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Transistor Small Signal AC Models
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Transistor Small Signal AC Models
Transistor Small Signal AC Models
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1
Question
What is the purpose of a transistor model in AC analysis?
Answer
A transistor model is an equivalent circuit that represents the AC characteristics of the transistor, using circuit elements to approximate its behavior.
2
Question
Name the two commonly used transistor models in small signal AC analysis.
Answer
The two commonly used transistor models are the re model and the Hybrid equivalent model.
3
Question
How does the re model represent a bipolar junction transistor (BJT) and what is its main disadvantage?
Answer
The re model uses a diode and a current source to duplicate the behavior of the transistor, reflecting that BJTs are current-controlled devices. Its main disadvantage is sensitivity to the DC operating point, making it specific to particular circuit conditions.
4
Question
In the common-base configuration using the re model, how is the input impedance (Zi) calculated and what is its approximate output impedance (Zo)?
Answer
Input impedance Zi is approximately equal to re, and the output impedance Zo is approximately infinite (∞ Ω).
5
Question
What is the voltage gain (AV) expression for the common-base configuration in terms of re and RL?
Answer
The voltage gain AV ≈ α (RL / re), where α is the current gain factor.
6
Question
For common-emitter configuration, how is the input impedance Zi related to β and re?
Answer
The input impedance Zi ≈ β × re, where β is the current gain and re is the intrinsic emitter resistance.
7
Question
What does the output impedance Zo approximate in the common-emitter configuration?
Answer
The output impedance Zo ≈ ro, which is approximately infinite (∞ Ω).
8
Question
Give the general expression for voltage gain (AV) in a common-emitter configuration.
Answer
Voltage gain AV ≈ - (RL / re), where RL is the load resistance and re is the intrinsic emitter resistance.
9
Question
What is the current gain (Ai) for the common-emitter configuration?
Answer
The current gain Ai ≈ β, and output impedance ro ≈ ∞.
10
Question
In the common-collector configuration, how is the input impedance Zi expressed?
Answer
Input impedance Zi ≈ (β + 1) × re.
11
Question
What is the expression for the output impedance Zo in the common-collector configuration?
Answer
Output impedance Zo ≈ re in parallel with RE (re || RE).
12
Question
How is voltage gain AV calculated in the common-collector configuration?
Answer
Voltage gain AV ≈ RE / (RE + re).
13
Question
What are the four hybrid parameters used in the Hybrid Equivalent Model?
Answer
- hi: input resistance - hr: reverse transfer voltage ratio (Vi/Vo), approximately 0 - hf: forward transfer current ratio (Io/Ii) - ho: output conductance
14
Question
Which hybrid parameters are specifically emphasized in the Simplified General h-Parameter Model?
Answer
Input resistance hi and forward transfer current ratio hf.
15
Question
Relate the re model parameters to h-parameters in the common-emitter configuration.
Answer
hie = β × re (input resistance), and hfe = β_ac (forward current gain).
16
Question
Relate the re model parameters to h-parameters in the common-base configuration.
Answer
hib = re (input resistance), and hfb = -α ≈ -1 (forward current gain).
17
Question
What is the hybrid p model typically used for and how does it relate to the re model?
Answer
The hybrid p model is most useful for high-frequency transistor analysis. At lower frequencies, it closely approximates the re parameters and can be replaced by them.