Fundamentals of Electric Circuits
Fundamentals of Electric Circuits
6th Edition
ISBN: 9780078028229
Author: Charles K Alexander, Matthew Sadiku
Publisher: McGraw-Hill Education
Question
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Chapter 19, Problem 92P
To determine

Calculate the voltage gain, current gain, input impedance, and output impedance for the amplifier shown in Figure 19.131 in the textbook.

Expert Solution & Answer
Check Mark

Answer to Problem 92P

The voltage gain, current gain, input impedance, and output impedance for the amplifier are 16.3476_, 89.3542_, 25.649kΩ_, and 192.5kΩ_, respectively.

Explanation of Solution

Given Data:

Refer to Figure 19.131 in the textbook for the amplifier circuit.

hie=4kΩhre=104hfe=100hoe=30μS

From the given amplifier circuit, the internal resistance (Rs) is 1.2kΩ, emitter resistance (Re) is 240Ω, and the load resistance (RL) is 4kΩ.

Formula used:

Refer to Equation 19.73 in the textbook and write the expression for voltage gain of a amplifier in terms of hybrid parameters as follows:

Av=VcVb        (1)

Here,

Vc is the collector voltage, which is the output voltage and

Vb is the base voltage.

Write the expression for current gain of the amplifier as follows:

Ai=IcIb        (2)

Here,

Ib is the base current and

Ic is the collector current.

Write the expression for input impedance of the amplifier as follows:

Zin=VbIb        (3)

Calculation:

Redraw the given circuit as shown in Figure 1.

Fundamentals of Electric Circuits, Chapter 19, Problem 92P , additional homework tip  1

From Figure 1, write the expression for emitter current as follows:

Ie=Ib+Ic        (4)

Write the expression for base voltage from the circuit in Figure 1 as follows:

Vb=hieIb+hreVc+(Ib+Ic)Re        (5)

Write the expression for collector current as follows:

Ic=hfeIb+VcRe+1hoe        (6)

Write the expression for collector voltage as follows:

Vc=IcRL        (7)

From Equation (7), substitute (IcRL) for Vc in Equation (6) as follows:

Ic=hfeIb+(IcRL)Re+1hoe(1+hoeRLRehoe+1)Ic=hfeIb

Rearrange the expression as follows:

IcIb=hfe1+hoeRLRehoe+1=hfe(1+Rehoe)1+Rehoe+hoeRL

IcIb=hfe(1+Rehoe)1+(Re+RL)hoe        (8)

From Equation (2), substitute Ai for IcIb as follows:

Ai=hfe(1+Rehoe)1+(Re+RL)hoe

Substitute 100 for hfe, 30μS for hoe, 240Ω for Re, and 4kΩ for RL as follows:

Ai=(100)[1+(240Ω)(30μS)]1+(240Ω+4kΩ)(30μS)=(100)[1+(240Ω)(30×106S)]1+(240Ω+4000Ω)(30×106S)=(100)(1.0072)1.1272=89.3542

From Equation (6), substitute (hfeIb+VcRe+1hoe) for Ic in Equation (8) as follows:

hfeIb+VcRe+1hoeIb=hfe(1+Rehoe)1+(Re+RL)hoe

Rearrange the expression as follows:

hfeIb+VcRe+1hoe=hfe(1+Rehoe)1+(Re+RL)hoeIb[hfe(1+Rehoe)1+(Re+RL)hoehfe]Ib=VcRe+1hoe[hfe(1+Rehoe)hfe(Re+RL)hoehfe1+(Re+RL)hoe]Ib=hoeVc1+Rehoe

Ib=[1+(Re+RL)hoe]hoeVc(1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe]        (9)

Rearrange the expression in Equation (5) as follows:

Vb=hieIb+hreVc+(IbRe+IcRe)

Vb=(hie+Re)Ib+hreVc+IcRe        (10)

From Equations (7) and (9), substitute (VcRL) for Ic and [1+(Re+RL)hoe]hoeVc(1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe] for Ib as follows:

Vb=(hie+Re){[1+(Re+RL)hoe]hoeVc(1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe]}+hreVc+(VcRL)Re={(hie+Re)[1+(Re+RL)hoe]hoe(1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe]+hreReRL}Vc

Rearrange the expression as follows:

VcVb=1{(hie+Re)[1+(Re+RL)hoe]hoe(1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe]+hreReRL}

Substitute 100 for hfe, 4kΩ for hie, 104 for hre, 30μS for hoe, 240Ω for Re, and 4kΩ for RL as follows:

VcVb=1((4kΩ+240Ω)[1+(240Ω+4kΩ)(30μS)](30μS)[1+(240Ω)(30μS)]{(100)[1+(240Ω)(30μS)]100(240Ω+4kΩ)(30μS)(100)}+104240Ω4kΩ)=1((4000Ω+240Ω)[1+(240Ω+4000Ω)(30×106S)](30×106S)[1+(240Ω)(30×106S)]{(100)[1+(240Ω)(30×106S)]100(240Ω+4000Ω)(30×106S)(100)}+104240Ω4000Ω)=1{(4240Ω)(1.1272)(30×106S)(1.0072)[(100)(1.0072)100112.72]+1040.06}=1{0.1434(1.0072)(112)+1040.06}

Simplify the expression as follows:

VcVb=16.3476

From Equation (1), substitute Av for VcVb to obtain the voltage gain of the amplifier.

