Transistor electrical spec(2N3903)

Adjuster

Joined Dec 26, 2010
2,148
The generator impedance is on the other side of C1 from the transistor: it is separate from the stage input impedance.

The amplifier input impedance does of course include an effect from the biasing resistors R1 and R2.
 

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Thevenin's Planet

Joined Nov 14, 2008
183
It seem that you are stating that the Generator impedances are not included in this Equation in regarding to the statement, ("separated from Biasing resistors R1 and R2").Z(total) = Rg1 || Rg2 || R1 || R2 || B+1(re'+Re), is this the formulae that should be used to calculate the input impendance,from generator's Z to re' + Re ?
B+1(re'+Re)=105*6.788+150=16,462.74,which is only the transistor internal impedance.
 

Adjuster

Joined Dec 26, 2010
2,148
If you simply want to determine the total impedance measurable across the generator terminals, at a frequency high enough for the coupling capacitor reactance to be negligible, then indeed all these elements would be considered as being in parallel.

Having said that, to determine the coupling capacitor corner frequency, the generator impedance should be added in series with the amplifier input impedance. It is worthwhile to understand the difference.

Thus, you calculate Rg = Rg1||Rg2, and the amplifier input impedance Rin = R1 || R2 || (β+1)(re'+Re)

The input coupling -3dB corner frequency is 1/(2*pi*(Rg+Rin))

 

Thread Starter

Thevenin's Planet

Joined Nov 14, 2008
183
Finally,the last part about the input overall of the input parameter,is that,since the capacitor nature is to have a leading current to it's voltage,this will cause a Phase shift in the input signal ? If so ,I should use
the complex form or the Rectangle method ? I would have to review that data though.
 

Adjuster

Joined Dec 26, 2010
2,148
Of course the capacitor will cause a phase shift, or at least it will if the frequency under consideration is low enough for the capacitor reactance to be significant. You may analyse that using whatever appropriate method you are familiar with for handling RC circuits.

I have to say however that in following this thread it seems to me that you may be struggling to see the wood for the trees.
(An English expression, meaning that when we focus on details, the wider context may not be clear to us.) While it is necessary to understand the detail of theory, it is also necessary to understand what is happening on a functional basis, and to have some idea of the relative importance of different aspects of the system.

For instance, here we have an amplifier with two AC coupling capacitors. For most purposes it will suffice to determine the necessary capacitor values approximately, if for no other reason than that the capacitors used are unlikely to be very close tolerance. The amplifier at present has component values that make its bias point quite sensitive to current gain, and we might wonder if effort might be better spent in improving that rather than analyzing its performance in minute degree. (Unless perhaps it is a homework question or worked example, in which case the student must of course base his answers on the values given. )

Another problem that seems to trouble many people who come to the subject is the sort of complaint you made about the inadequacy of transistor data: things that do not seem to fit in with how we might expect them to be. I have seen somewhat similar comments along the lines of "how can X possibly be so", where X is some surprising but nevertheless true part of circuit behaviour, such as a transistor collector potential falling below the base during saturation.

Here we need to accept that some things are as they are. Usually there will be explanations for these things which we may strive to understand, but even if we cannot understand them, that does not mean that they are necessarily untrue.
 

Thread Starter

Thevenin's Planet

Joined Nov 14, 2008
183
Well, you have been very informative and I am very grateful that I have access to folks at AAC and no doubt there will be more conversation on these subjects of electricity and electronic in the coming future, until then ....
 

Thread Starter

Thevenin's Planet

Joined Nov 14, 2008
183
Of course the capacitor will cause a phase shift, or at least it will if the frequency under consideration is low enough for the capacitor reactance to be significant. You may analyse that using whatever appropriate method you are familiar with for handling RC circuitsThe amplifier at present has component values that make its bias point quite sensitive to current gain, and we might wonder if effort might be better spent in improving that rather than analyzing its performance in minute degree. (Unless perhaps it is a homework question or worked example, in which case the student must of course base his answers on the values given. )

"Bias is quite sensitive to current gain,is this related to feed-back thru a emitter resistor? Additionally,a pull of greater load current that would effect bias ? or an increase in base bias current ? If so what would you suggest ?
 

Adjuster

Joined Dec 26, 2010
2,148
The general aim is to reduce the dependency of the collector current to current gain to an acceptably low level. Feedback via an emitter resistor plays a part in this in the standard four-resistor bias circuit: generally the voltage dropped in the emitter resistor would not be less than several hundred millivolts. Often the voltage is larger, but here there is a compromise, as this voltage reduces the possible output voltage swing. Having a relatively big voltage across the emitter resistor is desirable, as it reduces the effect of small voltage changes on the emitter current.

The base bias potential divider has to provide a quite stable voltage, not too much affected by how much base current flows - this is quite variable because of the wide range of β. The resistors might be chosen so that the total current is about ten times the typical value of the base current. (Or five times the maximum base current, or... this is a rough rule if thumb,and may require modification to fit particular circumstances.)

Another approach to this is to say that the parallel combination of the base potential divider resistors should be much less than the value given by multiplying the emitter resistor by (β+1).

Lower bias resistor values do reduce the input impedance of the stage, but this may be an acceptable price for obtaining predictable DC conditions.

This is all standard stuff, and you will probably find many good descriptions of it in textbooks or on the internet. Reading these at some length may help you to understand the matter better. Here are some examples, you should easily be able to find more:

http://www.allaboutcircuits.com/vol_3/chpt_4/10.html

http://en.wikipedia.org/wiki/Bipolar_transistor_biasing#Voltage_divider_biasing

http://www.eece.ksu.edu/~wkuhn/useful_stuff/BJTbiasing.PDF
 

Thread Starter

Thevenin's Planet

Joined Nov 14, 2008
183
E-book is not giving anything regarding temperture control concerning the size of components if temperture rise or decreases. The dissipation of heat in regard to component sizes. It does give Boltzn constant resistance.
 

Adjuster

Joined Dec 26, 2010
2,148
E-book is not giving anything regarding temperture control concerning the size of components if temperture rise or decreases. The dissipation of heat in regard to component sizes. It does give Boltzn constant resistance.
Please try to explain what you mean more clearly. What is "Boltzn constant resistance" - a garbled reference to Boltzmann's constant? Chapter references would help identify what you are referring to.

If you think that the E-book would benefit from references to thermal stability on transistor biasing, please feel free to comment about this in the Feedback and Suggestions section.
 

Thread Starter

Thevenin's Planet

Joined Nov 14, 2008
183
Thermal stability is somewhat spoken about in the E-book reference that you refer me to in the above thread, in the sense
of regarding the collector-emitter feed back and the emitter-feed back.I was wonder about a specific procedure that can be followed to achieve enough parameter in the components to handle temperature changes that deviated from normal room temp (whatever that may be ) in Fahrenheit or Celsius.Also keeping the bias within the active region.
 
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