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# AE_Lecture5_PartA_Low FrequencyAnalysisofCE_Amplifier

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_Lecture5_PartA describes the short Circuit Time Constant Method of determining the lower -3dB frequency of RC coupled CE Amplifier.

AnalogElectronics_Lecture5_PartA_Low Frequency Analysis of CE Amplifier(Final)

Figure 1. RC-coupled CE Amplifier.

Lower -3dB frequency of CE Amplifier is determined by Short Circuit Time Constant Method.

Coupling Capacitors and Emitter By-pass capacitor are responsible for lower -3dB frequency(fL).

We consider the time constant associated with each capacitor with the remaining capacitors shorted. Suppose the time constants associated with CC1, CC2 and CE are τ1S , τ2S and τ3S . Then the overall time constant associated with the amplifier due to combined effect of CC1, CC2 and CE is τL where:

Here Time Constant associated with each capacitor is τ = RC where R is the equivalent resistance seen by each Capacitor.

Figure 2. Low Frequency Incremental model

Figure 3. Low frequency Incremental Model with CE and Cc2 shorted. Equivalent resistance seen by Cc1 is RS + (RB||(rx+rπ))

Figure 4. Incremental Model with Cc1 and Cc2 shorted. Ce sees the equivalent Resistance R2s= vo/io

In Figure 4 , a voltage source vo is connected in place of CE .

Current drawn from the source is : io= ib + βfo. ib

where ib = vo/(rπ+ rx+ RB||RS) therefore io = ib(1 + βfo)= (1 + βfo).vo/(rπ+ rx+ RB||RS);

Therefore vo/ io = (rπ+ rx+ RB||RS)/ (1 + βfo)= R2s ;

By detailed analysis,

Here there are two poles corresponding to two capacitors CC1 and CE . The second coupling capacitor is considered to be infinity. The highest pole decides lower -3dB frequency. So 0.188Krads/sec decides the lower -3dB frequency which comes out to be 30Hz.

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