be using it for upcoming circuits.

Before building the next circuit, we're going to introduce a new element: the potentiometer.  Potentiometers are made of some resistive material that has fixed connections at either end and a movable contact point.  This is shown in the schematic diagram as a resistor with an arrow pointed partway along its length:

A photograph of your potentiometer, with connections annotated. The 1, 2, and 3 refer to the three pin connections. This one has been soldered to a tiny bit of PCB to make it easier to use.	The schematic symbol for a potentiometer

Find a 1k potentiometer (or 10k or 100k, the specifics aren't critical) and place it in your breadboard.  Connect three wires to the three terminals, and then use your multimeter to test its behavior.

Does the resistance between terminal 1 and 2 increase, decrease, or stay the same when the screw is turned?

Does the resistance between terminal 1 and 3 increase, decrease, or stay the same when the screw is turned?

Okay, we can adjust a resistor.  Let's use it in our amplifier and see what happens!

Consider the circuit shown below:

An adjustable amplifier circuit.  Note the blue numbers indicate the potentiometer pins

Predict what gains you can achieve with this circuit, and briefly explain

As you observed with the potentiometer circuit, we can make a non-inverting amplifier such that it has a gain of 1.  Like its transistor counterpart, this is called a follower circuit.  Let's make one intentionally by connecting our op-amp as follows:

Under what conditions does the op-amp circuit follow?

How are the requirements different from transistor followers?

Next, let's construct an inverting amplifier circuit, shown below:

An Inverting Amplifier circuit.

For the input, use a sine wave with the following parameters:

• 1 kHz frequency
• 1V pk-pk amplitude
• 0V offset

Don't forget to include a sketch or screenshot of the two signals in your report
Find the gain $g = \dfrac{V_{out}}{V_{in}}$ for this circuit.  Is it close to what you'd expect?

Why is this circuit called an inverting amplifier?

Suppose that you swapped the position of the 22k and 10k resistors. 

Predict what the gain would be for this instance, and briefly explain

By replacing one of our resistors with a circuit whose current depends on physical parameters, we can use the op-amp to convert this current to a voltage.  Since measuring voltages is typically easier than measuring current (you don't have to break a circuit and put a meter in), this sort of circuit can be useful to us when we want to do something like measure light levels with a phototransistor.

As you might guess from the name, phototransistors act like bjt transistors that react to light.  Specifically, photons are able to pass through the clear packaging and strike the base layer of the transistor, knocking electrons free via the photoelectric effect.  These electrons create a small base current, which is then amplified to a much larger current through the collector and emitter.  More photons yield more current so this is a proportional measurement, unlike the photoresistor.

Our phototransistor and its schematic symbol.  Note the flattened edge belongs to the emitter side.

With this in mind, let's see what happens when we replace the 10k resistor in our last circuit with a phototransistor, as shown below:

A light to current converter, constructed with an op-amp.  Note that the phototransistor's base isn't connected to anything.

In what way does $V_{out}$ vary as the light level increases? (You may want to use your phone's flashlight here).  Is it what you'd expect?

Predict how this circuit's sensitivity to light would change if the resistance was increased.

Choosing an op-amp

• Go to Digikey.com and search for “op-amp”
• See that the top category of Instrumentation, Op Amps, Buffer Amps has about 20k listings
  • Op-Amp Evaluation Boards is where you'd go for a pre-built circuit that shows off the capabilities of an op-amp, makes it easy to drop into a circuit, or makes it so you don't have to deal with RF design issues.
• Click on the big scary one anyways
• Scroll down to the search entry that says op-amp and de-select it with the x.
  • Now you have 30,000 problems!
• We'll start with some basic filters in the Common Attribute column
  • Select In-stock and exclude Marketplace Products
    • The marketplace is for thrid party sellers, generally not what you want.
  • In the Product Status column select Active
  • In the Mounting Type column select Through-Hole
    • Surface Mount parts are more common, but don't fit on our protoboards
  • Click Apply All to add these filters and take you down to >1000 options
    • Notice that as you apply filters, the remaining column options will change. For instance, there aren't any through-hole chips with 32 op-amps in one package, so that option is no longer there.
• We've still got too many options, so let's filter a bit more
  • Change the Amplifier Type to Standard (General Purpose) and the Number of Circuits to 1
  • Below all the filtering options, find the Price column and sort by ascending (the up arrow)
• This is a good place to get looking. First off, you'll probably notice that there are multiple entries for variations on a chip, such as a MCP6241-E/P, a MCP6231-E/P and a MCP601-I/P.
  • In this instance, the primary difference between these chips is a trade-off between slew rate and supply current. The faster chip the chip, the higher the passive power usage here.
  • You'll also see that the Voltage - Supply Span (Max) column tends to be 5 to 6 V for a lot of chips. This means that while they'd be okay for working with a USB power supply, we wouldn't be able to handle a 20V amplitude signal with them.
• Let's filter based on the supply span by going to the filter column for Voltage - Supply Span (Max) and highlighting everything 30 V or more.
  • You can just click and drag for this!
• At this point, the cheapest options are probably in the 741 family
  • They're basically a dinosaur compared to even the 411s we're using.
• The next option that pops up is the NE5534P/NE5534AP
  • At over $1 each, this seems like a bit much. They'd work, but we might've filtered too hard before now.
• Go to the Applied Filters section and remove the Amplifier Type option
• With this, the TL071CP pops up as an option for even less than the 741
  • Its cousin, the TL071IP seems to be designed to work in harsher temperatures
• You can click on the little .pdf icon just to the left of the picture of the chip to open up the datasheet
  • There's way too much info here to go through right now.
  • We can verify the critical features that it will:
    • 1: operate on up to +-20V rails
    • 2: work in the same circuit
      • Figure 4.3 shows that the pinout is the same as the 411

If you can find the part you're already using, you can use Digikey's compare feature to cross-check the most critical parameters, as shown below:

A comparison between the LF411, tdhe TL071, and the UA741 op-amps. The LF411 has the lowest input bias current & voltage input offset, but costs 3-4x as much. So unless you really need the precision, you might as well go with a cheaper option. It also passively draws more current, making it a worse choice for battery-powered devices.

Alternatively, you can:

• Go to the page for the LF411
• In the Product Attributes section, check Filter Similar Products
• Check the following options:
  • Part Status
  • Number of Circuits
  • Package/Case
• This gets ~500 results
  • Filter by In Stock and exclude Marketplace to get rid of half of the options
  • Filter the Voltage - Supply Span (Max) to 30V or more
• You're now at a similar (but not the same) place for looking at new parts.

Now, try checking out the cost of specialization.

• If you sort by Voltage - Input Offset ascending, it will run you \$13 per chip to get a 0.5 $\mu$V offset
• If you sort by Slew Rate descending, it will cost $11 to get a chip whose output can change by 4V every nanosecond
• If you sort by Operating Temperature ascending, it will cost $4 per chip that operates anywhere from -55 C to 125 C
  • You could leave it outdoors at the south pole and still have it work… sometimes.
• If you sort by Price descending, you'll find the AMP02EPZ for nearly $50 each
  • This gets you an amplifier that is extremely precise, easy to use (only needs 1 resistor) and will tolerate substantial abuse to its inputs without being damaged.
  • The datasheet says it is low cost but that was back in 2003; it is probably a mostly used for repairs or in equipment with strict certification requirements (e.g. medical devices)

Portions of this page are adapted from “Flexible Resources for Analog Electronics” by Stetzer and Van De Bogart.