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Op Amps

Rob_K

Sep 20, 2013
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Hi Guys,

Could someone please explain to me what you would use an op amp for, like real world application? I have been studying them for 3 years, and can give you the circuit diagrams of inverting and non inverting op amps, I can calculate the voltages and gains of them, but no one seems particularly happy to tell you what they are used for. and now it has come to the point that I get asked that in a job interview, and I don't know the answer.

Please help me on this one.
 

BobK

Jan 5, 2010
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Off the top of my head:

Oscillators (sawtooth, square wave, sine wave)
Audio preamp
Active filters
Tone controls
Scaling and offsetting a voltage
Constant current sources
Voltage regulators
Voltage to current / current to voltage converters
Schmitt trigger
Ideal rectifier
Ramp generator
Driver stage for push-pull output of audio power amp

Basically, almost any analog circuit can use op amps.

Bob
 

Rob_K

Sep 20, 2013
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At last, a great understandable answer! That's perfect, I finally have something to work with now, thank you Bob.
 

Rob_K

Sep 20, 2013
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Ok, I have a curious question now, as I am really not up on signal processing, but what would be the point of having the output of an op amp going into a Digital to Analogue Converter?

The output of an op amp is, I assume essentially analogue. The only thing I can think of is that from what I have learned about Op amps is that they can produce a square wave of sorts, could this then be used as a source for binary to then be processed for a clean signal? is it used as a buffer of sorts? and yes I did mean DAC not ADC.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Ok, I have a curious question now, as I am really not up on signal processing, but what would be the point of having the output of an op amp going into a Digital to Analogue Converter?

Can you show us an example?
 

Rob_K

Sep 20, 2013
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I can, but I need to let you know that it was for a question to complete in order to get a job interview which I am turning down based on the fact that my electronics needs a bit more study.

Disclaimer out of the way, this for me is a good place to know what level I should be as a graduate. My degree was robotics by the way, not electronics. I was asked to describe the function of this circuit, so I will give you as far as I could get.

All these op amps are the same and all configured as non inverting op amps, the two directly in parallel appear to have a gain of 2, the next one in the series on the bottom line I think has a gain of 250ish? and the last one a gain of 4. They all feed into a Multiplexer and the three op amps with the lowest gains all have outputs that branch into DAC, the high gain one just goes to earth.

At a guess this circuit is some kind of filter maybe, I would like to know.
 

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(*steve*)

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I'm guessing, but is this an ADC using the DAC as part of the feedback.

It's not a excessive approximation. Maybe someone else can help.
 

KrisBlueNZ

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The device at the right side of the diagram is not a DAC. It's a DG4051A, Vishay Siliconix's enhanced version of the CD4051 eight-to-one analogue multiplexer IC. Its output, on pin 3, is not shown. There is no DAC in that diagram. And without seeing the rest of the design, it's impossible to guess at what it does!

DACs do have analogue inputs though. A multiplying DAC has a signal input, and a voltage-output DAC has a reference voltage input.

There is a huge amount of helpful, explanatory information available on op-amps. Google op-amp applications or look for application notes on the web sites of the big manufacturers - ti.com, onsemi.com, fairchildsemi.com, st.com etc.
 

Rob_K

Sep 20, 2013
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Yes, you are right, this whole circuit goes into a multiplexer, what I was referring to are the outputs from the three lowest gain Op amps U13D, U17A and U17C which each have a branch to something that says DAC3 and DAC4. What would these refer to in that case?

Also I am curious as to why you would use U17B and U17C like that, they have a gain or 250 and 4 respectively, could you not just do that with one op amp?
 

BobK

Jan 5, 2010
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Also I am curious as to why you would use U17B and U17C like that, they have a gain or 250 and 4 respectively, could you not just do that with one op amp?
Not if you exceeded the GBP.

Bob
 

KrisBlueNZ

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The "DAC3" connections into U13D and U17A feed into their inputs, on pins 13 and 2 respectively. Likewise, the DAC4 connection into U17C feeds into its input on pin 9.

The way these circuits are arranged, my guess would be that the DAC outputs are being used to provide an adjustable DC offset on the signal.

U17B is a non-inverting amplifier with no DC offset; U17C provides the adjustable DC offset with a gain of 4. If the designer had applied the DC offset to the first stage, he/she would have had to attenuate it, to scale the DAC output value to the signal range. That would be possible but it's simpler to do it that way.

Also as Bob says, the gain-bandwidth product could be the reason, but I doubt it; if that was the reason, the designer would have been more likely to use two stages with equal gain (sqrt(1000) x sqrt(1000)).
 

Rob_K

Sep 20, 2013
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Thank you very much guys, this is all very helpful and gives me something to work with in more ways than one. I may have some more questions at some point.

Kind regards

Rob
 

Arouse1973

Adam
Dec 18, 2013
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Also as Bob says, the gain-bandwidth product could be the reason, but I doubt it; if that was the reason, the designer would have been more likely to use two stages with equal gain (sqrt(1000) x sqrt(1000)).

I agree Kris, not studied the circuit much but your right with it all I think. The last output stage and I am assuming this is a dc circuit which is why GBP wouldn't be an issue, is probably done this way to reduce output impedance Rout = Ro*Acl/Ao = Ro*1+(Rf/R)/Ao, increase phase margin for driving capacitive loads, reduce gain error and limit overshoot. That is why the second stage has a lower gain I think.
Thanks
Adam
 
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