# Operational amplifier questions

Discussion in 'General Electronics Chat' started by sharanbr, Jul 4, 2015.

1. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
1
Hello All,

I would like to get some clarifications related to operational amplifiers, as listed below,

Ringing effect: in the analysis of an op-amp, it is seen that when an input is step then output will not smoothly follow input but
there can be ringing effect around the max value, before output settles to input value. Of course, this is with appropriate amplification.

Now, if input is varying (say a simple sinusoidal input) then this means that as input keeps varying, output does follow input with appropriate scaling.
However, output also does not follow input smoothly but with distortions. Is this correct?

I have uploaded a sample image (named capture) that has input and output. Please note that, I have kept gain = 1 to keep things simple.

Phase response: the phase of of amplifiers reduces as frequency increases. I don't know if phase lag is an absolute value or a function of frequency itself.
But if input contains multiple frequency components (which is the case with natural signals), as they pass through the amplifier, each frequency component undergoes varying phase lag. So, each frequency component does get amplified but is output with varying delays.
Hence effectively the shape of the signal would change if one were to assume that the effect of the amplifier is same as adding each of these frequency components after amplification

Integrator: conceptually, is an integrator used in amplifier a sort of accumulator, where output is not proportional to current input but
is current value + delta?
Delta is proportional to input.

Time constant: what really is a time constant of a system?

Order of the system: what is an order of a system and how it can be computed?

Infinite input/output resistance: in description of amplifiers, reference is made to input and output resistances being infinite.
I am not able to appreciate what this means. Can I get some simple explanation with an example?

Amplifier design: in certain amplifier description, the design itself contains another amplifier. I believe this is done for integrator or voltage buffer (I am not too sure). So, how does it work? An amplifier needs another amplifier?

Amplification: this is more of a conceptual (and probably philosophical) question. It one were to amplify an input, one cannot do this using just an input alone. For example, if an input current needs to be amplified then one would need additional current source to do this and input alone is not sufficient.

A simplistic amplifier shows integrator as the key block that amplifier input value. But since this is just a passive element, where does additional current/voltage that is present in the output come from?

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2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,921
600
op amp uses external power supply

some op amps use one external power supply
other op amp use two external power supplies

3. ### #12 Expert

Nov 30, 2010
17,829
9,160
Your questions would require more than one chapter of a book about operational amplifiers, or a couple of weeks in school. Most people here don't have time to write a book for you. If you could reduce your requirements to one question at a time, you might get better answers.

4. ### ramancini8 Active Member

Jul 18, 2012
461
129
Read "Op Amps for Everyone"; free on TI web.

5. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
1
Actually, I am going through a 40+ set of video lectures and these questions are coming from there.

I am mainly looking for hints and directions. Rest I can try to figure out by going through Analog IC text books which I use for cross reference.

Apr 5, 2008
16,965
2,958
7. ### atferrari AAC Fanatic!

Jan 6, 2004
2,828
927
Hola Sharan

Reread post #4 immediately above yours last. He not only recommends it; he wrote it! Reading that paper you will have much less questions to post again and with more specific doubts.

absf and GopherT like this.
8. ### GopherT AAC Fanatic!

Nov 23, 2012
7,111
5,766
I read that document over and over when I started in electronics. Thank you.

Thanks for pointing this out.

Apr 13, 2009
76
1

10. ### MrAl Distinguished Member

Jun 17, 2014
3,605
754
Hi,

These were rather easy questions but since there were so many i did not quote your individual lines just copied them, and my responses are shown enclosed in brackets like: [This is my response].

[START]

Hello All,
[Hello there]

I would like to get some clarifications related to operational amplifiers, as listed below,

Ringing effect: in the analysis of an op-amp, it is seen that when an input is step then output will not smoothly follow input but
there can be ringing effect around the max value, before output settles to input value. Of course, this is with appropriate amplification.

Now, if input is varying (say a simple sinusoidal input) then this means that as input keeps varying, output does follow input with appropriate scaling.
However, output also does not follow input smoothly but with distortions. Is this correct?
[There is some distortion in every amplifier, however you can get special low distortion op amps too]

I have uploaded a sample image (named capture) that has input and output. Please note that, I have kept gain = 1 to keep things simple.

