Can anybody explain Emitter-Follower easily?

Thread Starter

livelongpranav

Joined Oct 18, 2017
17
I know basic electronics but am totally unable to understand the Emitter-Follower configuration of transistor. Can anybody explain it in easy to understand terms? Consider the following scenario:

-I give a NPN transistor a base voltage of +5V. Voltage at collector is +12V and Emitter is connected to common ground. There is NO LOAD. (I know that if there is load on the collector side the transistor will simply act as a switch.) But if I'm not wrong, when there is no load, the voltage between Emitter and Ground is 4.2V (5V - Voltage Drop), and not 12V (or 11.2V accounting for drop)! Why is this happening? When the transistor is saturated, shouldn't current flow from Collector to Emitter and voltage should be 12/11.2 V?

-Also, the primary application of this config is to change high input impedance to low output impedance. However I am unable to understand impedance at all!

Any help will be highly appreciated. Sorry if the questions are too silly.
 

Ramussons

Joined May 3, 2013
797
-I give a NPN transistor a base voltage of +5V. Voltage at collector is +12V and Emitter is connected to common ground
These is not possible. The Emitter Voltage WILL be .5 to .7 volts less than the Base voltage.
The statements made are not possible.
How do you have Collector at 12 volts when there is no load or Emitter resistor? You will have a burnt out Transistor.

Please go through https://www.tutorialspoint.com/amplifiers/amplifiers_emitter_follower_and_darlington.htm.

Please ask a question with a schematic so all of us will be clear about your question.
 

kubeek

Joined Sep 20, 2005
5,603
Show your circuit. What do you mean by no load? If you have 5V at base, 12V at collector and 0V at emitter the transistor will self destruct very quickly.
 

LvW

Joined Jun 13, 2013
851
The "emitter follower" circuit s identical to the common-collector configuration.
Then, where is the resistor which is required for transferring collector current fluctuations into a voltage?
Answer: Between the emitter node and ground!
 

BobTPH

Joined Jun 5, 2013
2,028
What you have described is not an emitter follower. Perhaps that is why you cannot understand it. As others have said, the circuit you describe is a smoke machine.

Bob
 

MrChips

Joined Oct 2, 2009
19,280
Here is your basic NPN transistor circuit.
This is for illustrative purposes only. Ignore the current and voltage values for the moment.

upload_2019-10-18_9-24-54.png

Resistors R1 and R2 apply the base bias current and voltage for proper circuit operation.
Rc is the collector load.
Re is the emitter load.

The current and voltage readings given describe the DC operating point, or Q-point of the circuit.
The behaviour of the circuit in response to a changing input signal is called the AC response.

Insert a suitably sized capacitor (C2) in parallel with Re and you have a common emitter amplifier.


Adjust Rc and Re to attain the desired AC gain. Note that the output signal is 180° out of phase from the input signal.

Now, instead of a capacitor in parallel with Re, put the capacitor in parallel with Rc. You now have a common collector amplifier (aka emitter follower).
Again adjust Rc and Re to suit your requirements. You may even reduce Rc to zero. You still need a suitable value for Re otherwise you will destroy the transistor.

upload_2019-10-18_9-37-9.png

A common collector amplifier provides current gain. The voltage gain is less than unity. There is no phase inversion.
 

MrAl

Joined Jun 17, 2014
6,504
I know basic electronics but am totally unable to understand the Emitter-Follower configuration of transistor. Can anybody explain it in easy to understand terms? Consider the following scenario:

-I give a NPN transistor a base voltage of +5V. Voltage at collector is +12V and Emitter is connected to common ground. There is NO LOAD. (I know that if there is load on the collector side the transistor will simply act as a switch.) But if I'm not wrong, when there is no load, the voltage between Emitter and Ground is 4.2V (5V - Voltage Drop), and not 12V (or 11.2V accounting for drop)! Why is this happening? When the transistor is saturated, shouldn't current flow from Collector to Emitter and voltage should be 12/11.2 V?

-Also, the primary application of this config is to change high input impedance to low output impedance. However I am unable to understand impedance at all!

Any help will be highly appreciated. Sorry if the questions are too silly.

Hi there,

My reply will seem a little redundant but i want to drive two points home.

First, an "emitter follower" transistor circuit is called that because the emitter voltage follows the base voltage to some degree of accuracy (it's an approximate statement but usually acceptable).. That means if the base voltage goes up by 1 volt then the emitter voltage goes up by 1v. That means the emitter can not be connected to ground. Also, the transistor must be biased properly for this kind of operation.

Second, since the circuit description you gave is very questionable, you must show your exact circuit so everyone here knows exactly what you need. Without a circuit it will take days or weeks to figure out. So post a circuit and if you dont know how then just ask here first.
 

Bordodynov

Joined May 20, 2015
2,401
Is that "smoke machine" in Belorussian? I like it!
No, it's a smoke machine from the Republic of Belarus! After all, the file is called "Goodbye NPN".
Once I did a really similar experiment. I measured the voltages in the radio circuit and accidentally closed the transistor's base for 9 V power supply. There was a small smoke from the transistor (KT315A) in a plastic case.
 

MrAl

Joined Jun 17, 2014
6,504
No, it's a smoke machine from the Republic of Belarus! After all, the file is called "Goodbye NPN".
Once I did a really similar experiment. I measured the voltages in the radio circuit and accidentally closed the transistor's base for 9 V power supply. There was a small smoke from the transistor (KT315A) in a plastic case.
Hi,

From the dust the transistor cometh, and to the dust the transistor returneth. (ha ha) :)
 

Papabravo

Joined Feb 24, 2006
12,407
No, it's a smoke machine from the Republic of Belarus! After all, the file is called "Goodbye NPN".
Once I did a really similar experiment. I measured the voltages in the radio circuit and accidentally closed the transistor's base for 9 V power supply. There was a small smoke from the transistor (KT315A) in a plastic case.
Everybody has one of those moments sooner or later. Mine was a defective "big blue" electrolytic capacitor that pop'ed its little plastic cork and made a plume about 25 feet (7.65 meters) high.
 
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