Av=16.3476

From Equation (9), substitute (1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe][1+(Re+RL)hoe]hoeIb for Vc and from Equation (8), substitute hfe(1+Rehoe)1+(Re+RL)hoeIb for Ic in Equation (10) as follows:

Vb={(hie+Re)Ib+hre(1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe][1+(Re+RL)hoe]hoeIb+hfe(1+Rehoe)1+(Re+RL)hoeIbRe}={(hie+Re)+hre(1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe][1+(Re+RL)hoe]hoe+hfe(1+Rehoe)1+(Re+RL)hoeRe}Ib

Rearrange the expression as follows:

VbIb={(hie+Re)+hre(1+Rehoe)[hfe(1+Rehoe)hfe(Re+RL)hoehfe][1+(Re+RL)hoe]hoe+hfe(1+Rehoe)1+(Re+RL)hoeRe}

Substitute 100 for hfe, 4kΩ for hie, 104 for hre, 30μS for hoe, 240Ω for Re, and 4kΩ for RL as follows:

VbIb={(4kΩ+240Ω)+(104)[1+(240Ω)(30μS)][(100)(1+(240Ω)(30μS))100(240Ω+4kΩ)(30μS)(100)][1+(240Ω+4kΩ)(30μS)](30μS)+(100)[1+(240Ω)(30μS)]1+(240Ω+4kΩ)(30μS)(240Ω)}=4240Ω+(104)(1.0072)(100.7210012.72)33.816μS+(100.72)(240Ω)1.1272=4240Ω35.7417Ω+21445Ω=25649ΩVbIb25.649kΩ

From Equation (3), substitute Zin for VbIb to obtain the input impedance of the amplifier.

Zin=25.649kΩ

Consider output voltage (Vc) as 1 V and redraw the circuit in Figure 1 as Figure 2 to find the output impedance of the amplifier.

Fundamentals of Electric Circuits, Chapter 19, Problem 92P , additional homework tip  2

Apply KVL to the input loop for the circuit in Figure 2 as follows:

Ib(Rs+hie)+hreVc+Re(Ib+Ic)=0

Substitute 1 for Vc as follows:

Ib(Rs+hie)+hre+Re(Ib+Ic)=0

Ib(Rs+hie+Re)+hre+ReIc=0        (11)

Apply KCL at the output node for the circuit in Figure 2 as follows:

Ic=VcRe+1hoe+hfeIb=hoeVc1+hoeRe+hfeIb

Substitute 1 for Vc as follows:

Ic=hoe1+hoeRe+hfeIb

Rearrange the expression as follows:

Ic=hoe+(1+hoeRe)hfeIb1+hoeReIb=(1+hoeRe)Ichoe(1+hoeRe)hfe

Ib=Ichfehoe(1+hoeRe)hfe        (12)

From Equation (12), substitute [Ichfehoe(1+hoeRe)hfe] for Ib in Equation (11) as follows:

[Ichfehoe(1+hoeRe)hfe](Rs+hie+Re)+hre+ReIc=0[(Rs+hie+Re)hfe+Re]Ic=(Rs+hie+Re)hoe(1+hoeRe)hfehre[(Rs+hie+Re)+hfeRehfe]Ic=(Rs+hie+Re)hoe(1+hoeRe)hfehre(1+hoeRe)hfeIc=(Rs+hie+Re)hoe(1+hoeRe)hfehre(1+hoeRe)(Rs+hie+Re)+hfeRe

Substitute 100 for hfe, 4kΩ for hie, 1.2kΩ for Rs, 104 for hre, 30μS for hoe, 240Ω for Re, and 4kΩ for RL as follows:

Ic=(1.2kΩ+4kΩ+240Ω)(30μS)[1+(30μS)(240Ω)](100)(104)[1+(30μS)(240Ω)](1.2kΩ+4kΩ+240Ω)+(100)(240Ω)=0.1632(1.0072)(100)(104)(1.0072)(5440Ω)+24000Ω=0.153129479Ω=5.1935×106S

Write the expression for output impedance of the amplifier as follows:

Zout=VcIc

Substitute 1 for Vc and 5.1935×106S for Ic to obtain the output impedance of the amplifier.

Zout=15.1935×106S=192500Ω=192.5kΩ

Conclusion:

Thus, the voltage gain, current gain, input impedance, and output impedance for the amplifier are 16.3476_, 89.3542_, 25.649kΩ_, and 192.5kΩ_, respectively.

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