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AnalogElectronics_Lecture4_PartB_low and high frequency model of CE BJT.
INCREMENTAL MODEL OF CE BJT FROM T MODEL OF CB BJT
Figure 11a. Low Frequency Incremental T Model of CB BJT.
Let us re-orient this as CE configuration.
Figure 11b. The reoriented T Model to represent CE BJT.
It should be noticed that in T-Model under normal orientation has controlled current(α f i e ) coming out of collector node since base current is coming out of base node. But in reoriented T Model controlled current(α f i e ) coming into collector node since base current is coming into base node.
Input Mesh Equation:
Where g m = trans conductance = I C /V T whereas r e = V T / I E and I C = α F I E
The output current is =
Here
Figure 12. Bode Plot of beta and alpha and location of ω β and ω α .
Beta cutoff frequency = ω β and alpha cutoff frequency = ω α .
Cut-off frequency = -3dB frequency
= this is the frequency where parameter falls to 0.707 of its flat band value or midband value
= corner frequency
= half power frequency
Figure 13. Low Frequency Hybrid-π Model of CE BJT.
This is due to EARLY EFFECT or due to Base Width Modulation. A parameter Early Voltage V A is used for determining the output impedance of the hybrid –π Model. This output impedance is 1/h oe . The definition of Early Voltage V A is given in Figure 14.
Figure 14. The definition of Early Voltage for a CE BJT.
COMPARISON BETWEEN HYBRID-pi MODEL OF CE BJT AND T MODEL OF CB BJT
| T-model of CB BJT | Hybrid-pi Model of CE BJT |
| Unilateral model | Non Unilateral model |
h
rb =
 |
 |
In CB BJT,if we consider base spreading resistance to be zero then
For CE BJT if we consider r µ to be infinity then:
Incremental model at high frequency
Figure 16. High frequency T-Model of CB BJT.
Here τ t is the transit time taken by the minority carriers to cross the base width. The mechanism of transit is both diffusion and drift.
Now let us consider CE BJT at high frequency:
Here β f (short circuit current gain in CE BJT) is arrived at in exactly the same manner as α f was arrived at in CB BJT.
Figure 17. High Frequency Hybrid-π Model of CE BJT.
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