Grounding Op-Amp circuit exercise

[Weaver]

Homework Statement

2. Homework Equations
3. The Attempt at a Solution
For the first set of questions:

I've worked through to part 6), at which is I encountered my first problem. I'm not entirely sure what the question is asking. Is it as if there would be a capacitor between Q1 and X1? Is the voltage to be reduced the one calculated in part 5)?

Moving on from that

I am also unsure about part 9) and 10). We haven't really covered MOSFETs in lectures so these two questions are a bit lost on me. Do I use the frequency, current and resistance to get the average value?

For the second answer sheet for I haven't worked out 5), 7), 9), 12) However I am working on this right now. I will probably have questions later on this

I don't want the answers, but rather help on what methods I should be doing or perhaps pointers to resources that would help

Also, I'm not even certain what I have done is correct

I appreciate any help

Thanks in advance,

 

[anorlunda]

Those images under attempt at a solution are too poor quality to read. You probably won't get any answers until you post something more legible.

 

[Weaver]

Those images under attempt at a solution are too poor quality to read. You probably won't get any answers until you post something more legible.

Thanks for that. I'm at uni, so I don't have access to a scanner. I will update them later tonight

 

[Weaver]

Thanks for that. I'm at uni, so I don't have access to a scanner. I will update them later tonight
]

I've decided to make smaller threads asking about the problems i have been having more specifically with typed solutions (my scanner broke) so that it is easier to follow. I would delete this thread, but I don't know how

https://www.physicsforums.com/threads/calculate-current-following-in-op-amp-to-ground.959831/

https://www.physicsforums.com/threads/decoupling-capacitor-for-pcb-track.959829/

https://www.physicsforums.com/threads/ground-current-for-voltage-regulator.959832/

 

[CWatters]

It was a mistake to have several threads running on the same problem.

Regarding the capacitor on X1 Q1... they are talking about a decoupling capacitor you need to add. It would be on the 24V rail. See my reply in your other thread where I mention ripple currents.

 

[Weaver]

It was a mistake to have several threads running on the same problem.

Regarding the capacitor on X1 Q1... they are talking about a decoupling capacitor you need to add. It would be on the 24V rail. See my reply in your other thread where I mention ripple currents.

Apologies for that. I thought it might help break up the question. I won't do it again. Thanks for replying in the other forum

 

[CWatters]

I haven't looked at your working for 1-6 properly but..

For 9. It asks you to calculate the voltage drop down a typical PCB trace due to the 10mA FET gate current. Use the frequency you calculated in 8.

EDIT: In case it's not obvious this current flows in and out of the MBED power/ground pins on it's way to/from the FET gate.

For 10: Similar to Q6

 

[Weaver]

I haven't looked at your working for 1-6 properly but..

For 9. It asks you to calculate the voltage drop down a typical PCB trace due to the 10mA FET gate current. Use the frequency you calculated in 8.

EDIT: In case it's not obvious this current flows in and out of the MBED power/ground pins on it's way to/from the FET gate.

For 10: Similar to Q6

Thank you very much! I get and I think I have those out figured. When it asks average AC, does that mean RMS?
I’ve followed through with the rest of the questions. The last one I have a problem with the op amp ground current

Should there be a value in a data sheet for this? Or is it calculated?

If it’s calculated, at worse case, the op amp will have 0V at it’s In(+) so no voltage appears at the output. So at this point it is just the supply current into the op amp?

 

[CWatters]

Thank you very much! I get and I think I have those out figured. When it asks average AC, does that mean RMS?

That's what I would answer. Not sure why they ask for that as it's the peak to peak that more important.

Edit: Earlier they suggest you model the output as a 10mA current source feeding the gate capacitance of the FET. So perhaps think carefully about the waveform and how you do the rms calculation.

 

[CWatters]

The last one I have a problem with the op amp ground current

Should there be a value in a data sheet for this? Or is it calculated?

If it’s calculated, at worse case, the op amp will have 0V at it’s In(+) so no voltage appears at the output. So at this point it is just the supply current into the op amp?

I agree.

 

[Weaver]

That's what I would answer. Not sure why they ask for that as it's the peak to peak that more important.

Edit: Earlier they suggest you model the output as a 10mA current source feeding the gate capacitance of the FET. So perhaps think carefully about the waveform and how you do the rms calculation.

Right, so the waveform is generated from the resistance and inductance of the the PCB with the ripple current produced from the MOSEFT. In this case is the Vripple close to a square
waveform as it is the MOSFET switching?

(By the way, I really appreciate all the help you've given)

 

[CWatters]

No isn't this Q9 where they ask about the MPEG average AC current? Eg due to the FET gate current not the source/drain current.

What I was hinting at is that a constant current (10mA) driving a capacitor (FET gate) produces a triangle wave

 

[CWatters]

Actually forget that. The gate voltage will be a triangle wave but the ripple on the supply voltage will be a square wave as it's the constant 10mA * PCB trace impedance.

 

[Weaver]

Actually forget that. The gate voltage will be a triangle wave but the ripple on the supply voltage will be a square wave as it's the constant 10mA * PCB trace impedance.

Okay, so as it's a square wave, the Vrms is just the peak voltage in this case?

 

[CWatters]

Yes that would be my answer for that question.

 

[Weaver]

Yes that would be my answer for that question.

Great! Thank you for all the help you've given. Definitely helped improve my understanding! Really appreciate it