# Help requested to identify common emitter, collector base bjt circuits

#### iouae

May 14, 2023
24
Hello all

I have meticulously studied hundreds of examples of BJT transistor circuits organised into common emitter, common collector and common base configurations. I find that all these tutorial sites are written way above my pay grade, and when they start showing formulae and discussing how this b current joins with this c current to form e current, my eyes glaze over. Could someone tell this thick-o if the following simple rules are true...

1. If the base is connected to the ground with no resistor, it's a common base circuit.
2. If the collector is connected to the V+ with no resistor, it's a common collector circuit.
3. If the emitter is connected to the ground with no resistor, it's a common emitter circuit.
4. If the out is tapped off the collector it is generally a common emitter circuit (exception being rule 1).
5. If the out is tapped off the emitter it is generally a common emitter circuit.
6. If the input circuit and output circuit share say a collector then it is a common collector circuit.

Take the above circuit. I find rule 6 meaningless in real working circuits since the in and out circuits in the above diagram seem to share both emitter and collector. From rule 2 I recognise this as common collector but I cannot see why they say the collector is common to both in and out. What am I missing.

Thank you.

#### iouae

May 14, 2023
24
Try this ...............might shake up the brain cells....

https://byjus.com/physics/characteristics-of-a-transistor/
Thank you Bluejets.

The site you recommended gave the simpler template circuit for a common emitter, and another site I was looking at gave a working circuit. I attached both files.
I can actually see how they both have the emitter to 0V rail in common with the input and output closed circuits.

But then I look at the circuit in my first post which is a common collector, and it too has the emitter to 0V rail in common with the input and output closed circuits.

Thank you for the help but I am still utterly confused.

ps. The only way I would have been able to GUESS that the working circuit (with detailed values) was common emitter was by seeing that its 220 ohm resistor in series with 1.2K ohm is negligible, so the emitter is effectively grounded, making it a common emitter (see rule 3). But this contorted reasoning cannot be the way to figure it out.

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Feb 19, 2021
783
Dont get hung up on stuff like no resistor in base for Common Base, you can in
fact have a CB circuit with an R in its base.

Simple way is whats common leaves the other two terminals to function as
input / output.

So CB means Emitter and Collector are the the connections for in/out, each
can be either. Note there is a "preferred" input and a preferred output, but
either can be used, but properties change due to choice.

So CC means Emitter and Base are the ......

So CE means Base and Collector are the ....

Generally the common element is thought of as AC ground, generally. Note below simplifications
do not show biasing stuff, eg. resistor in a common element, but portray basic interpretation. Note
below show the "preferred" input / output connections, but nothing protects one from reversing
the preferred, although results dramatically different, like no gain in circuit.....

Regards, Dana.

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#### iouae

May 14, 2023
24
Dont get hung up on stuff like no resistor in base for Common Base, you can in
fact have a CB circuit with an R in its base.

Simple way is whats common leaves the other two terminals to function as
input / output.

So CB means Emitter and Collector are the the connections for in/out, each
can be either. Note there is a "preferred" input and a preferred output, but
either can be used, but properties change due to choice.

So CC means Emitter and Base are the ......

So CE means Base and Collector are the ....

Generally the common element is thought of as AC ground, generally. Note below simplifications
do not show biasing stuff, eg. resistor in a common element, but portray basic interpretation. Note
below show the "preferred" input / output connections, but nothing protects one from reversing
the preferred, although results dramatically different, like no gain in circuit.....

Regards, Dana.

View attachment 59094
I think I am beginning to get it now.

So CC means Emitter and Base are the ...... connections for in/out

So CE means Base and Collector are the ....connections for in/out

So in the attached circuit below each are CE because the Base and Collector are the connections for in/out.

Thanks Dana (and Bluejets)

I will keep looking through example circuits and see if I get stuck.

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#### iouae

May 14, 2023
24
I made the attached solar tracker today and it seemed to work fine. The motor turns and winds a string which rotates a parabolic cooker and it also moves the LDR into the shade of an upright bit of cardboard. So as the sun moves, it moves sunlight back onto the LDR and switches on the motor the motor, which again pulls the LDR back into shadow..

As it is drawn, this configuration is common collector, which maximises current, which is good, and does not increase voltage which is also good..
If I placed the motor on the collector side it would be in common emitter configuration which increases voltage and current. It is a low voltage motor so I want as small a voltage as possible.

Is the above the correct sort of reasoning when designing a circuit such as the one attached.?

