Morning Eric.

Yes, I'm not confident but I'll have one more shot at September.

Anyway, yes, my main difficulty is analysing some circuits circumstances where I don't know if I can write a certain closed loop equation or what happens with voltage at a certain point or which directions current will flow.

Yesterday night I have solved one of the exam's problem with help of a friend of mine that has some experience with electronics. I told him about what I was not sure if it could be done and why and he explained me why I couldn't, how I should do it and what would happen if a small detail would be changed. And it really makes sense to me but when I'm on my own, I just can't answer to my own questions because I just don't know exactly what happens with electricity in some situations.

I must be honest and say that I haven't read all the PDF's you suggested, from the very beginning to the very end, but I read some paragraphs. Please understand that I don't have time to read everything that all people that helps, suggests to me!

Ok, getting back to this thread's topic, the circuit I posted at the first post, was the circuit that our teacher chose as the amplifier exam problem.

I have found that around the internet, it's hard to find tutorials using the same hybrid model that we use in classes. Apparently, in classes, our teacher uses some kind of simplified H-model. Not the Hybrid-pi nor the T_model. So, it's hard to find examples that looks like the ones our teacher gave us at classes.

Another issue that I had, was to understand the main (generic) procedure to analyse amplifiers depending on the type of assemble, I mean, CC/CD, CE/CS, CB/CG amplifiers.

I think the procedure is, for instance, for Vi, is to write a closed loop equation that excludes exactly the point where our Vi is, I mean, Vi to be equal to all voltage drops from that point forward to the right until the end of the circuit. I don't know if I have explained my self correctly.

Anyway, the way Jony130 tried to help me, I think is a bit different from what we have learned at classes.

I'll post here in a bit, what our teacher told us what would be the 4 parameters for a basic Common-Base BJT amplifier!

 

Ok, for the basic example of a Common-Base amplifier, this is what we have from our teacher at classes notebook:


\(\displaystyle {A_{v}=\frac{v_{o}}{v_{i}}=\frac{-hfe\cdot i_{b}\cdot R_{Load}}{-hie\cdot i_{b}}=\frac{hfe}{hie}\cdot R_{Load}}\)

\(\displaystyle {A_{i}=\frac{i_{o}}{i_{i}}=\frac{-hfe\cdot i_{b}}{-\left ( hfe + 1 \right )\cdot i_{b}}=\frac{hfe}{hfe + 1}}\)

\(\displaystyle {R_{i}=\frac{v_{i}}{i_{x}}=\frac{-hie\cdot ib}{-\left ( hfe+1 \right )\cdot i_{b}}=\frac{hie}{hfe+1}}\)

\(\displaystyle {R_{o}=\frac{v_{x}}{i_{x}}=\infty}\) because \(\displaystyle {\overset{\underbrace{i_{x}}}{\frac{hie+ib}{R_{s}}}+i_{b} + \overset{\underbrace{i_{c}}}{hfe\cdot i_{b}}=0\Leftrightarrow i_{b}=0}\)

Now, based on this, we must solve the amplifier at the 1st post of this thread!

 

Hi, you forget about hie on your small-signal diagram. And in expression for Rin shouldn't you have Vin = -hie*Ib ? Do you even know why we have a minus sign there?
Also you should be able to solve all of this without even knowing that this is a transistor circuit. Because at this stage of your education you already should know how to solve the circuit with dependent sources in it based on network analysis skills.

 

hi psy,
Add this Basic BJT Amps pdf to your database.
Page 435 Section 6.7 deals with CB BJT amplifiers.
E

 

Hi, you forget about hie on your small-signal diagram. And in expression for Rin shouldn't you have Vin = -hie*Ib ? Do you even know why we have a minus sign there?
Also you should be able to solve all of this without even knowing that this is a transistor circuit. Because at this stage of your education you already should know how to solve the circuit with dependent sources in it based on network analysis skills.

Previous post corrected. Added forgotten hie and corrected the .asc file!
About Ri, probably yes. Maybe I missed it when writing on my notebook what teacher wrote at the board.

Well, maybe you're right when you say that I should know how to solve this even without knowing this is a transistor, based on network analysis skills. The true is that we don't have time, in my opinion to dig it better than what we do! And somehow, I have the felling this is not as straight forward as a simple resistive circuit analysis, but probably I'm wrong.

hi psy,
Add this Basic BJT Amps pdf to your database.
Page 435 Section 6.7 deals with CB BJT amplifiers.
E

Ok, I'll try to read that. Hope the content of that pdf matches what I'm used to see on my classes! Otherwise I might be confusing my mind even more!

 

hi psy,
Add this Basic BJT Amps pdf to your database.
Page 435 Section 6.7 deals with CB BJT amplifiers.
E

Eric, I've noticed that that PDF also doesn't use the same model that we use at classes! I don't know the exact name but it's not Hybrid-Pi nor Hybrid-T model. So I don't know if it would be profitable to be reading something that I can't actually use later at my exam!

 

In your post number 1 you called the model, Hybrid equivalent. Could it be that model combined some of the resistances used in the model illustrated in the info that Eric sent you?

Look at figure 6.63 specifically page 436.

As far as your specific circuit in post 1, it is very similar to the one on page 465 of the text with the exception of the base capacitor.

 

In your post number 1 you called the model, Hybrid equivalent. Could it be that model combined some of the resistances used in the model illustrated in the info that Eric sent you?

Look at figure 6.63 specifically page 436.

As far as your specific circuit in post 1, it is very similar to the one on page 465 of the text with the exception of the base capacitor.

That one is Hybrid-Pi, I think. Yes, in terms of schematic is very similar, but they use different parameters! I didn't want to be mixing them because probably will be worse for me!

 

Morning Psy,
The fact that different parameter names and models are often used, is one of the reasons I suggested that you get to know the basics of the components, transistors in this example, the equations/parameters/models you are using to solve the lesson problems would be more meaningful.

If you understood the basic operation and construction of a transistor you could quickly transpose the different models and parameters to fit the problem you are trying to solve.
Rather than simply 'plugging' values into an equation, try to understand the derivation of the equation.

E

BTW: all your 'helpers' want you to be successful in your studies, most of us have been where you are today.

 

Eric, I've noticed that that PDF also doesn't use the same model that we use at classes! I don't know the exact name but it's not Hybrid-Pi nor Hybrid-T model. So I don't know if it would be profitable to be reading something that I can't actually use later at my exam!

The small-signal model that you and also your teacher are using is called "the h-parameter model". Of Course you have a simplified version of it hre = 0 and hoe = ∞ also notice that hie ≈ rpi and hfe = gm*rpi and all this small-signal models are equivalent to each other.
Please take a look at chapter 6.2.5 on page 390
https://en.wikipedia.org/wiki/Bipolar_junction_transistor#h-parameter_model

Also do you try to solve the circuit? Can you show as your work?

 

Hi...

That chapter and page are from Eric's link? The wikipedia link has no such reference or page!

Maybe later today I'll give another try on this circuit. But no, I have not tried it out again since last Friday!

 

Hi...

That chapter and page are from Eric's link?

Yes.

 

Can you guys tell me the book where you're extracting those PDF's?

 

Sure we can
http://highered.mheducation.com/sites/0073380644/information_center_view0/index.html

 

Sure we can
http://highered.mheducation.com/sites/0073380644/information_center_view0/index.html

Is that the same as this?


Edited;

Forget it. I found it!

 

The small-signal model that you and also your teacher are using is called "the h-parameter model". Of Course you have a simplified version of it hre = 0 and hoe = ∞ also notice that hie ≈ rpi and hfe = gm*rpi and all this small-signal models are equivalent to each other.
Please take a look at chapter 6.2.5 on page 390
https://en.wikipedia.org/wiki/Bipolar_junction_transistor#h-parameter_model

Also do you try to solve the circuit? Can you show as your work?

The simplified version of h-parameters has hoe = 0; hoe is a conductance, not a resistance, as you know.

The description of the hybrid pi model from Wikipedia: "The hybrid-pi model is a linearized two-port network approximation to the BJT using the small-signal base-emitter voltage

and collector-emitter voltage

as independent variables, and the small-signal base current

and collector current

as dependent variables." sounds like the description of a y-parameter model.

A person could take any set of parameters (https://en.wikipedia.org/wiki/Two-port_network)--h-parameters, z-parameters, etc., and convert them to y-parameters, then use them in the hybrid pi model.

It looks to me as though his class instruction is using the hybrid pi model (https://en.wikipedia.org/wiki/Hybrid-pi_model), but with h-parameters. A little odd, but it works.

 

The simplified version of h-parameters has hoe = 0; hoe is a conductance, not a resistance, as you know.

Yes, I made a stupid mistake. Maybe because I always think in terms of ro in Ohms not in Siemens .

The description of the hybrid pi model from Wikipedia: "The hybrid-pi model is a linearized two-port network approximation to the BJT using the small-signal base-emitter voltage

and collector-emitter voltage

as independent variables, and the small-signal base current

and collector current

as dependent variables." sounds like the description of a y-parameter model.
It looks to me as though his class instruction is using the hybrid pi model (https://en.wikipedia.org/wiki/Hybrid-pi_model), but with h-parameters. A little odd, but it works.

Well my book about semiconductor disagree with this.
h-parameters model comes from two port network theory where they treat BJT as a black box. And this model has nothing to do with semiconductor physics. On the other hand the for example the hybrid pi model is highly related with semiconductor physics. Hybrid-pi/T-model are called a "physical model" because they more or less accurately reflect the "physics phenomenons" that occur inside the BJT.

 

Hello...

I'm going to try to start this from the very beginning, once more!

Can I think about what should be the first thing to calculate or does this also depends on the circuit?

For instance, is it better to start by Vi or by Ri or is it irrelevant?

 

I think that you should start with Vin, because you need Vin for voltage gain and also Vin is needed for Rin = Vin/Iin.

 

I think that you should start with Vin, because you need Vin for voltage gain and also Vin is needed for Rin = Vin/Iin.

I will consider Rs = 0 and hoe = 0 (1/hoe = \(\displaystyle {\infty}\)), at least for now, to try to keep things similar to what my teacher has taught us!

Ok, so one question I have. I know that when we have 2 resistors at a parallel setup, the voltage drop across one resistor is the same as the voltage drop across the other one. So, can I say that my Vi is either the voltage drop across Re or across hie, is this correct?

If this is correct isn't Vi = ib*(hfe + 1)*Re or Vi = -hie*ib????