Common Collector Circuit

[kmdineshece]

how common collector has high input impedance than others?

is this circuit correct?...

Base-emitter--->Reverse
Collector-Base--->Forward Bias

 

[Audioguru]

Your circuits are upside down. Usually the positive voltage is at the top and the negative voltage is at the bottom.
You show VEE at the wrong terminal of the battery. VEE is the negative for a Cmos circuit. The long bar on the symbol of a battery is its positive terminal.

Your transistors are missing very important resistors so your transistors will instantly burn out.

The base-emitter of a transistor is normally forward-biased and the collector-base is reverse-biased.

 

[chuckey]

For a common collector amplifier (emitter follower), the DC bias is exactly the same as a common emitter amplifier, its just the signal in/out is different.
Frank

 

[goldsmith]

how common collector has high input impedance than others?

is this circuit correct?...

Base-emitter--->Reverse
Collector-Base--->Forward Bias

Hi kmdineshece

A Cc amplifier has a large value of input impedance and it is dues to the AC equivalent circuit of a transistor in CC mode . are you familiar with that ? are you familiar with concept of beta and hfe ? and are you familiar with the reason behind the word transistor ? why we call it transistor ? ( transformer resistor ) ?

Best Wishes
Goldsmith

 

[LvW]

For a common collector amplifier (emitter follower), the DC bias is exactly the same as a common emitter amplifier, its just the signal in/out is different.
Frank

For clarification, I think it helps to state that the DC bias PRINCIPLE is exactly the same as for a common emitter stage.
For further clarification: The overall input resistance is identical to the parallele combination of the voltage divider resistors (DC bias, R1||R2) and the dynamic input resistance rin at the base node.
In common collector configuration rin is rather large due to heavy negative feedback resulting in a gain very close to unity.

 

[kmdineshece]

Yes Sir!....Thank you for your reply!........
ya I know little bit transistor operation, Hfe and all!........
High Input Impedance---Drives low Current,
But , How Common Collector Having this?

I think based on doping, if Base-Collector as Input (Base & Collector -Lightly doping region), then it restricts flow of current heavily!......so CC is called as high input impedance circuit!.....

Am I Correct Sir?

 

[chuckey]

The equivalent circuit is:- The base terminal is connected to the internal junction via a 3K resistor. From this junction there is a current generator of Hfe X Ib connected out to the emitter junction via a 20 ohm resistor. The result of a CC connection is that something like .98 Vin ( Vbe + Ve) appears at the emitter, so for Vin the current in the 3K is 1 - .98 or about 50 times less then would be expected.
Frank

 

[LvW]

Am I Correct Sir?

No. As mentioned already in post#5 - the most important reason for such a large input resistance of a CC combination is NEGATIVE FEEDBACK caused by the emitter resistor.

 

[colin55]

The COMMON COLLECTOR stage is also called the EMITTER FOLLOWER stage.
The input is the base and the output is the emitter. The collector is connected to the supply rail.
This stage is classified as having a HIGH INPUT IMPEDANCE because the transistor allows you to deliver a high current to the load by supplying a very small current via the base.
In other words the transistor amplifies your effort by about 100 times.
This is due to the gain of the transistor.
When you are doing this, the load appears to be a much-higher value of resistance as the transistor multiplies the resistance of the load by a factor of about 100 times.
That's why the circuit is classified as having a HIGH INPUT IMPEDANCE.

 

[LvW]

........When you are doing this, the load appears to be a much-higher value of resistance as the transistor multiplies the resistance of the load by a factor of about 100 times.
That's why the circuit is classified as having a HIGH INPUT IMPEDANCE.

Colin55, everything OK - however, WHY is the transistor doing this amazing multiplication by a factor of 100?
You shouldn´t keep secret the reason: Negative feedback.
It is a well-established knowledge that negative current-controlled voltage feedback reduces the gain and increases the input resistance - all with the same factor: (1+ magnitude of loop gain.)

