Calculate the coupling capacitor CC required in Fig. 11.20 to provide a low frequency 3 dB point at 125 Hz if Rs = 600 Ω, hie = 1 kΩ, hfe = 60, R1 = 5 kΩ and R2 = 1.25 kΩ. For (a) an ideal bypass capacitor CE, (b) a practical bypass capacitor with RCE = 25Ω.

Question

Calculate the coupling capacitor [latex]C_{C}[/latex] required in Fig. 11.20 to provide a low frequency 3 dB point at 125 Hz if [latex]R_{s}[/latex] = 600 Ω, [latex]h_{ie}[/latex] = 1 kΩ , [latex]h_{fe}[/latex] = 60, [latex]R_{1}[/latex] = 5 kΩ and [latex]R_{2}[/latex] = 1.25 kΩ . For (a) an ideal bypass capacitor [latex]C_{E}[/latex], (b) a practical bypass capacitor with [latex]R_{CE}[/latex] = 25Ω .

Accepted Answer

The lower 3 dB frequency, [latex]f_{1}=\frac{1}{2\pi (R_{s}+R_{i}^{′})C_{C}}[/latex]

(a) [latex]R_{i}^{′}= R_{1} || R_{2} || h_{ie}[/latex]

[latex]C_{C}=\frac{1}{2\pi \int{1}(R_{s}+R_{i}^{′}) }=\frac{1}{125 imes 2\pi imes [600+5000\parallel 1.25 imes 10^{3}\parallel 1000]}[/latex]

[latex]=\frac{1}{125 imes 2\pi imes [600+5000]}[/latex]

[latex]C_{C} = 1.15 \mu F[/latex]

(b) [latex]R_{i}^{′}= R_{1} || R_{2} || [h_{ie} + (1 + h_{fe}) R_{CE}][/latex]

[latex]= 5000 || 1.25 × 10^{3} || [1000 + (61) (25)] = 716.31 \Omega[/latex]

[latex]C_{C}=\frac{1}{2\pi \int{1}(R_{S}+R_{i}^{'}) }[/latex]

[latex]=\frac{1}{125 imes 2\pi imes [600+716.31]}=0.97 \mu F[/latex]

Question 11.9: Calculate the coupling capacitor CC required in Fig. 11.20 t...

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