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AnalogElectronics_Lecture4_PartC_RC coupled Amplifier Design Procedure.

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: Part C of Lecture 4 gives the design procedure of RC coupled Amplifier for maximum symmetrical swing and for maximum voltage gain.

AnalogElectronics_Lecture4_PartC_RC coupled Amplifier Design Procedure.

CE RC COUPLED AMPLIFIER

Figure 1
Figure 1 (Picture 1.png)

DC equivalent circuit ;

Figure 2
Figure 2 (Picture 2.png)

We draw the thevenin equivalent circuit for the input port 1-1'

VB=Thevenin Equivalent voltage at 1-1'

Figure 3
Figure 3 (Picture 3.png)

Output mesh equation

Figure 4
Figure 4 (Picture 4.png)

Under this condition 100% stability of Q point is achieved with respect to aging, replacement and temperature.

In the expression

IC=βF(VB- VBE)/[RB+(1+βF)RE],

βF and VBE are temperature sensitive as well as BC reverse biased junction leakage current ICEO is also temperature sensitive.

Here βF(T)= βF(300K)2(T-300)/80,

Figure 5
Figure 5 (Picture 5.png)

ICEO(T)= ICEO(300)2(T-300)/10 ;

All these three temperature dependences are causes of Q point instability and thermal runaway. So if DC operating point is independent of these three terms we achieve 100% stability and prevent thermal runaway.

But practically it is impossible to meet the two conditions given in Eq.A .

Generally IC is taken in the range of 1mA to get the best performance out of the BJT in terms of βF and transit frequency ωT .

Figure 6
Figure 6 (Picture 7.png)

RB= (1/5) (1+βF)RE and VB = 3V. Eq. B

This gives a satisfactory stability of Q point and prevents thermal runaway. In self-bias, there is current-series feedback which is responsible for Q point stability.

Figure 7
Figure 7 (Picture 9.png)

Since we get optimum performance at 0.5mA < IC < 5mA, therefore we choose IC = 1mA.

For maximum symmetrical swing we take the voltage across RC = VCE = 5 V and we take voltage across RE =VRe = 2V for a decent stability.

In Figure 22, it is shown how distribution of voltage as shown in Figure 21 ensures maximum symmetrical swing.

Under static condition or under drift condition Q point traces the locus along static loadline and under signal condition it traces the dynamic loadline.

This gives RC = 5kΩ and RE = 2kΩ

Figure 8
Figure 8 (Picture 10.png)

Since Eq.B demands

Figure 9
Figure 9 (Picture 11.png)
;

Therefore RB= 40kΩ if βF = 100;

R1R2/( R1+R2)= RB=40k Eq.1.

VB= IBRB + 0.7V + 2V and IB= 1mA/100;

Therefore VB= 3.1V= VCCR2/( R1+R2) Eq.2

We have two simultaneous equations and two unknowns.

Solving them we get R1= 154k and R2= 54k;

Figure 10
Figure 10 (Picture 12.png)

Figure 11
Figure 11 (Picture 13.png)

The internal voltage gain

= vout/vin= -gmRC=-192.3;

Here we have not included the source resistance.

The input resistance=

Rin= rx+rπ= 100Ω+ 2.6kΩ= 2.7kΩ;

So here including RS will reduce the Voltage gain only slightly since RS may either be 50Ω or 600Ω which is negligible compared to 2.7kΩ.

The Band Width calculation will be seen in next section.

In CB BJT, input impedance is :

Rin = re= 26Ω for IE= 1mA.

Here RS = 50Ω or 600Ω is going to considerably reduce the Voltage Gain if RS is included while measuring the voltage gain. In this case internal voltage gain is going to be significantly higher.

What is difference between hybrid-parameters, hybrid-pi parameters and T-Model?

Hybrid Model is a mathematical model and abstract model whereas Hybrid-π or T Model are physical models. In data sheet both the parameters are given. We may work in terms of either model.

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