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In class we are designing a few different circuits and it uses some diodes and opamps. Everything is fine on paper and everything makes sense. These are only ever refereed to as a "diode" or "opamp".

So then I made a simulation on pspice. However depending on what diode or opamp I choose the results I got were completely different. There where many opamps and diodes to choose from in the component list.

Until now I just thought a diode is a diode or opamp is an opamp as there's never been any more specifics on them. Nothing like a resistor or capacitor where you have to choose the correct value component to get it all to work.

So I was wondering when people say "use an opamp" is there a standard common / specific opamp that is the general one used.

Same with the diodes. Is there a standard go to diode that is used in all circumstances ..unless otherwise stated.

After thinking about it..what about transistors also?

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    \$\begingroup\$ It's worth noting that in real life, there's more to selecting a capacitor than just the component value. Not all capacitors are created equal.. \$\endgroup\$ – Li-aung Yip Sep 27 '13 at 10:55
  • \$\begingroup\$ The same with resistor, you need to watch for the power rating. Inductors are even worse. \$\endgroup\$ – lucas92 Feb 9 '17 at 16:00
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Here are the types I immediately think of when someone says "diode", "op-amp", ...

  • Op-amp : LM741. The first "easy to use" op-amp IC on the market.
  • Diode : 1N4001. General purpose silicon diode good up to 50V blocking voltage and 1 amp current. The 1N4002, 1N4003, etc. are similar diodes with higher voltage ratings.
  • Transistor: 2N2222. NPN bipolar junction transistor. The 2N2907 is apparently the PNP equivalent.
  • (Linear) Voltage regulator : LM78xx series, i.e. the LM7805 for 5 V, the LM7812 for 12 V.
  • Digital logic, i.e. NAND gates and so on: 7400 series and 4000 series.

These are extremely common, basic parts. If you walked into a hobby store and asked for a hundred transistors, without specifying anything else, you would probably get a bag of 2N2222's.

This is not to say these parts are useful for everything - they have limitations on voltage, current, speed, accuracy, and so forth. But if you have to pick a type of component for the purposes of a SPICE simulation, these will work fine.


Edit: For reference, here are the "default parts" you get in CircuitLab:

  • Op-amp TL081
  • Diode 1N4148
  • Zener diode 1N4733A
  • NPN BJT 2N3904
  • PNP BJT 2N3906
  • N-channel MOSFET IRF530
  • P-channel MOSFET IRF9530
  • N-channel JFET J310
  • P-channel JFET J271
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  • \$\begingroup\$ Unless they carry this. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 27 '13 at 10:43
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    \$\begingroup\$ I disagree with the 1N4001. If you need a standard diode in an opamp circuit it makes a lot more sense to use a 1N4148. \$\endgroup\$ – radagast Sep 27 '13 at 12:16
  • \$\begingroup\$ Oh god why the 741? It's an ancient POS. Use a TL084 or another more recent, nicer op-amp at minimum. \$\endgroup\$ – Connor Wolf Sep 28 '13 at 0:23
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    \$\begingroup\$ @IgnacioVazquez-Abrams 'this' asks you what country you're from then dumps you to the company's home page, where you can buy wire rope, arduinos and 3D printers. Which of those do you suggest to use for an op-amp? \$\endgroup\$ – Pete Kirkham Sep 29 '13 at 12:39
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    \$\begingroup\$ @PeteKirkham: The link points to a 100 transistor set containing various BC and BD transistors. I don't know why that website is so horrible. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 29 '13 at 21:04
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See TUP TUN DUS DUG for a list of "universal" small-signal transistors and diodes often used interchangeably in example circuits published (e.g.)

Here is a link to a scan of an original page by Elektor Magazine, who coined the phrase TUP TUN DUS DUG. They hardly use it today (and some of the parts may have become obsolete), but it is still a valid concept and it's good to know where it came from. If you're planning a design, today, with second source parts in mind, you're doing essentially the same thing.

What people think of as the "common" or "basic" transistor is generally an NPN small-signal BJT but the exact type varies from place to place and over time. As an occasional hobbyist I used to use BC108 then BC547 but I'd buy anything cheap (e.g.) and I'm used to seeing 2N3704 and translating that to BC547 with the leads in the wrong order.

There doesn't seem to be an equivalent "universal" small-signal MOSFET?

By comparison, the 1N4148 is much more uniformly found in examples.

The 741 opamp seems to hold a similar position even though apparently it is usually not a good choice anymore.

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    \$\begingroup\$ TUPTUNDUSDUG - brings back fond memories of Elektor magazine (a good source of 'old school' circuits). \$\endgroup\$ – JIm Dearden Sep 27 '13 at 14:46
  • \$\begingroup\$ Link is broken... \$\endgroup\$ – Bradman175 Feb 9 '17 at 0:46
  • \$\begingroup\$ @Bradman175: Thanks, new link found and inserted into answer. \$\endgroup\$ – RedGrittyBrick Feb 9 '17 at 12:11
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When referring to a generic (rather than standard) op amp, diode, transistor etc. its about the basic function of the device without considering specific circuit criteria such as voltage range, power consumption, speed of operation etc.

For example. If you take the 'op amp' you would expect a device to have two inputs (inverting and non inverting), have a high open loop gain, have high impedance inputs and a low impedance output. You would also expect it to perform predictably in 'standard circuits' such as inverting / non inverting amplifier, integrator / differentiator, comparator etc.

In other words virtually any op amp can be used as a plug in replacement and still work.

For specific applications it might be important that the output has full range or the frequency bandwidth has a high value or it can be used a low single supply voltage. In that case you would specify the device type to be used in the circuit.

