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# Tag Info

62

There are many good reasons not to use the 1968-vintage LM741: - Minimum recommended power supply rails are +/- 10 volts Modern op-amps have power supplies that can be as low as 1.8 volts. Input voltage range is typically from -Vs + 2 volt to +Vs - 2 volt Modern op-amps can be chosen that are rail-to-rail Input offset voltage is typically 1 mV (5 mV ...

49

Here is a \$35 kit you can make, which ends up being the equivalent of a 741 op-amp using discrete 13 2N3904 and 7 2N3906 transistors. It has eight binding posts representing the eight pins of the device. Here is a link to the datasheet, which includes the schematic for the kit (shown below) and a BOM. Compare that to a "real" 741 out of the TI datasheet: ...

43

The short answer: input impedance is "high" (ideally infinite). Output impedance is "low" (ideally zero). But what does this mean, and why is that useful? Impedance is the relationship between voltage and current. It's a combination of resistance (frequency-independent, resistors) and reactance (frequency-dependant, inductors and capacitors). To simplify ...

40

It seems like you are running into Slew Rate Limitations, and your output is presenting what is called 'Slew-Induced Distortion' - the Op-Amp's Output Swing is limited by the Slew Rate, so as frequency increases the limit for maximum output swing without 'Slew-Induced Distortion' decreases - typically Op Amps Datasheet have an 'Output Swing vs Frequency' ...

38

The 741 is an old piece of junk, primarily used to teach basic electronics for cheap. I seem to remember reading somewhere that if every 741 ever made were to be collected, there would be enough to give every person on earth 6 or 8 of them. Modern op amps fall into several categories. General Purpose - These op amps are not very fast, have bad non-ideal ...

34

The first OP-amp is actually creating the circuit ground. The 7810 creates a stable 10 volt, which is then divided by the voltage divider R2 and R3, filtered by C3 to make a stable 5 volt level relative to the most negative level. The OP-amp then buffers this, and the rest of the circuit uses its output as the reference ground. Remember that ground in a ...

33

Op amps are pretty close to being ideal differential amplifiers. So the real question is, what's so great about amplifiers? There are (at least!) three answers. First, the obvious -- amplifiers let you change the amplitude of a signal. If you have a small signal (say, from a transducer), an amplifier lets you raise its voltage to a useful level. Amplifiers ...

32

The basic feedback equation doesn't require any calculus or advanced math, only simple algebra. It should be well within high school level math. I find equations work a lot better if you first describe what is going on in words, then follow that up by writing the equation. You can even invite students to come up with the equation by modeling the verbal ...

31

I believe your analysis to be good. I've made sallen-key 4th order filters that cut-off around 3 MHz with absolutely no worry about performance. I don't see that 10 MHz is unachievable. It's all about op-amp choice. For a unity gain stage it's easy to ascertain where the gain starts dropping below (say) 0.99 and regard that as the limiting frequency. On the ...

30

Bipolar opamps like the 741 or the LM324 have different tradeoffs than FET opamps. For one thing, they were designed many years ago when FET IC technology was less advanced relative to bipolar IC technology. It's unfair to call the 741 junk; it was something wonderful in its time. Its close derivative, the LM324, is still in volume production today, so ...

29

This is classic crossover distortion in LM324. Add a load resistor to output until crossover distortion disappears. Start with 1K to V- or gnd if using single supply. This occurs with capacitive loads or next stage pullup R bias to Vcc/2 perhaps. Increase R load values as alternative if so... ( by design in order to minimize idle current) Datasheet ...

28

You seemed to have actually found a reasonable circuit on the internet. I heard there was out there somewhere. The equations you cite are overly strict. Instead of just telling you the values, it's better to explain what each part does. R1 and R2 are a voltage divider to make 1/2 the supply voltage. This will be the DC bias the opamp will operate at. ...

28

It's possible, but it won't work well. Firstly, there is the problem of combining the two outputs, with one scaled precisely 1/256 of the other. (Whether you attenuate one by 1/256, amplify the other by 256, or some other arrangement, *16 and /16 for example, doesn't matter). The big problem however is that an 8-bit DAC is likely to be accurate to ...

28

While I agree with @pipe and in fact upvoted his answer, an additional nuanced reply is that a ground is more than "just" a reference. What I mean by this is a ground is not just a voltage, but something that can source & sink current and stay at the same potential. The ground created by that op-amp can both source and sink current and remain roughly ...

28

High speed with a small difference is difficult to get. Note that not only do comparators tend to have higher input offset voltages than opamps, but also much higher effective noise, as to get high speed they are wideband beasts. Oliver Collins produced a paper a couple of decades ago showing that you get much better results, that is less time jitter, if ...

