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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
Figure 1 (Picture 1.png)

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:

Figure 2
Figure 2 (Picture 2.png)

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

Figure 3
Figure 3 (Picture 3.png)

Figure 4
Figure 4 (Picture 4.png)
Figure 5
Figure 5 (Picture 5.png)

Figure 2. Low Frequency Incremental model

Figure 6
Figure 6 (Picture 6.png)

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

Figure 7
Figure 7 (Picture 7.png)

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 ;

Figure 8
Figure 8 (Picture 8.png)
Figure 9
Figure 9 (Picture 10.png)
Figure 10
Figure 10 (Picture 11.png)

By detailed analysis,

Figure 11
Figure 11 (Picture 2.png)

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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