Generic diodes are either small signal types used for detecting AC signals or rectifier types - used for power supply AC/DC conversion. Even here you usually have to state Silicon or Germanium type.

Specific diodes will be chosen by voltage, current, frequency, construction etc.

Generic transistors - (NPN or PNP) are sorted initially by power rating - small signal, medium power or high power. It will be assumed that the gain for a small signal type will be a least 100 and the high power type will have a gain of about 10. A typical small signal (NPN) type could be a 2N2222

Of course for specific circuits you need to consider voltage ratings, frequency range etc.

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The general, standard component types that you are looking for are more precisely termed "ideal diode," and "ideal opamp." Ideal Components may be used to represent real electrical components, and do not exist in the real world. Analytical equations and intuition are often greatly simplified by using idealized components instead of more realistic models. When discussing or simulating circuits on an ideal level, there should be no specific device or model number that comes to mind. When people say "use an opamp" in a theoretical setting they are usually referring to an ideal opamp. This is what it means when we say "ideal opamp":

Ideal opamps

An ideal opamp is usually considered to have the following properties:

  • Infinite open-loop gain
  • Infinite voltage range available at the output
  • Infinite bandwidth with zero phase shift and infinite slew rate
  • Infinite input impedance and so zero input current and zero input offset voltage
  • Zero output impedance
  • Zero noise
  • Infinite Common-mode rejection ratio (CMRR)
  • Infinite Power supply rejection ratio.

These ideals can be summarized by the two "golden rules":

  1. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero.
  2. The inputs draw no current.

The first rule only applies in the usual case where the op-amp is used in a closed-loop design (negative feedback, where there is a signal path of some sort feeding back from the output to the inverting input). These rules are commonly used as a good first approximation for analyzing or designing op-amp circuits.

None of these ideals can be perfectly realized. A real op-amp may be modeled with non-infinite or non-zero parameters using equivalent resistors and capacitors in the op-amp model. The designer can then include these effects into the overall performance of the final circuit. Some parameters may turn out to have negligible effect on the final design while others represent actual limitations of the final performance that must be evaluated.

enter image description here

This diagram shows an equivalent circuit of an operational amplifier that models some resistive non-ideal parameters. From the ideal opamp properties above, an ideal opamp would have:

  • \$ R_{in} = \infty \$
  • \$ R_{out} = 0 \$

If you are using a tool such as PSPICE, there is usually an ideal opamp model (maybe OPAMP). If not, it is quite simple to build one using idealized components. Don't forget that real op-amps differ from the ideal model in various aspects.

Keep in mind the distinction between ideal circuit models and realistic circuit models. All basic electronic components have some ideal model that can be used for simplicity. If the component has a model number, it models a real component rather than an ideal component. Usually design tools name ideal models with the generic name, e.g. "RESISTOR," "CAPACITOR," "OPAMP," etc.

Source: diagram and explanatory text from Wikipedia.

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There is no "standard" op-amp, diode or transistor.

There is "common" devices though. For example: 741 op-amp configuration is kind of "classical".

Anyway, it is perfectly fine that your results differ for different components. The rate of difference depends on the configuration of the circuit you're implementing. For example: the open-loop gain of an op-amp becomes insignificant once you employ it in closed loop with negative feedback.

I remember myself frustrated when I discovered that analog electronics does not obey the simplified models and equations developed in under-graduate and graduate classes. Ask specific questions on this forum and the community will help you in overcoming real difficulties.

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  • \$\begingroup\$ I'm talking about switching the components in the same circuit. We were creating a precision rectifier and depending on what opamp or diode I selected the results all somewhat resembled the expect output but had differences. For example the first opamp I choose from the list and diode combination creates an output that was a few volts less than the input and every so often had massive spikes in it. One combination charted an output of 1kv! When input was only 5v and supply was only 15v. After trying some others finally we got the full 5v half rectified output with no peculiarities. \$\endgroup\$ – binarysmacker Sep 27 '13 at 11:21
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    \$\begingroup\$ @binarysmacker, I answered your initial question about "standard" components. It seems that you're encountering difficulties with design/simulation of one particular circuit. I suggest you post a specific question about your circuit to the forum. Make sure to add schematic and describe the problem, and I'm sure you'll get a lot of advices in no time. \$\endgroup\$ – Vasiliy Sep 27 '13 at 22:45
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When folk say "use an op-amp" there is a hidden statement that assumes the application won't mind if the real op-amp hasn't got: -

  • Infinite gain and slew rate
  • No spurious phase shift input to output
  • Zero input offset voltage
  • Zero input bias and offset currents
  • Infinite input impedance
  • Perfect common mode rejection
  • Perfect power supply rejection
  • Zero current and voltage noise generation
  • Zero output impedance
  • The ability to drive voltages to either supply rail from the output
  • The ability to input voltages to either supply rail

There are probably a lot more I've forgotten.

Many op-amp applications don't mind about these things but there are also many op-amp configurations that need pretty low noise or pretty high gain and slew rate etc. You have to then do the long trawl of looking through data sheets to find what you need. Of course simulators help and that's where you uncovered variations that mean one application will work with op-amp A but not op-amp B.

For op-amps I don't care abut paying a bit more - I always default to an OP4177 quad - probably the best quad op-amp available for low to low-medium speeds. If I want rail-to-rail features, medium speed and a low voltage supply I degault to the AD8606.

For diodes, voltage rating, current rating and reverse recovery times are usually the first things I look for but, in some applications I'll choose schottky because of their low forward volt-drop.

BJTs and FETs are the same as op-amps - there are a lot of parameters but my default small signal BJT is the BC547 and for high frequencies it's the BFR92.

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