Transistors configuration.

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Hello everybody !


I'm confused by many thing lately, but the transistors configuration is one of them.

I can't understand how to identify in which configuration the transistor is working on.

For example this one :

1657230900817.png

I can't say which configuration is that with this photo :

1657230934057.png

In some websites they say that common emiter is then when we ground the emiter and common collector is when collector is grounded but then I've this picture that says it is common collector while emiter is grounded.

1657231030209.png

I don't know how to identify the configuration.

Thanks for any advice :)
 
Last edited by a moderator:

Suncalc

Joined Mar 23, 2021
10
It's a simple common emitter configuration SE voltage amplifier. The bias is set by the voltage of the zener diode and the current through the stage is roughly (Vzener-0.7v)/500Ω
 

Papabravo

Joined Feb 24, 2006
18,981
Hello everybody !


I'm confused by many thing lately, but the transistors configuration is one of them.

I can't understand how to identify in which configuration the transistor is working on.

For example this one :

View attachment 270958

I can't say which configuration is that with this photo :

View attachment 270959

In some websites they say that common emiter is then when we ground the emiter and common collector is when collector is grounded but then I've this picture that says it is common collector while emiter is grounded.

View attachment 270960

I don't know how to identify the configuration.

Thanks for any advice :)
It does not look like a valid configuration because there is no DC power supply or biasing. There is also no indication of where the output might be. did you make this circuit up, or did you just haphazardly copy it from somewhere?
 

Thread Starter

Xenon02

Joined Feb 24, 2021
355
It does not look like a valid configuration because there is no DC power supply or biasing. There is also no indication of where the output might be. did you make this circuit up, or did you just haphazardly copy it from somewhere?
I took this circuit from this site where is the simulation (falstad).

Edit .
Which circuit if you mean this first one then it is from (falstad).
The others are also from other sites.
It's a simple common emitter configuration SE voltage amplifier. The bias is set by the voltage of the zener diode and the current through the stage is roughly (Vzener-0.7v)/500Ω
I don't understand
 

Papabravo

Joined Feb 24, 2006
18,981
I took this circuit from this site where is the simulation (falstad).
...
Take another look and see if you missed something. Try to identify the two nodes that the input signal is connected to. Then try to identify the two nodes the output signal is connected to. The node which is common to both the input and the output will tell you the configuration
 

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Take another look and see if you missed something. Try to identify the two nodes that the input signal is connected to. Then try to identify the two nodes the output signal is connected to. The node which is common to both the input and the output will tell you the configuration
So in this circuit :

1657232977516.png


Which one I should assume as output ?

Here is how I selected this circuit (it is in polish) but I know I didn't modify anything in this circuit that was made by this site.
2022-07-08.png
What is it supposed to do?
As I remember it is supposed to be a voltage follower
 

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Or maybe let's start with different circuit for example this:

1657233672552.png
Why some websited says this is common collector configuration ?
As I see output is connected to emiter - ground, and input I guess base- ground
 

Papabravo

Joined Feb 24, 2006
18,981
Great. that translates as

Voltage stabilizer with vertiver​
Which still is not very helpful, but it looks like it could be a voltage regulator with a zener diode as a reference. If this is the case it is an emitter follower, also known as a common collector circuit. Normally this circuit would be used with a DC source, so the presence of an AC source was confusing.
 

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Great. that translates as

Voltage stabilizer with vertiver​
Which still is not very helpful, but it looks like it could be a voltage regulator with a zener diode as a reference. If this is the case it is an emitter follower, also known as a common collector circuit. Normally this circuit would be used with a DC source, so the presence of an AC source was confusing.

Why is it common collector how do you indentify it ? Is there some kind of rule besides this one with common node input/output ?
 

Papabravo

Joined Feb 24, 2006
18,981
Or maybe let's start with different circuit for example this:

View attachment 270965
Why some websited says this is common collector configuration ?
As I see output is connected to emiter - ground, and input I guess base- ground
The node which is common is the collector. Remember voltage can be take with respect to ANY reference point. GND is NOT a required reference point. In the AC sense V+ and GND are the same point.
 

Thread Starter

Xenon02

Joined Feb 24, 2021
355
The node which is common is the collector. Remember voltage can be take with respect to ANY reference point. GND is NOT a required reference point. In the AC sense V+ and GND are the same point.
As I understand input nodes are collector - base, output nodes are collector - emiter ?

Because I don't understand why GND is not required and why in AC V+ and GND are the same point.
 

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Yes, that is the idea.
Hmmm okej, but why GND is not required ? And why for AC V+ and GND are the same ?

And why :


Great. that translates as

Voltage stabilizer with vertiver​
Which still is not very helpful, but it looks like it could be a voltage regulator with a zener diode as a reference. If this is the case it is an emitter follower, also known as a common collector circuit. Normally this circuit would be used with a DC source, so the presence of an AC source was confusing.
This is common collector ? I don't see here output like in usual configurations.


I don't know how the input is linked, is it collector - emiter ? But there is no output so how did you assume the configuration ?
 

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Or here :

1657234850347.png

On the left is common emiter on the right common collector.
There is no output but somehow we can tell which one is which, so how am I supposed to know which is the configuration?
 

Papabravo

Joined Feb 24, 2006
18,981
Hmmm okej, but why GND is not required ? And why for AC V+ and GND are the same ?

And why :




This is common collector ? I don't see here output like in usual configurations.


I don't know how the input is linked, is it collector - emiter ? But there is no output so how did you assume the configuration ?
Talking about the circuit in post #8
  1. GND is required for the circuit to function.
  2. The collector is the node which is common to both the input and the output
  3. The output signal is taken from the junction between the emitter and the load resistor. You can measure it from emitter to ground, or you can measure it from emitter to V+. Since there is a fixed relationship between V+ and ground it does not matter which one you choose.
  4. The input goes to the base in the circuit of post #8 and it can be measured with respect to either node because there is a fixed relationship between the nodes.
  5. Reread the section on AC analysis, where you set all DC sources to zero. That is because DC sources have no effect on the AC behavior of a circuit.
Talking about your original circuit, the identification of input and output nodes is complicated by having an AC source in place of the usual DC source. It would make more sense to have a DC source in series with an AC source if you are trying to understand the operation of a voltage regulator. Standard transistor configurations apply to amplifiers, and a voltage regulator is NOT an amplifier. It is a different kind of species.
 

Papabravo

Joined Feb 24, 2006
18,981
Or here :

View attachment 270966

On the left is common emiter on the right common collector.
There is no output but somehow we can tell which one is which, so how am I supposed to know which is the configuration?
There is an output for both, but again these are not amplifiers, they are being used as switches.
Also keep in mind that a BJT (Bipolar Junction Transistor) is NOT a voltage device, It is an input current that control an output current. It could care less where and how you are measuring your voltages.
 

dl324

Joined Mar 30, 2015
14,905
There is no output but somehow we can tell which one is which, so how am I supposed to know which is the configuration?
There is output. The examples are poor because we don't know the operating points and they're more likely to be saturation mode.

For common emitter circuits, the output is taken from the collector. For common collector, the output is taken from the emitter.
 

Papabravo

Joined Feb 24, 2006
18,981
There is output. The examples are poor because we don't know the operating points and they're more likely to be saturation mode.

For common emitter circuits, the output is taken from the collector. For common collector, the output is taken from the emitter.
You left out common base, where the base can be connected to GND or to V+.
 
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