 

[colin55]

Colin55, everything OK - however, WHY is the transistor doing this amazing multiplication by a factor of 100?
You shouldn´t keep secret the reason: Negative feedback.
It is a well-established knowledge that negative current-controlled voltage feedback reduces the gain and increases the input resistance - all with the same factor: (1+ magnitude of loop gain.)

There is no such thing as NEGATIVE FEEDBACK in this case and you should not be using the term.
Negative feedback is when the output signal is re-directed back to the input to reduce the gain and reduce the distortion.
In this case the transistor is acting like a fork-lift truck. You put a small amount of effort on the lever and the forks lift the heavy load.
In this case the small amount of current you deliver into the base is multiplied by the transistor and a current about about 100 times is passed through the collector-emitter leads.
This current produces a voltage-drop across the emitter resistor and this is what raises the transistor.

 

[goldsmith]

here is no such thing as NEGATIVE FEEDBACK in this case and you should not be using the term.

Hi colin55
Sorry but i'm completely disagree with your statement .

Negative feedback is when the output signal is re-directed back to the input to reduce the gain and reduce the distortion.

Of course we all know what negative feedback is . but you must understand real concept of the transistor . have you ever though why we call it transistor ? because it transfer resistance . how ? one of the most usual ones is , from emitter to the base or vice versa . so it means something is reflecting from the out put into the input . so what you call this ? isn't this a feedback ??!!! of course it is !
So you must use the word feedback too !


Best Luck
Goldsmith

 

[colin55]

Hi colin55
Sorry but i'm completely disagree with your statement .


Of course we all know what negative feedback is . but you must understand real concept of the transistor . have you ever though why we call it transistor ? because it transfer resistance . how ? one of the most usual ones is , from emitter to the base or vice versa . so it means something is reflecting from the out put into the input . so what you call this ? isn't this a feedback ??!!! of course it is !
So you must use the word feedback too !


Best Luck
Goldsmith

You are completely wrong in your reasoning.
I have been teaching electronics for 40 years and written 25 books with sales of over 750,000 copies. My website has reached 22,000,000 visitors.
No-one has yet said I have been wrong.
To start with, negative feedback REDUCES the overall effect.
Show me one book where it states the emitter resistor provides NEGATIVE FEEDBACK.
You are getting confused with the common emitter stage where the emitter resistor provides EMITTER DEGENERATION.

 

[FvM]

I believe there are different ways to analyze a common collector respectively emitter follower circuit, depending on how the transistor is modelled in the equivalent circuit. I don't remember a specific text book, but describing the circuit behaviour as amplifier with internal (negative) feedback sounds quite familiar to me. Isn't it an intuitive explanation for the stable voltage gain and high linearity of this circuit?

I have no problems if you prefer a different approach to explain the operation of the circuit. But no reason to fight other explanations.

 

[Audioguru]

An emitter-follower is a piece of wire that has current gain.

 

[LvW]

You are completely wrong in your reasoning.
I have been teaching electronics for 40 years and written 25 books with sales of over 750,000 copies. My website has reached 22,000,000 visitors.
No-one has yet said I have been wrong.
To start with, negative feedback REDUCES the overall effect.
Show me one book where it states the emitter resistor provides NEGATIVE FEEDBACK.
You are getting confused with the common emitter stage where the emitter resistor provides EMITTER DEGENERATION.

I really cannot believe that somebody who has written 25 books (on electronics?) does not know about negative feedback in a transistor circuit.
If "no-one has yet said" you "have been wrong" - well, evreything happens the first time. And in this specific case - you are wrong. Sorry.
Colin55 - may ask you one simple question:
What is the difference between an emitter resistor Re - regarding the signal input resistance - in a common-emiiter configuration if compared with the task and the effect of such a resistor in a common collector configuration?