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#### Delta Prime

Jul 29, 2020
2,042
Is the above the correct sort of reasoning when designing a circuit
I hope this helps.

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#### iouae

May 14, 2023
24
I hope this helps.
I worked my way through the paper you recommended. Probably a first for me to bother with the mathematics surrounding a transistor. I award myself a Vc (Victoria cross) that I did it and survived. I had to look up what KVL means and am quite pleased with myself that I was able to work out V(RE) for myself in 4.7.2

But I know that I have missed the point the paper was trying to convey. For instance in 4.7.1 the LED and resistor are together, and in 4.7.2 they are on either side of the transistor. Is this important at all?

In 4.7.1 there is a base resistor, and 4.7.2 there is none. Is there some point to that?

As a personality flaw, I despise details, which probably explains why I hate maths. I just need someone to tell me, does it matter when placing a motor and transistor together in series whether the motor is on top (common emitter) or on the bottom (common collector). I will take their word for it (without mathematical proof) if they say yes it matters, one generally places a motor in common collector mode.

The following is what shako.ai said in answer to my question "which is a better way to connect a transistor and motor in series, common emitter or common collector mode?". "In general, common emitter mode is better for driving low-current loads, such as LEDs or sensors, while common collector mode is better for driving high-current loads, such as motors or speakers."

Yet when I do a Google search for "dc motor and transistor circuit" almost every circuit has the motor in common emitter mode.

Thanks again, it was a new experience for me.

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#### Delta Prime

Jul 29, 2020
2,042
As a personality flaw, I despise details, which probably explains why I hate maths
Not a problem this is more specific to your application.

#### iouae

May 14, 2023
24
If anyone is following this thread and like me is trying to get a handle on identifying what is common, then I just read the best way of identifying what is common.

Look for the transistor pin attached to the (most) unvarying voltage. Dead easy.

So if one pin is attached to the Vcc rail or V0 then that never varies so that is the common pin.

The only exception I have seen to this rule is where both collector and emitter have resistors attached to them, then the smallest resistor is the common rail, since this smaller resistor drops voltage less.

Feb 19, 2021
783
But I know that I have missed the point the paper was trying to convey. For instance in 4.7.1 the LED and resistor are together, and in 4.7.2 they are on either side of the transistor. Is this important at all?

In 4.7.1 there is a base resistor, and 4.7.2 there is none. Is there some point to that?

4.71 is a NPN transistor as switch, 4.72 is PNP. Both have load, LED and R, in the collector of
their respective transistor. Both have a base resistor.

Regards, Dana.

#### iouae

May 14, 2023
24
4.71 is a NPN transistor as switch, 4.72 is PNP. Both have load, LED and R, in the collector of
their respective transistor. Both have a base resistor.

Regards, Dana.
I really did study the article and it was an interesting first for me to see such detail.

I guess it missed my interest which is to apply my new found knowledge of common c,b,e when using a motor.

Studying dozens of circuits using a Google search "transistor motor circuit" I see that virtually all use a common emitter with the motor on top of a npn.

### does it matter if a motor is connected to the collector or emitter​

PERPLEXITY
Answer: It is generally recommended to connect a motor to the collector of a transistor when using it as a switch. Most tutorials on driving a DC motor connect the motor between the power source (battery) and the transistor's collector pin
1
. In addition, in lab experiments, high-current loads such as motors are attached to their power source and then to the collector of the transistor
2
. This is because the emitter is in the controlling circuit (base current) and in the controlled circuit (collector current), which is called common emitter mode. It is the most widely used arrangement for transistors so it is the one to learn first
3
. However, it is possible to connect a motor to the emitter of a transistor, but this can cause complications as the base current depends on the motor and will vary all over the place as a result
4
. In summary, it is generally recommended to connect a motor to the collector of a transistor when using it as a switch."

I would value someone doing the same detail as the previous article we discussed on motors and transistors explaining say point 3 above and why motors connected in common emitter mode are preferable.

But I thank you and all who have contributed to my learning to date.

#### Delta Prime

Jul 29, 2020
2,042
I really did study the article and it was an interesting first for me to see such detail
No one suggested you didn't study.
explaining say point 3 above and why motors connected in common emitter mode are preferable

Nomenclature! BJT transistors operating as a switch.
You turn a mechanical switch On
You drive a transistor into Saturation
Same thing .
You turn a mechanical switch off
You drive a transistor into Cut Off
Same thing.
Transistors must be driven fully on or fully off
Transistors must be driven into saturation or cut off
Same thing.
There's no way around mathematics there's something called Beta,
And will not be included in this discussion
In short the common emitter configuration can both amplify voltage and current it is considered to be a general purpose configuration for both AC and DC operation.

