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For bjt in common emitter mode , base to emitter capacitance ( Cπ) is ten times the collector to base capacitance. Transistor is biased at quiescent collector current Icq= 1 mA and its short circuit unity gain frequency is 0.909. What is the Cπ value?
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For bjt in common emitter mode , base to emitter capacitance ( Cπ) is ...
Explanation:
In a BJT amplifier, the base-emitter junction acts as a capacitor and is denoted by Cπ. The collector-base junction also acts as a capacitor and is denoted by Ccb. The given problem states that Cπ is ten times Ccb.
Step 1: Calculation of Short Circuit Unity Gain Frequency (fT)
fT is the frequency at which the transistor's current gain drops to unity. It can be calculated using the following formula:
fT = gm/2πCπ
Where gm is the transconductance of the transistor and can be calculated using the formula:
gm = Ic/Vt
Where Ic is the quiescent collector current and Vt is the thermal voltage (25mV at room temperature).
Substituting the values, we get:
gm = 1mA/25mV = 40mS
fT = 40mS/2πCπ = 0.909
Step 2: Calculation of Cπ
Substituting the value of fT and the given relation between Cπ and Ccb, we get:
Ccb = Cπ/10
fT = gm/2πCπ
0.909 = 40mS/2πCπ
Cπ = 40mS/2π(0.909) = 7.03pF
Therefore, the value of Cπ is 7.03pF.
In a BJT amplifier, the base-emitter junction acts as a capacitor and is denoted by Cπ. The collector-base junction also acts as a capacitor and is denoted by Ccb. The given problem states that Cπ is ten times Ccb.
Step 1: Calculation of Short Circuit Unity Gain Frequency (fT)
fT is the frequency at which the transistor's current gain drops to unity. It can be calculated using the following formula:
fT = gm/2πCπ
Where gm is the transconductance of the transistor and can be calculated using the formula:
gm = Ic/Vt
Where Ic is the quiescent collector current and Vt is the thermal voltage (25mV at room temperature).
Substituting the values, we get:
gm = 1mA/25mV = 40mS
fT = 40mS/2πCπ = 0.909
Step 2: Calculation of Cπ
Substituting the value of fT and the given relation between Cπ and Ccb, we get:
Ccb = Cπ/10
fT = gm/2πCπ
0.909 = 40mS/2πCπ
Cπ = 40mS/2π(0.909) = 7.03pF
Therefore, the value of Cπ is 7.03pF.
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For bjt in common emitter mode , base to emitter capacitance ( Cπ) is ten times the collector to base capacitance. Transistor is biased at quiescent collector current Icq= 1 mA and its short circuit unity gain frequency is 0.909. What is the Cπ value?
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