Operational Amplifier : where do saturation occurs? - Page 1
Forum section: electronics / beginners

author: ratatax
author_rank: Regular Contributor
timestamp: July 18, 2025, 05:11:07 pm
content: Hi there,I have an audio signal of large amplitude (10V) going into an operational amplifier in inverted mode, working with only 5V as power supply.I can easily set the feedback/input resistor to have like 0.3 gain to bring the output level well in range, but I don't know, will that work?I searched the net but cannot find the answer, does saturation in opamps happen at the input no matter how feedback and input resistors are configured, or does it happen at the output?

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2025, 05:21:18 pm
content: So long as the output has not saturated, the voltage difference between inverting and non-inverting input pins should be almost zero.If you use the inverting configuration, where the non-inverting input is "zero", you can use a large input resistor and smaller feedback resistor to reduce the voltage swing at the output and avoid saturation.Note that the "zero" at the non-inverting input is the reference for how the input voltage (before the input resistor) can swing.If you use split supplies (e.g., +/- 15 V), that "zero" is ground, but with a single supply (e.g., +5 V), you need to establish a DC reference (such as +2.5 V) for the non-inverting input and swing the input about that voltage.The simplest way to do that, when usable, is to put an appropriate capacitor in series with the input resistor: it will remove any offset between the natural reference for the input voltage (which might be ground) and the reference voltage, but will remove any effect from the input DC level.

author: ratatax
author_rank: Regular Contributor
timestamp: July 18, 2025, 06:55:05 pm
content: Thanks. Yes I am using a DC coupling capacitor because the input will swing between -5/+5V while my opamp is 0/+5V.But I'm not sure to understand. With a low enough gain (say for example 0.5 and my opamp is rail to rail), I will turn my -5/+5V signal to 0/+5V without clipping?

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2025, 07:11:16 pm
content: Obviously you are pushing the output limits with those values, and “rail-to-rail” is not literally true, but the output capability is given by the actual limit, which defines saturation.When you operate instead in non-inverting mode, you are also limited by the operating range of the input circuit, but that voltage does not vary in inverting mode until the output saturates and the feedback can no longer force the inverting input to match the non-inverting input’s voltage.Slew rate is another issue, at high frequencies.

author: bson
author_rank: Unknown
timestamp: July 20, 2025, 08:07:02 pm
content: Check the input voltage range on the datasheet. If it were me I'd place a resistor divider prior to the opamp, to attenuate to stay within spec and not rely on the opamp to do it. The problem is all opamps have a phase margin due to their inherent delay, and if the input changes faster than this - even if only a glitch while attaching a connector for example - it may be induced to oscillation or locked up. Some won't, but it's better to design circuits not to depend on specific behaviors like this unless you absolutely have to.

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2025, 08:53:26 pm
content: Also, if you apply a high input voltage while the amplifier power is disconnected, you can get a relatively high voltage at the IC, since the feedback is inoperative.That is usually protected against with diodes from the inverting-input pin to the two power pins of the device.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 22, 2025, 02:21:47 pm
content: The headroom needed for an opamp is one of the reasons there are many different opamps. Some opamps need about a volt of headroom on the inputs, while others can manage to work with inputs that are even (a little bit) outside of the supply rails (Usually only on one side).For the outputs. Most older technology opamps need 1 to 2 V of headroom on the outputs, while modern opamps often optimize this into the milli volt region, but then it is also dependent on the output current.There are many different opamps, and I highly recommend you build a few circuits with them on a breadboard and do measurements to see how they behave. It's a good way to get some hands-on experience, and that is very important if you want to do more with electronics. Theory and simulators are nice, but they can't replace real-life experiments.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: July 22, 2025, 03:00:26 pm
content: When I taught electronics refresher courses at a previous job, I would proto-board a gain of 2 noninverting amplifier, using a RRo opamp (TLC2272) powered from 5 volt. I would increase the signal amplitude right before clipping.With the students observing, I would replace the TLC2272 with other common opamps, like the LM358 and TL072, so they could immediately observe the effects of a non-RR opamp.While we were at it, I would increase the supply voltage and then sweep the frequency from 1 to 30 Khz, and again on the scope compare the frequency response of the different devices.With these graphical examples they would gain a better understanding about opamp operation in half an hour.

author: MrAl
author_rank: Super Contributor
timestamp: July 22, 2025, 03:06:58 pm
content: Quote from: ratatax on July 18, 2025, 05:11:07 pmHi,To be absolutely sure you get the right answers you need to show the circuit.However, if it is a standard inverting amplifier then you have to apply an offset voltage to the non-inverting input, usually at one-half of Vcc. You also have to use a capacitor to help keep noise out of the bias node. Then you need an input coupling capacitor. You probably will also need an output coupling capacitor too.Saturation occurs when the input times the gain results in an output voltage that is either too high or is too low.With the setup as above, with -5v input the output will go to +1.5v relative to one-half of Vcc. Since one half of Vcc is 2.5v, the output will try to go to 4v which is probably too high for an op amp that can only reach 3.5v with a +5v input. With +5v input, the output will try to go to -1.5v relative to the half voltage bias of 2.5v, which means the output will go down to 1v, which is acceptable.Since the -5v input will not work right, something needs to be adjusted.If we bias to 2.0v instead of 2.5v, then the -5v input will produce an output of +1.5v relative to +2.0v, which is 3.5v which might work.If we bias to 1.5v then a -5v input will lead to an output of 3v, but now a +5v input will lead to an output of 0.00v which also will not work. Thus, going a little higher on the bias could be the key.If we bias to 1.75v then a -5v input will give us 3.25v on the output which should be ok, and a input of +5v will give us an output of 0.25v which also should be ok as long as there is not too much of a load on the output.If you can reduce the gain a little bit more, then it gets easier. A gain of 0.25 would be easier to work with and you can go through the above tests to see how it works.Also be aware of the power bandwidth of the cheaper general purpose op amps. A bandwidth of 1MHz does not mean you can run it up to 1MHz. The power bandwidth will be lower than the bandwidth.

author: Dasypus
author_rank: Contributor
timestamp: July 22, 2025, 03:41:28 pm
content: You could try simulating your circuit using LTSpice? I'm no expert, but doing this for my old 1987 HiFi amplifier, I learnt a lot and found the fault.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 23, 2025, 04:31:06 am
content: Quote from: UnijunctionTransistor on July 22, 2025, 03:00:26 pmYes indeed. Simple practical lab experiments such as this are easy to look over or disregard, but the interaction of all the senses, and actual real experiences really do help to really let the students understand what is happening, (compare that with quoting learned numbers from memory, but without understanding).Extending such experiments to other non ideal properties of opamps is also a good idea. Things like GBWP, offset and offset compensation (Shift in offset compensation with input change is a subtle one) Input impedance, bias and offset currents, etc.LM741 is especially nice as a learning tool, exactly because it's such a "bad" opamp. Comparing it with more modern opamps also helps the students to get a sense of progress that has been made by industry in the last 50 years.741 is from 1968, now 57 years ago.https://en.wikipedia.org/wiki/Operational_amplifier#741

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: July 23, 2025, 06:33:23 pm
content: Yet, as horrible as the 741 seems today, in the early 1970s it was the best thing since sliced bread.Specially when compared to even more primitive opamps like the 709, which had to have external compensation and could be easily destroyed.

author: TimFox
author_rank: Super Contributor
timestamp: July 23, 2025, 08:49:43 pm
content: I believe the biggest improvement in the 741 was abolishing the dangerous inversion of gain polarity under extreme conditions.