Feeling sad: I don't understand op-amps

Hey friends,

I have to know op-amps for A level physics. I have some handout and although there is a lot of information on it, I feel like I am missing the fundamental aspects of the op-amp.

Now I checked out www.coursework.info and I saw a really cool article on op-amp, but I have to pay to get full access.

Anyone can help me to understand op-amps.

Someone please point me in the right direction.

Thanks guys.
As always, all input appreciated.

Reply 1

InAbsentia

Hi,

Could really do with some more info about what more specifically you are struggling with. I'll go through the absolute basics.

An Op Amp is essentially a difference amplifier. It looks at the voltage on it's two inputs, and outputs the difference in voltage between the two, multiplied by some factor of gain. The two inputs are assigned as a noninverting input (symbol +) and inverting input (symbol -). The voltage out is given by:

[br]V_{out} = \left(V_{+} - V_{-}\right) \times \alpha[br]

where alpha is the gain factor.

The gain of an ideal operational amplifier is infinite, and in reality extremely large. To reduce the gain of the amplifier, negative feedback can be employed.

Hope this was helpful. Feel free to ask any questions. :smile:

Reply 2

Kyle_S-C

12

The first few paragraphs of the Wiki article seem to cover the basics, though the summary above is pretty much all there is to it.

I've only covered op amps briefly in physics, but it does seem that the whole point of them is negative feedback, whereby the output is fed back into one of the inputs (after being passed through a potential divider or some such). Wiki has more on that (though the usual caveats apply regarding Wiki, it may not be entirely reliable).

Reply 3

batboy

OP

Here is a question:

What is impedance?

"The ideal operational amplifier has the infinite impedance."

Reply 4

teachercol

9

The ac equivalent of resistance. ie the ratio of V/I

An ideal op amp doesnt draw any current

Reply 5

Drummy

12

the actual circuit inside an op amp is VERY complicated but what op amps do is incredibly simple. For that reason we take a "black box" approach, i.e. were not bothered about how it works just what it does.
You would never really use an op amp on its own (except perhaps as an impedance matcher) and there are only about half a dozen other circuits you need to know with the odd resistor and capacitor bunged here and there. i am sure they are in your text book.
Op amps have a very high input resistance (or impedance). This means that they draw very little current from wherever they get their voltage from. You probably know that if you draw current from a supply then its terminal p.d. falls. If this voltage is a "signal" then we do not want the voltage to fall (and we dont particularly want to draw a current). Bung an op amp after the voltage source. the voltage that goes in will be the same as the voltage that comes out but virtually no current will be drawn. This is often called impedance matching.
As always there's a bit more too in than that but thats all you get for free.

Reply 6

teachercol

9

You only need to know

comparator
inverting amp
non-inverting amp
(integrator - some syllabuses)

Reply 7

InAbsentia

Drummy

the actual circuit inside an op amp is VERY complicated but what op amps do is incredibly simple. For that reason we take a "black box" approach, i.e. were not bothered about how it works just what it does.
You would never really use an op amp on its own (except perhaps as an impedance matcher) and there are only about half a dozen other circuits you need to know with the odd resistor and capacitor bunged here and there. i am sure they are in your text book.
Op amps have a very high input resistance (or impedance). This means that they draw very little current from wherever they get their voltage from. You probably know that if you draw current from a supply then its terminal p.d. falls. If this voltage is a "signal" then we do not want the voltage to fall (and we dont particularly want to draw a current). Bung an op amp after the voltage source. the voltage that goes in will be the same as the voltage that comes out but virtually no current will be drawn. This is often called impedance matching.
As always there's a bit more too in than that but thats all you get for free.

Sorry, but this is complete misinformation.

What you describe is definitely not impedance matching, and impedance matching is definitely not achieved by "bunging in an opamp". What I think you're describing is just a simple unity gain amplifier, usually called a "voltage follower", whereby the entire output voltage is fed back round and used as negative feedback. Thus the opamp will produce an error voltage when there is a difference between the two inputs, eventually stabilising when the output is identical to the noninverting input.

Impedance matching is used where you have an output driving an input with different impedance, and want the output to see an input with an identical impedance to its own, but you also want the output to see an input impedance identical to its own. This is generally done to maximise power transfer and hence efficiency, and to reduce reflections from the output. An opamp may feature in some active impedance matching circuits, but the majority are passive.

The circuitry internal to an opamp is also rarely that complex.

Reply 8

InAbsentia

batboy

Here is a question:

What is impedance?

"The ideal operational amplifier has the infinite impedance."

As teachercol said, it is best at this stage to think of impedance as the ratio of V/I. It is the opposition to the flow of current, but it is not the same as resistance. It is the combination of resistance, which affects DC and AC signals, and something called reactance, which is like resistance but only acts on AC signals and at 90 degrees to resistance.

Reply 9

Drummy

12

sorry,
youre quite right, InAbsentia, that I was describing a voltage follower and that impedance matching is usually associated with gaining maximum power by matching the resistance of a source to a load.

It is always difficult to strike a balance between informing those who need to know what they want to know and sounding like a clever git.(which clearly I am not)

Containing at least 20 transistors and a dozen resistors I think the internal circuitry is complex.

Reply 10

Kyle_S-C

12

Try doing engineering...apparently they have to learn the insides of the op amp too...I don't. :biggrin:

Reply 11

batboy

OP

Sorry for not replying sooner guys!

Thanks Teachercol for that response. I don't see the word reactance appearing anywhere on my syllabus.

Yup, I need to know the comparator, inverting amp and non-inverting amp.

What freaks me out is that in the comparator, they say:
"When an op-amp is to be used in a circuit, it is usually connected to split power supplies to enable the output voltage to swing positively or negatively. The common link between the cells is termed the zero volt line. This forms the reference line to which all input and output voltages are measured."

What on earth? How can the line between the cells be zero volts. I know it is usual to take the -ve terminal to be zero (earth).

I am glad that I just have to know what the op amp does, not how it works. But still, my head is still confused.

Thanks again for clearing up that impedance.

Reply 12

Mehh

8

The best piece of advice I'd ever got from learning about op amps is that the input voltages are the same.
If there is a measurable difference in the voltages then the op amp will saturate and will output either +15V or -15V (if they are connected to a 15V dual rail system).
The idea is that since an op amp has a stupidly large gain, the input must be (very very very very nearly) the same for a resonable output to be possible.

So with that in your head look at how a voltage follower works. The +input is connected to an input. The output is connected to the -input.
Since +in = -in¹ AND -in = out² => +in = out

¹Since I said so....
²Since they are connected.

Hence the output follows the +input.

Reply 13

Drummy

12

if you connect the -'ve terminal of a 15V power supply to earth then the potential at the +'ve terminal is +15V.
If you connect the +'ve terminal to earth then the potential at the negative terminal is -15V.
Imagine having two 15V power supplies in series with the point between them earthed. This is a +/- 15V dual power supply.

You cant get a bigger voltage out of a d.c. amplifier than you supply it with. The voltage output limits of the above set up will therefore be +/- 15V. If you have a gain of, lets say 10x and the input is a sinewave with amplitude 5V then serious "clipping" is going to occur. The output will be a square wave with amplitude +/- 15V.

Feeling sad: I don't understand op-amps

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