28

It's all down to speed. What your circuit doesn't show is the self-capacitance of the photodiode: - Given that the signal produced by the photodiode is current (Iph shown above), if this is rapidly changing like in an optical data receiver, the junction capacitance will have a significant effect on rise and fall times. However, with a transimpedance ...

28

The power delivered to the load is from the Joule heating effect: \begin{equation} P=\dfrac{\Big(\dfrac{R_L}{R_L + 50}G\,V_{IN}\Big)^{2}}{R_L} \end{equation} So from differential calculus we know that a function reaches its maximum or minimum value when we differentiate and equal it to zero. In this case we'll have a maximum value, thus: \begin{equation} ...

27

In theory, the OpAmp should perform well no matter what the supply is doing. As we leave the theoretical model of an OpAmp (remember there aren't even supply pins on the basic symbol, just IN+, IN- and OUT), we have to consider more and more details brought in by the real circuit. Many will of course be obvious to you, but trust me - we'll eventually get ...

27

If there weren't loads of textbooks dating back decades that used the 741 as an example, I'd be surprised if many people knew of it now. It'd now be remembered like the OC71, which was the BC108 of the mid/late 60s. One of the reasons I think it persisted for so long beyond the mid-80s, when it was well superseded, was because of its many bad ...

26

They are not completely different. They are much more similar than you think. Technically, a comparator is just an op-amp optimized to operate in the saturation region (have the output saturate since it is always either LO or HI, but be able to leave saturation quickly for fast switching). Being linear in the linear region is not a design priority. An op-...

25

There are downfalls with choosing very large resistors and very small resistors. These usually deal with the non-ideal behavior of components (namely Op-Amps), or other design requirements such as power and heat. Small resistors means that you need a much higher current to provide the appropriate voltage drops for the Op-amp to work. Most op amps are able ...

25

It's not advisable unless the entire project is low voltage and battery operated. Anything mains operated could be dangerous even if transformer isolated. There are specific safety requirements for mains operated patient attached equipment to protect against excessive leakage currents and potential safety hazards due to equipment or component failures. ...

25

Gain/Bandwidth product, you want maybe 50KHz bandwidth at 60dB (1,000 times), so you need somewhere around 50MHz, gain/bandwidth product (And more would lower HF distortion)... Make it 80dB and now you need 500MHz GBP, which is getting difficult if you want low noise down near DC (And is getting really bad news to stabilise at low gain). Also consider that ...

25

Another reason not to use a 741 is that under certain conditions it can go into a latch-up state, where the output saturates and sticks to one of the supply rails until it is powered down. I can't find a reference specifically about the 741, but this page describes something similar: https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/op-amp-...

25

You can pick anything you want and call it ground. This power supply box has two power supplies in it. Shorting the + of one to the - of the other puts the two power supplies in series. That gives you a so-called "split" power supply. You can call the middle connection ground and then you have +15V, 0V and -15V wires. Or you can call the right connection ...

24

Brief answer to both questions: No, that's not correct. No, you do need to worry about that. Let's start from the beginning. There is no way you will ever deal with a literally 'DC' signal. Let's say you have a bench power supply, you use it to power your circuits, that's maybe some 5V DC, right? And what about when you turn it off? What about power ...

24

Yes, you are exceeding the specs. Look at table 6.1 of the DATASHEET on page 3: It clearly states the maximum difference between the V+ and V- pins is 16V. This means you can have +8V and -8V, as the difference is 16V. As you have it right now, your difference is 30V, which is almost double the maximum ratings. Chances are, you have damaged the op-amp and ...

23

According to this, the photodiode does indeed produce a current even when there is zero volts across it; it's the short circuit current. Note that the reference direction of $I_S$ in the question's diagram is opposite that of the $I_{SC}$ of the diode so the output voltage is: $V_{OUT} = - I_S \cdot R_F = I_{SC} \cdot R_F$ I found the above here. ...

23

You will rarely see a circuit with just one resistor as you show it; usually there will be another resistor (or equivalent source resistance) of the same value on the noninverting input, too. Most (nonideal) opamps have a finite input resistance, and this means that a tiny current flows into or out of the input terminals. This current is called "input bias ...

22

A unity gain buffer is what this article also suggests. Quote: The best thing to do with a surplus op-amp it is to use it. There are lots of places in an analog circuit where a buffer amplifier may improve performance - and a unity gain buffer uses no extra components. Grounding the non-inverting input is fine if you have a dual power supply. If you ...

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