Winterstone
Banned
Hi crutschow,
I believe, we are relatively close together.
This is because I think, YOU are using the equation Ic=beta*Ib as a possibility to interpret it as an indication for current-control (simplified view for engineering tasks).
In contrast, my opinion is that this is purely a mathematical relation between two currents, which is used in the course of BJT design only in the form: Ib=Ic/beta (see my examples below).
Thus, I see absolutely no necessity for another interpretation of the BJT principle - even during design and parts calculation.
Some further comments:
Quotation Crutschow: I was just trying to say that the view of an engineer is how a device acts as a black box when used in a design whereas a teacher emphasizes the device's inner workings (as it should be).
I must confess I cannot identify two different views.
We speak of a common emitter stage and its gain, right?
I am sure that both of us use exact the same formula in case of Re feedback) : G=g*Rc/(1+g*Re).
Where g is the transcundactance g=dIc/dVbe.
And, of course, we – as engineers - don’t use 2-port parameters for determining the transconductance but the following formula, which results from Shockleys equation : g=Ic/Vt.
Where is the problem? Where are different views?
By the way: Shockleys equation Ic=f(Vbe/Vt) clearly indicates voltage-control of Ic.
Quotation Crutschow: I believe you should confront the student/engineer with both views. The voltage-mode initially for analysis since that is the physics of the device. Then, when teaching circuit design, cover the current-mode model also, since that is a much easier way to view the transistor for many BJT design synthesis calculations.
Don’t you think we can use the relation Ib=Ic/beta as it is, just treating it as a mathematical formula without the necessity to define a „current-mode“? At which stage of the design do we need this relation?
My answer: Only as an indication for the order of magnitude of Ib – as an input for designing the bias network. We do not need the exact Ib value and – what’s important – we do not need to know if Ib has a controlling function or not. We need Ib only as a current, which equals a small percentage of Ic. More than that: We treat it as a parasitic parameter, which cannot be avoided. That's all.
In short: Why do you favor the „current-control“ mode for engineering practice? Where do you need it?
Quotation Crutschow: I think it would be very difficult to try to do a BJT circuit design without using the idea that it appears more like a current-mode device than a voltage-mode device for many of the design calculations.
Counter example: Design of a typical common emitter stage:
* Select/specify Vsupply, Rc, Re and Ic.
* Normally one does not know the value of beta (lets assume somewhere between 100 and 250)
* For the worst case assumption (beta=100) and Vbe=0.65 volts the resistive divider is calculated – based on the requirement that the current through the divider is (6...10) times the current Ib=Ic/100.
If the corresponding input resistance is to low a recalculation is necessary.
Question: Was it necessary to consider the BJT as current-controlled? No, I don’t think so.
I only have used the mathematical relation Ib=Ic/beta.
What is your engineering approach to design such a stage? I suppose, exactly the same.
Now – if somebody asks: Why Vbe=0.65 volts? My answer is: This value determines the current Ic.
And if somebody asks: Why factor (6...10)? My answer is: I try to make the source resistance of the provided bias voltage Vbe=0.65 as low as possible (approach of voltage-control!) – in accordance with the mentioned constraints.
My claim: Everybody who tries to design the bias network low-resistive (as low as allowed) exploits the voltage-control feature of BJT's (perhaps without knowing).
W.
I believe, we are relatively close together.
This is because I think, YOU are using the equation Ic=beta*Ib as a possibility to interpret it as an indication for current-control (simplified view for engineering tasks).
In contrast, my opinion is that this is purely a mathematical relation between two currents, which is used in the course of BJT design only in the form: Ib=Ic/beta (see my examples below).
Thus, I see absolutely no necessity for another interpretation of the BJT principle - even during design and parts calculation.
Some further comments:
Quotation Crutschow: I was just trying to say that the view of an engineer is how a device acts as a black box when used in a design whereas a teacher emphasizes the device's inner workings (as it should be).
I must confess I cannot identify two different views.
We speak of a common emitter stage and its gain, right?
I am sure that both of us use exact the same formula in case of Re feedback) : G=g*Rc/(1+g*Re).
Where g is the transcundactance g=dIc/dVbe.
And, of course, we – as engineers - don’t use 2-port parameters for determining the transconductance but the following formula, which results from Shockleys equation : g=Ic/Vt.
Where is the problem? Where are different views?
By the way: Shockleys equation Ic=f(Vbe/Vt) clearly indicates voltage-control of Ic.
Quotation Crutschow: I believe you should confront the student/engineer with both views. The voltage-mode initially for analysis since that is the physics of the device. Then, when teaching circuit design, cover the current-mode model also, since that is a much easier way to view the transistor for many BJT design synthesis calculations.
Don’t you think we can use the relation Ib=Ic/beta as it is, just treating it as a mathematical formula without the necessity to define a „current-mode“? At which stage of the design do we need this relation?
My answer: Only as an indication for the order of magnitude of Ib – as an input for designing the bias network. We do not need the exact Ib value and – what’s important – we do not need to know if Ib has a controlling function or not. We need Ib only as a current, which equals a small percentage of Ic. More than that: We treat it as a parasitic parameter, which cannot be avoided. That's all.
In short: Why do you favor the „current-control“ mode for engineering practice? Where do you need it?
Quotation Crutschow: I think it would be very difficult to try to do a BJT circuit design without using the idea that it appears more like a current-mode device than a voltage-mode device for many of the design calculations.
Counter example: Design of a typical common emitter stage:
* Select/specify Vsupply, Rc, Re and Ic.
* Normally one does not know the value of beta (lets assume somewhere between 100 and 250)
* For the worst case assumption (beta=100) and Vbe=0.65 volts the resistive divider is calculated – based on the requirement that the current through the divider is (6...10) times the current Ib=Ic/100.
If the corresponding input resistance is to low a recalculation is necessary.
Question: Was it necessary to consider the BJT as current-controlled? No, I don’t think so.
I only have used the mathematical relation Ib=Ic/beta.
What is your engineering approach to design such a stage? I suppose, exactly the same.
Now – if somebody asks: Why Vbe=0.65 volts? My answer is: This value determines the current Ic.
And if somebody asks: Why factor (6...10)? My answer is: I try to make the source resistance of the provided bias voltage Vbe=0.65 as low as possible (approach of voltage-control!) – in accordance with the mentioned constraints.
My claim: Everybody who tries to design the bias network low-resistive (as low as allowed) exploits the voltage-control feature of BJT's (perhaps without knowing).
W.
Last edited: