Determining h parmaters of bjt practically

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In summary, Jony has explained that the h-parameters are only used in small-signal analysis and designing amplifier circuits.
  • #1
techyash
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so i know that h11 is defined as v1/i1 when v2=0 and so on...but the thing is how do i do it practically...my professor told me that v1,i1,v2,i2 in h parameter equations are instantaneous values of current and voltage of ac in a bjt. he even wrote that to get h11, you have to set v2 of ac=0. but since, the lecture was a recorded one, i cudnt get it cleared.i have the following doubts-
1) how can v2 of ac be set to zero...if at all, say in ce config, i short the c and e terminals that is replace load resistance by a short circuit, how is it practically possible...
2) for h12, i will have to set i1=0 ie. in ce config, i will have to set i base as zero. which means i collector=0. which means that i2=0 which means h12 is infinity. not possible!
3) he told me to set v2=0, we use a capacitor... and for i1=0 we use an inductor at the base side...didnt get it.
4)on further reading on the net, i read that ib is kept constant so that i1 becomes zero..

i am all confused.pls help me asap
 
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  • #2
h-parameter are only used for small-signal analysis and designing a amplifier circuits. And don't forget that all h parameters are AC parameter.
So 0V means 0V ac voltage at given point. v2 = 0 V means that the collector AC voltage is 0V (Collector is short via large capacitor to ground).
Capacitor is a short for AC signal and coil is a open circuit for AC.
http://www.dtic.mil/dtic/tr/fulltext/u2/658138.pdf
But don't worry too much, in real world we don't use h-parameters to design an amplifier. Only EE students are forced by their mad professors to use it.
 
  • #3
Jony has explained it well.

1) shorting collector to ground using an appropriately large capacitor will set vce 0, for determining hie.

2) an appropriate inductor (RFC) is series with the base will still allow biasing current, but present an AC impedance →∞ . You then couple a signal generator to the collector and measure what fraction of its voltage appears at the base, for determining hre. Yes, you can apply input to the terminal you normally think of as output. Just keep the signal voltage low and Q1 won't be offended. ;)
 
  • #4
thank you people...nascentoxygen JONY[]...helped me a lot...
 
  • #5


First of all, it is important to understand that the h parameters of a BJT (bipolar junction transistor) are small-signal parameters that describe the linear behavior of the transistor. They are used to model the transistor in AC analysis and are not applicable for large-signal analysis.

Now, to address your doubts:

1) Setting v2 of ac to zero practically means applying a small AC signal at the base of the transistor and shorting the collector and emitter terminals. This can be done by connecting a signal generator to the base and using a shorting wire or a capacitor to short the collector and emitter.

2) For h12, you are correct that setting i1 to zero would result in h12 being infinity. However, this is not a practical scenario as it would mean that there is no base current flowing and the transistor is not functioning. In practical situations, there will always be some base current and h12 will have a finite value.

3) The use of a capacitor and an inductor to set v2 and i1 to zero, respectively, is a technique used in AC analysis to simplify the calculations. This is done by considering the capacitor as a short circuit for AC signals and the inductor as an open circuit. These components are not physically present in the circuit, but are used mathematically to simplify the analysis.

4) In AC analysis, ib is indeed kept constant at a small value in order to set i1 to zero. This is because the base current is assumed to be small enough to have a negligible effect on the collector current. This allows us to simplify the calculations and focus on the linear behavior of the transistor.

I hope this helps to clear up your confusion. It is important to understand the theoretical concepts behind the h parameters, but also to remember that they are used for small-signal analysis and may not accurately represent the behavior of the transistor in large-signal situations.
 

1. What is the purpose of determining h parameters of BJT practically?

The purpose of determining h parameters of BJT (bipolar junction transistor) is to characterize the behavior of a BJT in a practical circuit. These parameters are used to design and analyze BJT circuits, and to ensure their proper functioning in electronic devices.

2. What are the most commonly used h parameters for BJTs?

The most commonly used h parameters for BJTs are hfe (common emitter current gain), hie (common emitter input impedance), hoe (common emitter output admittance), and hre (common emitter reverse voltage gain). These parameters describe the input, output, and transfer characteristics of a BJT.

3. How are h parameters measured experimentally?

The h parameters of a BJT can be measured experimentally by connecting the transistor in a specific configuration and applying known input signals. The resulting output signals are then measured and used to calculate the h parameters using specific formulas.

4. What factors can affect the accuracy of h parameter measurements?

There are several factors that can affect the accuracy of h parameter measurements, including the quality of the BJT, the test circuit used, and external noise sources. It is important to carefully select and calibrate the equipment used for measurements, and to minimize any sources of noise that can affect the results.

5. Can h parameter measurements be used for all types of BJTs?

No, h parameter measurements may not be applicable to all types of BJTs. The h parameters are most commonly used for small signal BJTs, but may not accurately represent the behavior of large signal or power BJTs. It is important to consult the datasheet of the BJT being used to determine its applicable parameters for accurate circuit design.

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