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Frequency Response of Common Emitter Amplifier

Frequency Response of Common Emitter Amplifier

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Frequency response is a plot of voltage gain vs the input signal frequency. The is maximum and constant in mid band region and amplifier should always be used in the mid band region. At lowest frequency gain decreases this is due to external capacitance or coupling capacitance cc1 and  cc2. At higher frequency  gain decreases due to  internal or practical or junction capacitances of bjt C𝝿  &  C𝝁

  • Low frequency Analysis

  • High frequency Analysis

Low frequency analysis of CE amplifier:

{\boldsymbol f}_{\mathbf C}\boldsymbol=\frac{\mathbf1}{\mathbf2{\mathbf{πR}}_{\mathbf{in}}{\mathbf C}_{\mathbf1}}

\mathbf{where}\boldsymbol\;{\mathbf R}_{\mathbf{in}}\boldsymbol\;\boldsymbol=\boldsymbol\;{\mathbf R}_{\mathbf1}\mathbf{\left\|R_2\right\|}{\mathbf h}_{\mathbf{ie}}

\boldsymbol\therefore\boldsymbol\;{\mathbf f}_{\mathbf c}\boldsymbol=\frac{\mathbf1}{\mathbf2\mathbf\pi\mathbf{\left({R_1\left\|R_2\right\|h_{\mathrm{ie}}}\right)}{\mathbf C}_{\mathbf1}}

{\mathbf f}_{\mathbf c}\boldsymbol=\frac{\mathbf1}{\mathbf2\mathbf\pi\mathbf{\left({R_s+R_{\mathrm{in}}}\right)}{\mathbf C}_{\mathbf1}}

{\mathbf f}_{\mathbf c}\boldsymbol=\frac{\mathbf1}{\mathbf2\mathbf\pi\mathbf{\left({R_s+R_{\mathrm{in}}}\right)}{\mathbf C}_{\mathbf2}}

High Frequency Analysis:

Miller's theorem:

 

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