Hello there,

Here is a step by step illustration of how to approach this problem. Credit goes to @sparky 1 for the original diagram.
An assumption is that the base emitter diode, when forward biased, is exactly 0.7 volts. You can easily change that if you wish.
You still have to do all the calculations yourself.

Since no value for Rc was given we can not compute a numerical value for the collector current which means we can only create a table for when the transistor is in saturation and when it is not.
Perhaps you forgot to mention the value for Rc, but if not, then you have to explicitly shown when the transistor is both in saturation and when it is not. If it is enough to show when it is in the active mode, then all the easier, no table would be needed only an expression for when it is in the active mode.

Note that the network is gradually transformed until we end up with more common components that make the circuit easier to solve.
If any of this is not clear you can state what is confusing here. Ignore the typo

Thank you for your post! I didn^t include the Rc value because I didnt realise it mattered. Rc=5 kohms.

 

Hello there,

Here is a step by step illustration of how to approach this problem. Credit goes to @sparky 1 for the original diagram.
An assumption is that the base emitter diode, when forward biased, is exactly 0.7 volts. You can easily change that if you wish.
You still have to do all the calculations yourself.

Since no value for Rc was given we can not compute a numerical value for the collector current which means we can only create a table for when the transistor is in saturation and when it is not.
Perhaps you forgot to mention the value for Rc, but if not, then you have to explicitly shown when the transistor is both in saturation and when it is not. If it is enough to show when it is in the active mode, then all the easier, no table would be needed only an expression for when it is in the active mode.

Note that the network is gradually transformed until we end up with more common components that make the circuit easier to solve.
If any of this is not clear you can state what is confusing here. Ignore the typo

I would recommend, as a first step, replacing the 15 V source and R1,R2 with it's Thevenin equivalent, which is 5 V in series with R1||R2, which is 216.7 kΩ. Makes things much easier.

 

Thank you for your post! I didn^t include the Rc value because I didnt realise it mattered. Rc=5 kohms.

See Post #12 above.

 

Thank you for your post! I didn^t include the Rc value because I didnt realise it mattered. Rc=5 kohms.

Wow! Now you tell us.

 

The quality of Electronics and Mathematics are well represented by those that participated.
I am the least qualified in an academic sense.

Is there an operating point that will ever work? not likely

A 5V base current at about 20uA with 10k impedance output has no current.
We can amplify some weak signals with the right base resistors,
The BJT transistors are current mode with impedance limitations I feel there are better transistors for weak signal.

I will read WBahn post again to see where I can improve on input and output impedance calculations.

 

I would recommend, as a first step, replacing the 15 V source and R1,R2 with it's Thevenin equivalent, which is 5 V in series with R1||R2, which is 216.7 kΩ. Makes things much easier.

Hi there,

I considered that but realized that often new people into the field don't have that kind of intuition developed yet even if they know how to do it, and he started with the same method so I thought that was the best way so that he would understand it right away. I believe we (or you) should show him that method too though so he starts to get the hang of it sooner. Yes, then it's easier thanks for the suggestion.

 

Thank you for your post! I didn^t include the Rc value because I didnt realise it mattered. Rc=5 kohms.

OH ok

So now can you do the problem from here yourself? I think you know how to do it now.
I made a typo in that post though it should have read "we cannot calculate the collector voltage", we can calculate the collector current but needed to know that value of RC to get the voltage.

After that you might want to think about the method given in post #22.

 

Hi there,

I considered that but realized that often new people into the field don't have that kind of intuition developed yet even if they know how to do it, and he started with the same method so I thought that was the best way so that he would understand it right away. I believe we (or you) should show him that method too though so he starts to get the hang of it sooner. Yes, then it's easier thanks for the suggestion.

In most curricula, Thevenin equivalents are covered well before diodes, let alone transistors, are introduced. Thevenin is part of basic DC linear circuit analysis in the first part of Circuits I, while diodes and transistors are usually covered in Circuits III (with Circuits II being transform methods in linear systems).

 

In most curricula, Thevenin equivalents are covered well before diodes, let alone transistors, are introduced. Thevenin is part of basic DC linear circuit analysis in the first part of Circuits I, while diodes and transistors are usually covered in Circuits III (with Circuits II being transform methods in linear systems).

I^ll see if I can find some examples of this on Youtube. I know about and use Thevenin in other circuits, but never used it with transistors. My book doesn^t cover it either. Thank you for that idea!

 

I^ll see if I can find some examples of this on Youtube. I know about and use Thevenin in other circuits, but never used it with transistors. My book doesn^t cover it either. Thank you for that idea!

The book shouldn't have to cover using Thevenin equivalents with transistors, just as it shouldn't have to cover using Ohm's Law over and over with each new concept it introduces. You are supposed to be building up a toolbox of knowledge and skills as you learn about different analysis techniques so that those techniques are part of your toolbox that you can draw upon as needed any time in the future.

This is rather like a student mechanic being shown how to use a torque wrench to torque head bolts to a specified value and then not using it to torque the main bearing bolts to their specified value because they weren't shown how to use a torque wrench when they later learned about main bearings.

That you need to go out to YouTube to find examples of using Thevenin equivalents for that biasing network indicates that you don't understand, fundamentally, the concept of Thevenin equivalent circuits, so that is really where you need to do back to.

 

I^ll see if I can find some examples of this on Youtube. I know about and use Thevenin in other circuits, but never used it with transistors. My book doesn^t cover it either. Thank you for that idea!

@ Krastar
I think, in your post#19 you came to a result already - without stressing Thevenin.

 

In most curricula, Thevenin equivalents are covered well before diodes, let alone transistors, are introduced. Thevenin is part of basic DC linear circuit analysis in the first part of Circuits I, while diodes and transistors are usually covered in Circuits III (with Circuits II being transform methods in linear systems).

Hi there,

That's understandable. What I go by is what I hear from the person that makes the post. If it sounds like they are very new to electronics I try to stay simple and introduce as little extra as possible. In other words, I try not to assume too much about their background education. From the post following yours, it sounds like this is the case although I think it's still good to advance the idea of network simplifications as well.