BJT transistors

[jrz126]

In my Microelectronics course, we're learning about BJT transistor operations and calculating the current's and voltages. The textbook is about 1200 pages and weighs ~10 pounds. It would probably be more useful as a paper weight than a text book.

I was wondering if anyone had any good sites or info on how to do these calculations.

I had to miss a couple classes due to being slammed with tests in all my other clases. So I'm alittle behind and would greatly appreciate your help.
Thank you.

 

[samcheetah]

i think you are studying from Microelectronic Circuits by Sedra/Smith, right ????

well, if thats the book you are reading i would recommend it as a pillow, seat, and everything that you can imagine of. but dont use it as a paper weight :lol: i seriously recommend this book for a microelectronics course.

and if you want to do calculations then you would definately need a textbook. most of the info on the net isnt that deep that you would normally find in a good textbook. you could find a number of articles and lecture notes from various universities but a book is the best thing.

i hope that helps

 

[jrz126]

i think you are studying from Microelectronic Circuits by Sedra/Smith, right ????

:shock: Yeah, but what edition am I using :lol:

well, if thats the book you are reading i would recommend it as a pillow, seat, and everything that you can imagine of. but dont use it as a paper weight :lol: i seriously recommend this book for a microelectronics course.

and if you want to do calculations then you would definately need a textbook. most of the info on the net isnt that deep that you would normally find in a good textbook. you could find a number of articles and lecture notes from various universities but a book is the best thing.

i hope that helps

After googleing for awhile I realized that this isnt covered too well like you said. I'll hit up the school library and see what they have. [/quote]

 

[sonaiko]

In my Microelectronics course, we're learning about BJT transistor operations and calculating the current's and voltages. The textbook is about 1200 pages and weighs ~10 pounds. It would probably be more useful as a paper weight than a text book.

I was wondering if anyone had any good sites or info on how to do these calculations.

I had to miss a couple classes due to being slammed with tests in all my other clases. So I'm alittle behind and would greatly appreciate your help.
Thank you.

what part of calculation do u need?
if u have a particular question dont hesitate to ask

 

[samcheetah]

if you want to understand the working of a BJT the article http://amasci.com/amateur/transis.html would be helpful but as i said before, for the calculations or simulations u need a good book which you already have.

i could suggest a number of other books in addition to the book you have. "The Art of Electronics" has a good discussion about BJTs. another easy book is "Electronic Devices" by Floyd.

you should also read the book "Analysis and Design of Analog Integrated Circuits" by Hurst, Lewis. this is of a little bit higher level but it is a great reference.

a visit to the library would be a really great idea. you would find a huge amount of info on BJTs in about every electronics book.

i have to tell you that you should work hard and concentrate alot on the operation of BJTs because the later courses will assume that you have perfected your knowledge of BJTs.

i hope that helps

 

[jrz126]

I have a pretty good idea of how it works, the moving holes and electrons and what not. It's the calculations that are getting me.

I can calculate the currents pretty good, using the ic=Bib ect. but it's getting the Vc Vb and Ve values.

We are mainly looking at the transistors in forward active mode.

well, if thats the book you are reading i would recommend it as a pillow, seat, and everything that you can imagine of. but dont use it as a paper weight

I think it will become a pillow, because I'll probably fall asleep reading it

 

[samcheetah]

I think it will become a pillow, because I'll probably fall asleep reading it

lolz :lol:

I can calculate the currents pretty good, using the ic=Bib ect. but it's getting the Vc Vb and Ve values.

We are mainly looking at the transistors in forward active mode.

Ic = BETA * Ib is just an approximation. actually Ic is dependent on the voltage across the base-emitter junction and the temperature and the saturation current (which also depends on temperature). but it is a great help in simplified analysis. try looking at the description of the Eber-Molls model in the book "The Art of Electronics" and the article i reffered to you and you will see what i mean.

actually the book that you have starts from a higher level. there are other books that give you a simple approach in calculating the voltages and currents for the common types of circuits like the base-biased amplifier, voltage-divider biased amplifier etc. i would recommend;

Electronic Devices
By Floyd

Electronic Devices and Circuits
by Bogart

 

[jrz126]

how do I modify a transistor in pspice to behave like the simplified models that I'm using for my calculations?

 

[jrz126]

What's the deal with this circuit here: we are supposed to solve for Vb (in between the bases and the 10k resistor) and Ve (shown as vo) when Vi= -5 and -10V.

Are the transistors in saturation mode? (atleast for the -5V case?) I remember building something similar to this in my circuits I class, we used op amps to make an audio D/A converter, and this was on the end to give enough current/power to the speaker.

 

[samcheetah]

i simulated these circuits in multisim and here is what i got

 

[Roff]

Sam, something is hosed in your sims. When the input is -5v, the PNP acts as an emitter follower, and the emitter voltage should be about -4.1v. The base will be at about -4.8v (both these values depend on PNP beta). When the input is -10v, the PNP saturates, and the emitter will be at about -4.9v. The base will be a Vbe drop lower, or about -5.6v. The NPN never turns on, so is not a factor in this circuit.
Simulators can be very misleading - although I ran this on SwitcherCAD III and got results very close to those I predicted.

Jrz126, this is a push-pull emitter follower, but it only works well for analog signals if you keep the input signal range well within the collector supply voltages (+/-5v in this case). It will have significant crossover distortion unless it is embedded in a feedback loop containing a fast slew rate op amp. As a stand-alone circuit, it is good for driving heavy loads with digital or switching inputs.

