Active elements

[beenuseren]

Hi,

I have a doubt.

This is with respect to transistor.

In CE configuration of a BJT, how does the increase in base current, reduces the width of depletion region in the CB junction? can anyone explain this theoritically?

And also why are the polarities of the EB junction voltage drop and CB junction voltage are opposite in polarity?

thanks,

 

[beenuseren]

Here is my another doubt,

I am very confused about Opamp.

This is what I know about an opamp.

An Opamp has 5 pins. Two of them are the inputs and another two of them are connected to the power supply, and the remaining pin is the output.

1) When the voltage at the positive input is higher when compared with the input at the negative input then the output goes positive and vice versa. Is it true?

2) How is that the feedback from the input to the output is responsible for the amplified output?

Please explain me this,

thanks,

 

[akg]

An Opamp has 5 pins. Two of them are the inputs and another two of them are connected to the power supply, and the remaining pin is the output.

it may can have more pins

1) When the voltage at the positive input is higher when compared with the input at the negative input then the output goes positive and vice versa. Is it true?

2) How is that the feedback from the input to the output is responsible for the amplified output?

first u should understand that an opamp has infinite gain (in theory) and it will amplify the signal in the inverting and non-inverting i/ps .if u know what is non inverting and inverting i/ps , an opamp o/p will zero if an equal voltage is applie to both pins. if there is a slight difference in the i/ps, opamp will amplify the difference infinite times so that the o/p will go to +Vcc or -Vcc .
pls refer to the analysis of opamp feedbacks.

 

[dboyer]

Here is my another doubt,

I am very confused about Opamp.

This is what I know about an opamp.

An Opamp has 5 pins. Two of them are the inputs and another two of them are connected to the power supply, and the remaining pin is the output.

1) When the voltage at the positive input is higher when compared with the input at the negative input then the output goes positive and vice versa. Is it true?

2) How is that the feedback from the input to the output is responsible for the amplified output?

Please explain me this,

thanks,

Much of what akg said was correct. An op amp is a difference amp with massive amounts of gain. so the output ends up being:
G * (V+ - V-)
With G being extremely large (lets say 100,000 V/V isn't that surprising) it is hard to use an opamp without some sort of negative feedback. Lets say that there is 1.000 V on the V- input, and 1.001 V on the V+ input. 1 mV * 100,000 = the positive rail. The opamp will be completely saturated, and not very useful as an amplifier. That is why circuits such as an inverting or a noninverting configuration use negative feedback to control the gain, and simply ground one of the inputs. They are easy to build, perform well, and are fairly easy to modify to fit special needs.

see this page:
**broken link removed**
it has lots of pages on all sorts of stuff, including opamps and transistors. It makes a decent introduction.

 

[beenuseren]

Also, why in the first place the OPamp gives infinite gain without the negative feedback?

thanks,

 

[dboyer]

Also, why in the first place the OPamp gives infinite gain without the negative feedback?

thanks,

Opamps have 'infinite' gain because they are designed that way. Many of the design tricks with opamps require this property.

 

[Nigel Goodwin]

That's a bit misleading! - opamps DON'T have infinite gain, but for the purposes of calculations it's high enough to consider it infinite.

The reason that this is important is that it lets you completely ignore the gain of the opamp in your calculations making nice simple equations you can use. For the same reason, you consider them to have zero output impedance, and infinite input impedance, but again this isn't true - but near enough to ignore and make things easy!.

 

[dboyer]

That's a bit misleading! - opamps DON'T have infinite gain, but for the purposes of calculations it's high enough to consider it infinite.

The reason that this is important is that it lets you completely ignore the gain of the opamp in your calculations making nice simple equations you can use. For the same reason, you consider them to have zero output impedance, and infinite input impedance, but again this isn't true - but near enough to ignore and make things easy!.

I don't think anyone has answered the question and said it has infinite gain. akg says "in theory" and I gave a practical and reasonable but completely made up gain in my first post.

In my experience, opamps are first presented as ideal difference amplifiers. Infinite gain and input impedence, zero output impedance, and equal voltage on both input pins. A few semesters later, we did a unit where poor choices of biasing resistors were chosen. It is amazing the kind of nonideal behavior you can coax out of them.

I think the OP is asking the wrong questions. Instead of why they have 'infinite' gain, (s)he should be wondering why a person WOULDN'T want 'infinite' gain. With an opamp that has a huge gain like that, you can build an amplifier around it with any arbiturary gain you could desire using just external cheap resistors (and a few basic equations).