Build something simple, useful, and practical.

Knowing and understanding how transistor circuits work is useful.

Now move on to working with common ICs, such as LMC555 timer, op amps such as LM324, voltage regulators such as LM7805.

i started with the 555 right now, then i will switch to others when i fully understand this

 

ok i give an example:
1) Lets say i wanna build a XNOR gate with transistors, how can i do that with knowing only how ''transistors works'' and basic configurations? like even if i know that i gotta copy a configuration that already exists and understand it no? but maybe someone more experienced knows how to build it from scratch, with knowing only what a XNOR gate needs to do

or its normal that u copy such circuits? and dont build anything really from scratch?

Okay, it happens that about the only digital circuit I've built from scratch with transistors for low power applications is an XOR, (such as lighting a tail-light).
Unfortunately the computer I run LTspice on, which has the schematics for that, is down.

But basically, you look at the transistor switching (digital) operation where it is off with no base-emitter signal, and saturated on when it has a signal (note that it's the base-emitter signal/voltage and polarity that's determines this, not just the base signal) and then, for an XNOR, try to configure a circuit that gives no output when either transistor is on, but an output when neither or both are on.
I came up with several configurations to do that for the XOR, some involving the use of additional diodes.
Note that an XNOR is just an XOR with an inverter on its output

You can use both NPN and PNP transistors, which may (or may not) help.
If you connect a PNP's emitter to a positive supply, then its output is the supply voltage (to its load) when you pull the base towards ground with a resistor, and off when the base voltage equals the supply voltage.

It's rather like solving a puzzle with more than one solution, and usually takes some time and dead-end circuits, before you get one that works.
It's a good exercise circuit to start with.

Have fun.

 

ok i give an example:
1) Lets say i wanna build a XNOR gate with transistors, how can i do that with knowing only how ''transistors works'' and basic configurations? like even if i know that i gotta copy a configuration that already exists and understand it no? but maybe someone more experienced knows how to build it from scratch, with knowing only what a XNOR gate needs to do

or its normal that u copy such circuits? and dont build anything really from scratch?

Here's an example building a XNOR gate with BJTs. My background in computer science so my method is a bit different than what you might typically see around here. I'll use the short version of the "sum of products" (SOP) algorithm to realise the circuit.

Original expression of XNOR:
Y = (A x B) + (A' x B')

This reads as Y is equal to 1 if (and only if):
(A AND B = 1) OR (NOT A AND NOT B = 1)

These are the logical conditions for the circuit to produce the XNOR truth table. Since we are only working with AND, OR and NOT gates, we need to find a way to express each sub-expression (inside brackets) using these gates. Luckily this example is already simplified and all we need to do is construct the expression one step at a time:

1) (A x B) - 1 AND gate
2) (A' x B') - 1 AND gate, 2 NOT gates
3) (A x B) + (A' x B') - 1 OR gate

Here's what we get. I labelled the boolean transformations as they are at the output of each gate.


Now that we have the boolean logic, we can replace the logic gates with real BJT circuits. I'll leave this an exercise for you. Use this as a reference to build your gates. Falstad circuit simulator works well for this example.



https://www.falstad.com/circuit/

Paste this code into the "Import from text" option in the File menu to prove to yourself the expression is correct.

$ 1 0.000005 10.20027730826997 50 5 50 5e-11
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150 240 304 336 304 0 2 5 5
152 368 256 480 256 0 2 5 5
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Now that we have the boolean logic, we can replace the logic gates with real BJT circuits.

That give a solution, but it's far from the simplest using BJTs.

 

The circuit is too simple.
There is no load in series with the BJT, so when the BJT is turn on then the Vcc and Gnd will be shorted.

Three conditions:
1. AND gate -- two BJTs are turn on.
2. OR gate -- anyone of BJT is turns on.
3. NOT gate -- the BJT is turn on.

 

real BJT circuits.

All three of your "real" BJT circuits place a dead short circuit from Vcc to GND.

It's called *R* T L for a reason.

ak

 

I don't need (or want) advice on how to answer the questions on this forum.
You don't like my approach to doing the answers, don't read them.

 

I personally find it much easier to learn something when someone gives a complete and practical solution

As opposed to what you posted, which was something guaranteed *not* to work.

In the same amount of time you could have whipped up a schematic like this.

OR - you could have taken even less time to post schematics that were correct.
ak

 

It's a peer review not advice bud

You're not my peer or your "bud" and it was not a review but a criticism of my post

doesn't mean you can get away with bullying me

If that's bullying, you are certainly easy to bully.

.

 

See

 

This is why I tried to get you to provide a worked example. I personally find it much easier to learn something when someone gives a complete and practical solution compared to reading about it in bits and pieces. It's especially relevant for an expert in mixed signals like yourself. If you leave it to ol' Art, it's going to be half baked and riddled with errors. Besides, aren't you retired with nothing better to do? Doing trivial exercises like this to completion will help keep your mind sharp.





That's why I said to use the image as a reference! You guys are killing me with some of these comments gwhahaha. In the same amount of time you could have whipped up a schematic like this. Well, this actually took me 3 minutes. Just image what you could do with 3 minutes or hell, 5 minutes!

View attachment 350070

Ye i got what u meant, like the overall schematic , u wanted to represent the logic gates modelling transistors as switch, clearly to make the circuit work u gotta modify it by adding the resistors like u did afterwards, dont worry, i got what u meant

 

chill guys we just trying to learn together so no worries

 

Ye i got what u meant, like the overall schematic , u wanted to represent the logic gates modelling transistors as switch, clearly to make the circuit work u gotta modify it by adding the resistors like u did afterwards, dont worry, i got what u meant

Before going into details and designing gates with BJTs, may I ask a question?
Do you know how and why a BJT can work as a switch?
Do you know how saturation is defined and how this state can be realized?

 

You're not my peer or your "bud" and it was not a review but a criticism of my post
If that's bullying, you are certainly easy to bully.

.

I'm definitely your peer and review is synonymous to criticism. Trying to gaslight me on this instead of returning to the topic IS bullying. Some of you regular members really blow my mind with the ego trips. I said my piece, do what you want.

chill guys we just trying to learn together so no worries

Would you like me to finish the XNOR example? And sorry about the drama.

 

I'm definitely your peer and review is synonymous to criticism. Trying to gaslight me on this instead of returning to the topic IS bullying. Some of you regular members really blow my mind with the ego trips. I said my piece, do what you want.



Would you like me to finish the XNOR example? And sorry about the drama.

sure thanks, if i understood correctly now what we gotta think about is how to connect a stage to next one? right?

 

sure thanks, if i understood correctly now what we gotta think about is how to connect a stage to next one? right?

Yup, but part of the challenge is making sure each BJT is properly biased. For fun I built the circuit using PNP transistors which is mirror opposite of NPN transistors.

Notice how I had to make the OR gate resistors 100k otherwise these inputs sink too much current and the output can never go high. Try 10k resistors and you'll see it doesn't work.

This problem gets increasingly worse as more logic gates are chained together. Can you figure out why this happens and how MOSFETs are mostly immune to this effect? Also, why didn't I include base resistors on the lower AND gate?

Feel free to ask questions about anything that is unclear.

$ 1 0.000005 10.20027730826997 50 5 50 5e-11
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Before going into details and designing gates with BJTs, may I ask a question?
Do you know how and why a BJT can work as a switch?
Do you know how saturation is defined and how this state can be realized?

It can function as switch cus it controls the current beetwen collector and emitter, to act as switch u gotta operate beetwen the 2 regions: cutoff and saturation ( active doesnt interest us cus we dont need an amplification ).

Lets start with the cutoff:
to obtain the transistor working in cutoff u gotta have a vbe below 0.7 ( no base current means no collector current, so there is now current flowing and the transistors acts like an open switch, to understand that u could also check the curve of the transistor itself ( by looking at the values u understand whats needed to obtain that condition )

to obtain the transistor working in saturation region u gotta have a vbe above 0.7 ( maximum current flows beetwen collector and emitter

 

I'm definitely your peer

Peer definition:

a person who is of equal standing with another in a group

I would certainly not claim to be @crutschow’s peer.

 

to obtain the transistor working in saturation region u gotta have a vbe above 0.7 ( maximum current flows beetwen collector and emitter

I think, it is not a bad idea to know how a BJT works (in the amplification region and in saturation) before studying circuits.

Here is the correct definition for saturation:
When the B-C junction starts to be forward biased the BJT enters the saturation region.
That means: The current Ic (and withit the voltage drop Ic*Rc) must be large enough to forward bias the B-C junction.
In this case, we have Vc<Vb and Vce<Vbe.
As a result, the base current now consists of two components (two junctions forward biased): Ibe and Ibc.
As a result, the current into the base will be much larger than anticipated by the factor B (which considers Ibe only).
Rule of thumb: Design base biasing so that Ib will be app. (5....10)% of Ic.

Note that this drastically increased base current Ib is the RESULT (a good indication) of saturation and NOT its CAUSE (as stated in some publications).
A current is always the result of a voltage - not vice versa.

(By the way - reading what you wrote ( "to obtain the transistor working in cutoff u gotta have a vbe below 0.7" ) I`ve got the impression that you think the value of 0.7V would be a kind of "threshold". This is not the case: The BJTs collector current is an exponential function of Vbe - and the voltage of Vbe=0.7V is something like an approximate value for some milliamps.)

 

Besides, aren't you retired with nothing better to do?

What an incredibly impolite statement. Being retired does NOT mean we have nothing better to do.