In both cases - you can create a feedback loop showing that the loop gain is (-gm*Re)
Hence, the closed-loop gain is Acc=Gm*Re/(1+gm*Re).
Perhaps you remember that the input of the transistor 4-pole is the voltage Vbe=Vb-Ve and that the output current produces a voltage across Re which is fed back to the inverting input (emitter node).
In this case, the transistor is considered, of course, as a voltage controlled current source (physical reality).

As another argument: It is well known that wideband common-collector circuits may tend to dynamic instability.
Colin - I suppose, you certainly know that such stability problems arise because of feedback.

 

[goldsmith]

You are completely wrong in your reasoning.
I have been teaching electronics for 40 years and written 25 books with sales of over 750,000 copies. My website has reached 22,000,000 visitors.

Here in Iran there is an old saying which says when a tree has more fruits , it's head is nearby the ground ! you know what does it mean ? it mean when somebody has a lot of knowledge and a lot of things he/she won't try to say it to a lot of people . and i guarantee that , here on EDA there a lot of pretty famous and absolutely more experienced persons in compare with you . ( of course i'm not one of them ) .so don't suppose that you are just one of the kind , ok ? . don't try to fight the idea of negative feedback in this case because you have not saw this before . okay ?
You simply say me and the other people are wrong but have you ever tried to think about what we are saying ?

Show me one book where it states the emitter resistor provides NEGATIVE FEEDBACK.
You are getting confused with the common emitter stage where the emitter resistor provides EMITTER DEGENERATION.


Did you find this post helpful? Click: Yes

I will try to show you some famous books about this issue . let me have a bit search on my text books , i'll let you know . but nevertheless , you think about this issue that there are many of the concepts which are new and are presented as new articles , can you say they are wrong ? of course not . i mean if you saw something new ( which in this case i believe it is not new , you just didn't face it but we all did ) it doesn't mean that it is not true . okay ?



Hope for your understanding


Best Luck
Goldsmith

 

[bigdogguru]

You are completely wrong in your reasoning.
...
...
Show me one book where it states the emitter resistor provides NEGATIVE FEEDBACK.
You are getting confused with the common emitter stage where the emitter resistor provides EMITTER DEGENERATION.

Reference:Transistor Circuit Techniques: Discrete and integrated, Chapter 4: Single-Stage BJT Amplifiers with Feedback, Page: 59-60

The emitter follower

The emitter follower or common-collector amplifier (Fig.4.4) is best considered as a series feedback amplifier in which the output is taken from the emitter rather than from the collector.

In simple terms, the output voltage is separated from the input voltage by the almost constant voltage drop across the forward biased base-emitter junction. As the input voltage varies, so does the forward biased base-emitter junction. As the input voltage varies, so does the output in almost exact sympathy except for the 0.7 V d.c. offset. The emitter is said to follow the base, hence the term emitter follower and the circuit is expected to have a voltage gain of unity (+1 since this no signal inversion between the base and emitter).

Analysis of the a.c. equivalent circuit is necessary to yield accurate expressions for voltage gain, input resistance and output resistance.

The procedure for determining the input resistance in the transistor r_in(t) is identical to that for the series feedback amplifier and yields the same result, i.e.

....
....

The text continues to derive the A_v or A_cc closed feedback loop gain equation previously indicated by LvW:

Acc=Gm*Re/(1+gm*Re)

I have several other texts in my library with a similar treatise of the common collector or emitter follower configurations which utilize the concept of feedback in their derivations.

BigDog

 

[colin55]

Reference:Transistor Circuit Techniques: Discrete and integrated, Chapter 4: Single-Stage BJT Amplifiers with Feedback, Page: 59-60


The text continues to derive the A_v or A_cc equation previously indicated by LvW:



I have several other texts in my library with a similar treatise of the common collector or emitter follower configurations which utilize the concept of feedback in their derivations.

BigDog

There is a big difference with the concept of FEEDBACK and NEGATIVE FEEDBACK. We are discussing NEGATIVE FEEDBACK

 

[godfreyl]

Well, if you concede that feedback is involved, but still insist that there's no negative feedback, then what are you proposing? That it's positive feedback?:shock: