Why do we need to zero the input voltage to calculate the output resistance of this BJT CE configuration?
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I've seen that image on the web before; can you provide a link to it?Why do we need to zero the input voltage to calculate the output resistance of this BJT CE configuration?
This is from Sedra and Smith's Microelectronic circuits book. I will provide the img of the statement later when I get home.I've seen that image on the web before; can you provide a link to it?
Also, can you provide a link to a statement that you need to "zero the input voltage"?
Your instructor is doing you a disservice. To find the resistance seen between to ports, you need two data points since resistance is the slope of the relationship between V and I. But if you turn off all the sources, you KNOW that one of those points is V=0V, I=0A so you only have to get one more. If any of the sources are turned on, the it is all but guaranteed that the V vs I line does NOT go through the origin.This is from Sedra and Smith's Microelectronic circuits book. I will provide the img of the statement later when I get home.
I went to my professor this morning and he says this is how we do it. I do remember zeroing voltage sources when trying to calculate the Thevenin resistance of a circuit. Thinking of it I never asked why, I just blindly zeroed the sources.
My best guess would be: voltage sources see other voltage sources as "short", and see other current sources as "open". But then again... I do not know why.
Yes this makes sense, thank you.I must confess that  due to language problems  I do not understand everything WBahn has explained in his last post.
Therefore, I try to do it with my own words (which I hopefully understand):
A resistor across a port is measured by connecting a voltage source across this port and measure the resulting current. Thus, it is clear hat the observed current must be caused by THIS voltage source only (and not by any other source in the circuit).
Therefore, the input signal must be zero.
But it does not HAVE to be zero  that just makes the computation easier.I must confess that  due to language problems  I do not understand everything WBahn has explained in his last post.
Therefore, I try to do it with my own words (which I hopefully understand):
A resistor across a port is measured by connecting a voltage source across this port and measure the resulting current. Thus, it is clear hat the observed current must be caused by THIS voltage source only (and not by any other source in the circuit).
Therefore, the input signal must be zero.
It seems to me that this sentence would make more sense with the addition of the word in red, or have I misunderstood something?But it does not HAVE to be zero  that just makes the computation easier.
Nope, you spotted a typo that really counts. Thanks! I've updated the original post.It seems to me that this sentence would make more sense with the addition of the word in red, or have I misunderstood something?
OK  of course, you are right. There are many cases for which we have two or even more alternatives for measuring circuit properies. Making two measurements for eliminating the unwanted influence is a kind of "compensation" measurement.But it does not HAVE to be zero  that just makes the computation easier.
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Please, be exact (to avoid misunderstandings). I suppose you speak about SIGNAL sources, correct? Of course, all DC supplies must remain switchedon.I'm thinking of another interpretation here. We know that ideal voltage sources have zero internal resistances. So when we calculate for output impedances we can short the voltage sources. Is that correct?
You are asking if the power source has to be disconnected? Of course, not.In real life situations though, when calculating the output impedance of any circuits, should we disconnect the power source of the circuit? or should we take into account the internal resistance of the source?
The initial question, very explicitly, was whether we NEED to zero in the input. This was again the main point in the followup in post #6 where the OP makes clear he isn't asking what the common method is, he is trying to understand WHY it is the common method and WHY zeroing the other sources is done. Otherwise, he would have been satisfied with his instructor's response that this is the way we do it.OK  of course, you are right. There are many cases for which we have two or even more alternatives for measuring circuit properies. Making two measurements for eliminating the unwanted influence is a kind of "compensation" measurement.
However, as you know, the method with Vin=0 is commonly used  and this was the initial question.
And, Mr Buggy, please make the distinction between calculating and measuring. Calculating is where you have the schematic and pull out a pen, some paper and a calculator and get to work. Measuring is where you pull out the device, some measurement tools such as a function generator and oscilloscope, and get to work.Please, be exact (to avoid misunderstandings). I suppose you speak about SIGNAL sources, correct? Of course, all DC supplies must remain switchedon.
In case the signal source has an internal resistance, the input node must remain terminated with this resistance (only Vsignal=0).
You are asking if the power source has to be disconnected? Of course, not.
And remember, the signal source has to be SHORTED (set to zero), NOT disconnected.
The initial question, very explicitly, was whether we NEED to zero in the input. This was again the main point in the followup in post #6 where the OP makes clear he isn't asking what the common method is, he is trying to understand WHY it is the common method and WHY zeroing the other sources is done.
For my opinion, this sounds a bit confusing.If you are calculating, you turn off ALL supplies. This is because turning them off only involves changing the schematic and we are assuming a linear system that doesn't mind having the supplies set to zero and/or shorted as far as the math is concerned. If you are measuring, then that is an option that is seldom available, and this is where what LvW is saying comes into play.
May I ask: What is the definition of a "large signal resistance"?Also, it generally depends on if you are looking for the... largesignal resistance....
Fine. You're absolutely right. The initial question clearly was limited to whether Vin needs to be set to zero in the method where Vin=0. So Vin must be zero. How foolish of me for even suggesting otherwise.* In post#8 I answered the OP`s question and told him WHY the signal source needs to be zero in case of calculating at the output the ratio Vout/Iout. Anything wrong?
* More than that, in my post#13 I agreed with your remark that there is another (more involved alternative).
* WBahn, what do you want in addition? What is your problem ?
But how did you get that small signal equivalent circuit that doesn't have any DC supplies in it? By turning off the DC supplies!WBahn, I am sorry but I must place some comments to your last post
For my opinion, this sounds a bit confusing.
a) If you calculate on the basis of the smallsignal model there are no dc supplies at all that could be switched off. This model assumes fixed dc bias conditions and, thus, a certain operating point.