Feb 19, 2021
783
When you use a transistor, bipolar, as a switch there is a rule of thumb.

When transistor is on you want it to drop very little V between Collector and Emitter.
This is called saturation, and there is a spec on Vcesat when it is on acting as a
switch. To get Vcesat one has to drive the base with a lot more current than "normal",
and the rule of thumb is force Ib = Ic / 10. This is called "forced beta" as designer is
actually constraining the gain. Here is a typical transistor when its beta is forced to 10,
and shows what the drop will be across transistor when it is on carrying load current.
Note each transistor usually has its own graph in the spec sheet.

Here is a set of curves that shows once you drive Ic / 10 into the base, more current does not
accomplish better results. Just Ic / 10, a forced beta of 10, is generally all you need to get minimum
Vcesat.

Regards, Dana.

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#### Delta Prime

Jul 29, 2020
2,042
When you use a transistor, bipolar, as a switch there is a rule of thumb
Well said!
This is the way I see it in my head. My analogy would be a popular one. A mechanical switch thrown to the on position dead solid contact no wiggle room you know it's not going to pop out of contact.
The mechanical switch caught in between on and off in the middle
Or slow turn- on -;slow turn off.
The mechanical switch gets warm dissipates heat causes inefficiency.
In either direction you would like a hard turn on or saturation .
Hard turn off or cut off.
That is the purpose in my opinion of the"Rule of Thumb"

#### iouae

May 14, 2023
24
Thank you Delta Prime and Dana for your explanations and your time.

On point 3 which says....
"3
. However, it is possible to connect a motor to the emitter of a transistor, but this can cause complications as the base current depends on the motor and will vary all over the place as a result"

Is it because gain across the transistor (saturation) depends on Vbe and not Vbc? When a motor is attached to the emitter the voltage at the emitter will change with the changing cycle of the motor (a DC motor is sometimes a dead short and sometimes an open circuit).
Thus Ve will vary between 0V and Vcc. This change in Ve thus changes Vbe four times with every cycle of the motor thus changing the degree of saturation of the transistor four times a turn.

Vbe has to be > 0.7V to turn the transistor on surely?

Am I on to something with the above explanation?

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#### iouae

May 14, 2023
24
Why try to re-invent the wheel?

Bluejets the diagram you posted showed, like almost every other circuit diagram with a DC motor, the transistor was in common emitter mode. I am not trying to reinvent the wheel, but trying to know why.

I think it is because common emitter mode means the emitter voltage is unvarying, being grounded in your diagram's case.
Thus Vbe is unvarying, making gain across the transistor constant since we control Vb.

I have been playing with motors and transistors for decades and throwing them together in circuits with no thought as to whether the motor needs to be attached to the collector or the emitter, though if I paid better attention to other folks circuit diagrams, I would have noticed that all people out there attach the motor to the collector and the emitter to the ground or common emitter mode. 3 days ago I was not familiar with these different modes, so I am now beginning to see the seriousness of not trying to re-invent the wheel.

I just love to experiment with electrical components, throwing them together without reference to others circuit diagrams, and just fiddling till they work (kind-of). It's play, more of a hobby than trying to make a serious application. Trouble is I dislike reading datasheets or anything vaguely mathematical, but maybe its necessary to force myself to pay attention to the theory.

#### AnalogKid

Jun 10, 2015
2,913
I skimmed through the thread and did not see this: Back to the original post, #5 is incorrect. #5 is common collector, just like #2.

There are pedantic, semantic, and qualifying issues with each statement, but I see the kind of truth table you are trying to construct. All of the subtleties of real circuit design notwithstanding, the table is ok for guidance.

Something you might add are the (low / medium / high) input and output impedances of each of the three circuit classes. Very handy in figuring out someone else's design decisions.

ak

#### Bluejets

Oct 5, 2014
6,988
but trying to know why.
My understanding, and I may need correcting here, BUT....as your diagram shows the load below the transistor emitter, as the transistor starts to turn on, it's emitter voltage would start to rise, turning the transistor off again.

These days, it's simpler, easier, quicker, just to use a mosfet.
No real calculations for base current, wattage dissipation etc. etc. as long as there is sufficient current handling capacity.
If using with uC use logic level.

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