Phase response: the phase of of amplifiers reduces as frequency increases. I don't know if phase lag is an absolute value or a function of frequency itself.
[Usually a function of frequency]

But if input contains multiple frequency components (which is the case with natural signals), as they pass through the amplifier, each frequency component undergoes varying phase lag. So, each frequency component does get amplified but is output with varying delays.
Hence effectively the shape of the signal would change if one were to assume that the effect of the amplifier is same as adding each of these frequency components after amplification
[True, but there are conditions when it is worse than with better conditions]

Integrator: conceptually, is an integrator used in amplifier a sort of accumulator, where output is not proportional to current input but
is current value + delta?
Delta is proportional to input.
[Yes, more or less. An integrator integrates the input signal over time in the continuous sense, and this can be viewed discretely by assuming steps of delta width, and the integrator sums them]

Time constant: what really is a time constant of a system?
[1 divided by the absolute value of the exponential exponent factor of t in the time response expression. There may be more than one. Example: for e^(-t/(R*C)) the time constant is R*C. Also defined as the time it takes for the response to reach to within about 36.8 percent of the final value]

Order of the system: what is an order of a system and how it can be computed?
[The order of a system is the order of the differential equation required to completely describe the system]

Infinite input/output resistance: in description of amplifiers, reference is made to input and output resistances being infinite.
I am not able to appreciate what this means. Can I get some simple explanation with an example?
[First, the input is estimated as infinite while the output is estimated as zero. You can appreciate the high input impedance by the fact that it does not load your previous stage output, and you can appreciate the low output impedance by the fact that the next stage does not load the output of your amplifer]

Amplifier design: in certain amplifier description, the design itself contains another amplifier. I believe this is done for integrator or voltage buffer (I am not too sure). So, how does it work? An amplifier needs another amplifier?
[This question is application specific. Different applications require more than one op amp for very different reasons so you would need to show a circuit for discussion]

Amplification: this is more of a conceptual (and probably philosophical) question. It one were to amplify an input, one cannot do this using just an input alone. For example, if an input current needs to be amplified then one would need additional current source to do this and input alone is not sufficient.
[This is actually a good question, which is seldom addressed. For most applications you need a source of power to run the amplifier. There are special cases when you only have to amplify voltage or current; for example, a transformer]

A simplistic amplifier shows integrator as the key block that amplifier input value. But since this is just a passive element, where does additional current/voltage that is present in the output come from?
[Almost every op amp amplifier has a power supply to power the op amp so it can work right. The extra power comes from that]

[ ]

Last edited: Jul 6, 2015
11. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
1
so, not only there is distortion, there is a change in shape of signal as different frequency components undergoing varying phase delays.

thanks.

Thanks. I have follow up question but first let me lookup some literature and come back here if needed.

thanks.

able to appreciate infinite input resistance but not zero output resistance.
Also, how is such a circuit designed. That is, infinite input resistance.

but where does additional current or voltage come from.?
It cannot be input alone. Right?

12. ### #12 Expert

Nov 30, 2010
17,829
9,160
And, that was the exception to my advice.

13. ### MrAl Distinguished Member

Jun 17, 2014
3,605
754
Hello,

Zero output impedance just means that the output appears to look like a voltage source. Infinite input impedance with zero output impedance is the ideal case, and real amplifiers always have less than infinite input impedance and slightly more than zero output impedance. The input spec is usually very high though, and the output spec is very low.

Some circuits are true power converters that change the basic nature of the power source without actually amplifying the power. They just amplify the current OR the voltage alone, but not both at the same time. When they only do one or the other, that is not power amplification it is just voltage or current amplification, so it is possible that a secondary power source is not needed.
For a transformer, the only power it receives is from the input source. That's because it CONVERTS the power, not amplifies it, although it may amplify the voltage. For example, 10 volts in and 100 volts out. We gain voltage but loose current, and for an ideal transformer the power in is the same as the power out.

14. ### Veracohr Well-Known Member

Jan 3, 2011
600
86
Technically yes, but the distortion in typical opamps, not even low-distortion opamps, is usually low enough that you won't notice.

Are you familiar with voltage dividers? A voltage-divider-like scenario develops between the output of an opamp and whatever it is connected to. If the input resistor of a divider is 0, there's no loss of voltage level. In practice opamps have some output resistance (sometimes intentionally added, sometimes not) and limited current capability. Most datasheets will specify a maximum output level based on the load. If the load is heavier (lower impedance), the maximum output of the opamp is lower.

No circuit is designed with infinite input resistance, but they can be designed with very high input resistance that can be approximated as infinite. With bipolar inputs this is more difficult and may involve darlington input stages (ex. LM358) and/or other techniques, but another option is to use FET input stages (ex. TL071). The input current of FETs is vanishingly low.