 

[samcheetah]

:cry: this keeps getting complicated. but i dont know why i just dont want to abandon my simulator.

anyway, why doesnt the NPN ever turn on ????

 

[Roff]

:cry: this keeps getting complicated. but i dont know why i just dont want to abandon my simulator.

anyway, why doesnt the NPN ever turn on ????

The NPN remains off because the load resistor (1k) is returned to a voltage (GND) that is more positive than the base voltage, so the base-emitter junction is never forward biased. The B-E junction must be forward biased for the transistor to be on.

 

[samcheetah]

oh ok

i forgot that the voltage at the base was more negative than the voltage at the emitter.

 

[jrz126]

Jrz126, this is a push-pull emitter follower, but it only works well for analog signals if you keep the input signal range well within the collector supply voltages (+/-5v in this case). It will have significant crossover distortion unless it is embedded in a feedback loop containing a fast slew rate op amp. As a stand-alone circuit, it is good for driving heavy loads with digital or switching inputs.

In my circuits lab, we had a +/- 15V on the ends of the transistors, so I guess it worked out ok. the way we did it was, we had a D/A converter hooked to the audio out of a cd player, then we converted it back. It was only 8-bit D/A, so it didnt sound that good anyways.

but anyhoo, when Vi is equal to -10V, would the PNP transistor be in saturation mode? If Vcb (or Vbc I cant remember which I used. but I did use the correct one for my hw.) <(-0.4V) then it is in saturation mode.
IIRC my Vcb for the -5V case was -0.38V, since it was very close to -0.4, I just assumed the -10V case was in saturation (I only had 10 minutes before i had to turn it in so I couldnt double check it). I calculated my forced beta (Ic/Ib) to be 99 and the actual beta was 100


Samcheeta, thanks for your help with that. I found an example of the circuit in my pillow :lol: , so I noticed that the NPN was cut off. Is that simulator program free/any good? I think it might come in handy.
Could Sam's simulation be messed up since the beta wasnt specified?

 

[Roff]

Jrz126, this is a push-pull emitter follower, but it only works well for analog signals if you keep the input signal range well within the collector supply voltages (+/-5v in this case). It will have significant crossover distortion unless it is embedded in a feedback loop containing a fast slew rate op amp. As a stand-alone circuit, it is good for driving heavy loads with digital or switching inputs.

In my circuits lab, we had a +/- 15V on the ends of the transistors, so I guess it worked out ok. the way we did it was, we had a D/A converter hooked to the audio out of a cd player, then we converted it back. It was only 8-bit D/A, so it didnt sound that good anyways.

but anyhoo, when Vi is equal to -10V, would the PNP transistor be in saturation mode?

Re-read my post.

If Vcb (or Vbc I cant remember which I used. but I did use the correct one for my hw.) <(-0.4V) then it is in saturation mode.
IIRC my Vcb for the -5V case was -0.38V, since it was very close to -0.4, I just assumed the -10V case was in saturation (I only had 10 minutes before i had to turn it in so I couldnt double check it). I calculated my forced beta (Ic/Ib) to be 99 and the actual beta was 100


Samcheeta, thanks for your help with that. I found an example of the circuit in my pillow :lol: , so I noticed that the NPN was cut off. Is that simulator program free/any good? I think it might come in handy.
Could Sam's simulation be messed up since the beta wasnt specified?

SwitcherCAD III is free and uncastrated, with no time or size limits. You do need to have some previous experience with spice, or be very good at using the help files.
Not specifying beta usually won't cause a problem when simulating with generic transistors, because most simulators default to a simple model which has beta as one of the parameters. For example, SwitcherCAD III uses beta=100 for the generic NPN.

Jeff, I see you are majoring in procrastinology. I majored in Pinochle my first two semesters at San Jose State, and came to the realization that it was too tough, and I was gonna flunk out if I didn't change my major. That's how I became an engineer.

 

[samcheetah]

Samcheeta, thanks for your help with that. I found an example of the circuit in my pillow

:lol: i told you it was a good book!!!!

the simulator i use is Multisim 7. you can download a demo version of Multisim 7 from **broken link removed** but there is one problem with the demo version. you cant save any file and the libraries have only a few parts.

i also have SwitcherCAD III but i havent used it much. you can download the free version of SwitcherCAD III from Linear Technology's website https://www.linear.com

ive said it a number of times that i personally that much in simulators. a hands-on approach is much better. and most of the work we do in the lab is on a breadboard. but when im in my house i dont have all the components at hand. so just to get an idea of the circuit i simulate it on Multisim.

i agree with the calculations made by Ron H. maybe there is something i dont know about Multisim that resulted in the wrong calculations. like some days back i started a discussion about opamp supply current. when i simulated a simple circuit in Multisim it seemed as if Multisim wasnt obeying Kirchoff's current law. but then Optikon pointed out that there would be some current in the node 0 of the opamp model which multisim is using. so there must be something simillar here.

 

[samcheetah]

i know that this is a very old topic that i am replying to. but today i was looking at a simillar circuit (the one in the book Microelectronic circuits which jrz pointed out). i remembered that multisim gave me wrong results for this circuit. so i simulated this circuit again on multisim and saw the output. but i was still getting wrong values. and there were really strange things happening with other circuits i had. so i did a reinstall of Multisim and voila!!!!!! everything works right now. the following are the results that i got after simulating this circuit in multisim. the results are simillar to those predicted by Ron_H

so just to set the record straight here are the results that i got