Approximate Hybrid Equivalent Circuits


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Again, the impedance looking into the output terminals is infinite so that

conductance is zero

.

Hence, the four h-parameters of an ideal transistor connected in CE transistor are The hybrid equivalent circuit of such transistor is shown in fig.8.

Approximate Hybrid Equivalent Circuits
(a) Hybrid CB Circuit In Fig. 9. ( a) is shown an NPN transistor connected in CB configuration. Its ac equivalent circuit employing h-parameters is shown in Fig. 9 ( b).
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The V/I relationships are given by the following two equations
These equations are self-evident because applied voltage across input terminals must equal the drop over hib and the generator voltage. Similarly, current ic the output terminals must equal the sum of two branch currents.
(b) Hybrid CE Circuit The hybrid equivalent of the transistor alone when connected in CE configuration is shown in Fig.10 (b). Its V/I characteristics are described by the following two equations.
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(c) Hybrid CC Circuit The hybrid equivalent of a transistor alone when connected in CC Configuration is shown in Fig.11( b). Its V/I characteristics are defined by the following two equations :
We may connect signal input source across output terminals BC and load resistance across output terminals EC to get a CC amplifier.
Typical Values of Transistor h-parameters
In the table below are given typical values for each parameter for the broad range of transistors available today in each of the three configurations.
Approximate Hybrid Formulas
The approximate hybrid formulas for the three connections are listed below. These are applicable when ho and hr is very small and R s is very large. The given values refer to transistor terminals. The values of rin(stage) or rin and ro(stage) will depend on biasing resistors and load resistance respectively.
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Common Emitter h-parameter Analysis
The h-parameter equivalent of the CE circuit of Fig.12 , no emitter resistor has been connected.
We will now derive expressions for voltage and current gains for both these circuits.
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l. Input Impedance When looking into the base-emitter terminals of the transistor, hie in series with hre no. For a CE circuit, hre is very small so that hre Vo is negligible as compared to the drop over hie. Hence, rin=hie. Now, consider the circuit of Fig.13. Again ignoring hre Vo we have
2. Output Impedance Looking back into the collector and emitter terminals of the transistor in Fig. (12 b), ro= l/hoe .
3. Voltage Gain
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4. Current Gain 5. Power Gain
Common Collector h-parameter Analysis
The CC transistor circuit and its h-parameter equivalent are shown in Fig.14
l. Input Impedance
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2. Output Impedance
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Conversion of h-parameters
Transistor data sheets generally specify the transistor in terms of its h-parameters for CB connection i.e. hib, hfb, hrb and hob. If we want to use the transistor in CE or CC configuration we will have to convert the given set of parameters into a set of CE or CC parameters. Approximate conversion formulae are tabulated over leaf :
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Approximate Hybrid Equivalent Circuits