doubts on transistor configuration??

[shash]

hey guyz,in the diagram there is ttl not gate given.i wanted to know in which configuration(CB,CE,CC) is the transistor Q2 connected and how to determine that.

 

[cks3976]

CE. Emitter is common to both Input and output sections. Also please note that CE configuration will produce a phase difference of 180 degrees between Input and Output, which is the property of an Inverter gate...

 

[shash]

sir but how to know from here that emitter is common to input and output is there any reason it can even be CC and what about Q3 and Q4.is Q1 connected in CB config?

 

[cks3976]

In BJT configurations, the term "COMMON" indicates the transistor terminal that is grounded. Often these configurations are also referred to as Grounded Emitter/Base/Collector configuration.

How do we identify the configuration ? carryout the following analysis:
Where the input signal is connected - in this case : BASE
Which terminal is outputting the signal ? : In this case : COLLECTOR

Next, is the BASE coming in the Output section - NO, or is COLLECTOR coming in the input section : NO. So you can see that the terminal that is present in both Input and Output sections is EMITTER (grounded). That gives the configuration its name.

Also please note that input/output signals are indicated 0 or 1 (GND or VDD) with respect to GROUND and NOT with reference to VDD....,

Hope this is clear

 

[shash]

"In BJT configurations, the term "COMMON" indicates the transistor terminal that is grounded. Often these configurations are also referred to as Grounded Emitter/Base/Collector configuration."

sir if that is true then in case of CC the collector isnt connected to ground.In the diagram the ouput is taken from emitter(for Q4) as well as collector(for Q3)

 

[LvW]

..............
sir if that is true then in case of CC the collector isnt connected to ground.In the diagram the ouput is taken from emitter(for Q4) as well as collector(for Q3)

* Yes, the collector is grounded in CC configuration - "grounded" always means: signal ground (remember the low internal supply impedance).
* For my opinion, the transistor is operated neither in CC nor in CE configuration (or if you like: in both modes at the same time) since the output is also taken from the emitter node. That's not a problem, because the three basic configurations are only special cases. Remember: It is very common to provide an emitter resistance and to use the collector as output. Thus, no terminal is "grounded". Is it still a "common emitter configuration"?
For my opinion, it does not matter at all how this mode is called. More important: How it works.

 

[shash]

so it is not necessary for a transistor to be connected only in CB,CC,CE, configuration

 

[betwixt]

Correct. The 'common' configuration is normally used in analog situations where the 'common' pin is the point where signals are referenced from. There are other configurations too and in this case the circuit is primarily digital anyway. In the schematic you show, Q2 is in phase splitting configuration, the emiiter follows the base voltage and the collector does the opposite.

Brian.

 

[shash]

i also wanted to know that can a BJT work without keeping one terminal common

 

[betwixt]

You are thinking analog but it's a digital circuit. Just think of Q2 as being a switch between C and E pins.

When the input is high no current flows into the input pin, Q2 is turned on by the base current through R1 so effectively it's C and E are joined together and current flows into the base of Q4 making the output low. When the input is low, Q2 switches off, it's C and E pins are isolated. Now Q4 has no base current so it also turns off but Q3 is made to conduct because of base current through R2 so the output goes high.

Brian.

 

[halee awan]

Ok if we consider that Mr."betwixt "'s statment

"Correct. The 'common' configuration is normally used in analog situations where the 'common' pin is the point where signals are referenced from. "
is true then what about the cct shown below ,Where/which is the reference pin
and furthermore if I add Resistor at Collector to VCC then whats is the answer ?

 

[betwixt]

Consider that as far as the signal is concerned the Vcc and Ground are at the same potential. Obviously this is not true for the DC conditions or it wouldn't work. So in that schematic it would be classed as "common collector" although the more common name is "emitter follower", particularly if you add a resistor in the collector line.

Brian.

 

[DartPlayer170]

How to Determine Configuration of Transistor Circuit

The best way to characterize the configuration of a BJ transistor circuit is to determine the input and output leads. The remaining lead is the common lead.
Clearly in your example, the base is the input and the emitter is the output. Hence it is CC. Using a collector resistor will not change this.

In the initial circuit of the thread, Q2 is has two outputs. However, only one of the two outputs are of importance at any given time, depending on the logical state of the Q2 switch. When the switch is on, it is used as CC and when the switch is off it is used as CE.

 

[halee awan]

Consider that as far as the signal is concerned the Vcc and Ground are at the same potential. Obviously this is not true for the DC conditions or it wouldn't work. So in that schematic it would be classed as "common collector" although the more common name is "emitter follower", particularly if you add a resistor in the collector line.

Brian.

Mr. Brian,
What does its mean of the above hi-lighted one ..Could you please share some more detail

 

[halee awan]

How to Determine Configuration of Transistor Circuit

The best way to characterize the configuration of a BJ transistor circuit is to determine the input and output leads. The remaining lead is the common lead.
Clearly in your example, the base is the input and the emitter is the output. Hence it is CC. Using a collector resistor will not change this.

In the initial circuit of the thread, Q2 is has two outputs. However, only one of the two outputs are of importance at any given time, depending on the logical state of the Q2 switch. When the switch is on, it is used as CC and when the switch is off it is used as CE.

Yeah the same thing and many other reasons I have been qouting ,but the Quest was asked, there is nothing seems common like we have Emmitter Lead is common in CE and Base lead is common In CB configuration (visually/physically)>>>>>
you got my point

 

[FvM]

In my view, the said terms CC, CE, CB are exactly valid for small signal equivalent circuits. Here the transistor is analysed as two-port and you need to define a common terminal in case of a three terminal component. You also abstract from DC potential differences in equivalent circuits.

When describing real transistor circuits, some can be still related to the basic configurations. Others neither can nor need to be.

P.S.: It has been asked, if lack of a clear "common" terminal is specific to digital circuits. You'll find similar analog circuits ("phase splitter") as well.

 

[betwixt]

Quote Originally Posted by betwixt View Post
Consider that as far as the signal is concerned the Vcc and Ground are at the same potential. Obviously this is not true for the DC conditions or it wouldn't work. So in that schematic it would be classed as "common collector" although the more common name is "emitter follower", particularly if you add a resistor in the collector line.

Brian.

OK, to expand a little:

The DC conditions in a circuit have to be set in order for the transistors to work properly. They need bias and collector current to work as amplifiers/oscillators and logic elements.

Looking at the circuit as though you were an AC signal to be amplified, it looks quite different though. Bear in mind that the signal does not care about DC cnditions, all it sees is resistive and reactive components.
So the input looks like a capacitor with it's reactance at whatever frequency you are dealing with, followed by three other 'big' loads, R1, R2 and the transistor junctions in parallel. There are also smaller loads like stray capacitance and inductances which really only become important at high frequencies. In an AC analysis, the ground and Vcc are considered to be the same potential because both should be free of signals, this is why R1 and R2 appear in parallel and also the reason why in good design a capacitor is always fitted between Vcc and ground, it's to make sure they really are at the same AC potentials.

Brian.

 

[Syncopator]

What does its mean as far as the signal is concerned the Vcc and Ground are at the same potential ... Could you please share some more detail

Voltage sources have internal impedances tending to zero ohms. For signal analysis all voltage sources are considered to be short circuits.

---------- Post added 09-12-11 at 00:01 ---------- Previous post was 08-12-11 at 23:58 ----------

CE configuration will produce a phase difference of 180 degrees between Input and Output

No it won't.

The signal at the collector is inverted with respect to the emitter. There is no phase shift.

 

[FvM]

"CE configuration will produce a phase difference of 180 degrees between Input and Output"

No it won't.
The signal at the collector is inverted with respect to the emitter. There is no phase shift.

Although the phase hasn't been shifted, an inverter "produces" a phase difference of 180 degree. You can argue about using more instructive terms, but not about the fact. The term difference doesn't imply a particular method how it's been generated.

 

[Syncopator]

Although the phase hasn't been shifted, an inverter "produces" a phase difference of 180 degree.

The two parts of your sentence are contradictory.

... about the fact.

The fact is that there is no phase shift, there is no phase difference.