NPN PNP OP-AMP Water analogies

1. Jan 3, 2010

sudar_dhoni

i have posted the pnp npn and op-amp water analogies.
Could someone explain me pnp and op-amp water analogies.I have understood the npn water analogies but i have many doubts in it which i will ask after i understand pnp and op-amp water analogies.
eventhough the explanation is given in the picture,
i can understand the explanation only for npn but not for pnp and op-amp.
For op-amp i also want an explanation on negative feedback also the water flow in it as the diagram doesnt show the water flow.

File size:
69.3 KB
Views:
476
File size:
61.4 KB
Views:
840
File size:
30.2 KB
Views:
844
2. Jan 3, 2010

skeptic2

PNP:
The PNP diagram is flawed. The text says that as the base-emitter door opens it causes the collector-emitter door to open but the diagram shows that as the base-emitter door opens, the collector-emitter door closes. The text is correct.

Opamp:
The water flow in the diagram is from top to bottom. There is no flow in the input circuit, only the effect of the water pressure. For negative feedback, imagine connecting the output of the opamp to the inverting input through a check valve so that input pressure cannot affect output pressure but output pressure can affect input pressure. The connection to the inverting input is made with a T connection. As the pressure increases in the inverting input, some of the output pressure cancels some of the pressure at the inverting input.

A better analogy to an opamp is a lever. The left side of the lever is the inverting input, the right side is the output and the fulcrum is the non-inverting input. The ratio of the lengths of the two sides of the lever is the ratio of the feedback resistor to the input resistor on the inverting input to the opamp.

Hold the fulcrum constant and move the left side (inverting input) of the lever and the output moves in the opposite direction multiplied by the ratio of the two sides. Hold the left side of the lever constant and move the fulcrum and the output moves in the same direction as the fulcrum amplified by the ratio of the sides.

A common misconception of opamps is that the output voltage is equal to the voltage gain times the difference of the voltages of the non-inverting and the inverting inputs. Vout = Av(Vin+ - Vin-). The actual formula is Vout = Av(Vin+ +(Vin+ - Vin-) or Av(2Vin+ - Vin-). Note that if the non-inverting input voltage is equal to the inverting voltage the output is not zero but equal to the non-inverting voltage similar, to the analogy of the lever.

3. Jan 3, 2010

The Electrician

Are you saying that the "Av(2Vin+ - Vin-)" formula applies to opamps generally, including those idealized to have zero common mode gain? Can you justify this assertion, either by showing a derivation, or with a link to an authoritative source on the web?

It's usual when first considering opamp operation to assume that the opamp is ideal, with zero common mode gain. The reason that "...the output is not zero but equal to the non-inverting voltage..." is that the lever analogy as you have described it has non-zero common mode gain.

4. Jan 3, 2010

Staff: Mentor

Sudar, could you please explain why you are spending time studying flawed/shorsighted analogies, instead of studying the actual physics behind these EE phenomenon? In my opinion (IMO), water analogies of electronics are extremely limited, and of very little teaching value. As you can see, even very experienced EEs on this forum are stuggling while trying to offere you some feedback on water analogies.

What level are you in school? What are you studying? What learning resources do you have at your disposal? What are your educational and academic goals?

5. Jan 4, 2010

sudar_dhoni

Berkeman
I am going to enter into engineering.
Why i am specific about water analogies is that i want to know how transistor amplifies current i.e how the electrons flow inside the transistor and cause amplification when signal is applied.No book is offering what i require and they keep on explaining in terms of conventional current and not explaining what occurs during amplification.
Infact first i was interested only in the physics involved in it and i have already asked in another thread
But i am not getting a proper explanation on it.

For OP-AMP i want to know what is happening inside an OP-AMP. But when i saw the circuit diagram inside it i was sure that nobody is going to explain the circuit inside and op-amp in detail ,so i chose the water analogy.If anybody is ready to explain how amplification occurs inside a transistor and op-amp then i will drop the water analogies

6. Jan 4, 2010

sudar_dhoni

How can the collector emitter door open when the base emitter door opens.
I thought that if you pull the rope on one side of the pulley the rope on your side goes down and on the other side it moves up.
Similarly here if the base emitter door is opened the rope connected to it moves up and the rope on the other side must move down and therefore the collector emitter door should close and not open.But the explanation gives that the door opens.

I cant understand your explanation marked which i have marked in blue.
Could you please mark the negative feedback and the water flow in the diagram .
Also without water flowing in the inverting input how can the pressure act on the inverting input. Hydrostatic pressure arises only due to contact and it is not like voltage or electric field which acts even at a distance.
I think you are giving the answer to my question but i cant grasp it immediatly

If possible could someone explain the entire working of Op-amp 741.
I have provided the circuit inside it.

Attached Files:

• 741_schematic.gif
File size:
8 KB
Views:
292
Last edited: Jan 4, 2010
7. Jan 5, 2010

Staff: Mentor

Yeah, I'd recommend forgetting about the water analogies, and focus on the solid state physics of what's hapenning instead.

I googled BJT amplification tutorial, and got lots of good hits. Try reading through a few of these to see if the amplification via the base current and Vbe starts to make sense:

There are also many LM741 explanations and tutorials:

There are several building blocks that make up an opamp, and reading through those links should help you start to understand each block.

8. Jan 5, 2010

Staff: Mentor

BTW, this classic opamp tutorial from National Semiconductor is a very good intro:

http://www.national.com/an/AN/AN-A.pdf [Broken]

.

Last edited by a moderator: May 4, 2017
9. Jan 6, 2010

sudar_dhoni

there are many websites
any specific one ?
But in none of these they explain whatas happening inside these devices.
THey only show the conventional direction, thats why i learnt the water analogies.
If possible please give me a website which focusses on whats happening inside these devices involving the electron movement.

10. Jan 6, 2010

sudar_dhoni

My humble request.
Even i dont like to study analogies but instead i want a indepth analysis in terms of electron movement of whats happening in all the solid state devices.But no website is explaining upto my expectations but simple keep on emphasising on the conventional current.Could you suggest any material book or website which explains whats happening inside the devices like the one i have given.

Also
If possible could you please give the water analogy for the leakage current which arises due to temperature changes in collector.
Also could you give the water analogy for the flow of minority carriers

Last edited by a moderator: May 4, 2017
11. Jan 6, 2010

The Electrician

The water analogy doesn't do a good job of explaining transistor physics because there is no such thing as separate minority and majority carriers in water. The transistor is simply treated as a valve, which is ok as far as it goes, but it doesn't explain the details, as you've discovered.

You should get a good text book such as one of these:

https://www.amazon.com/s/ref=nb_ss?...&field-keywords=semiconductor+physics&x=0&y=0

Last edited by a moderator: Apr 24, 2017