Chapter-14 Feedback and Oscillator Circuits

Example-14.1(a) Page Number-745

In [11]:
A=-100.0        #gain without feedback
Ri=10           #in kohm
Ro=20           #in kohm
beta=-0.1       #no unit
p=beta*A        #no unit
Zi=Ri           #in kohm
Zo=Ro           #in kohm

#for Voltage-Series feedback circuit:
Af=A/(1+p)
Zif=Zi*(1+p)
Zof=Zo/(1+p)

print "Voltage gain with feedback=",round(Af,2)
print "Input Impedance=",round(Zif,2),"kohm"
print "Output Impedance=",round(Zof,2),"kohm"

Voltage gain with feedback= -9.09
Input Impedance= 110.0 kohm
Output Impedance= 1.82 kohm

Example-14.1(b) Page Number-745

In [13]:
A=-100.0        #gain without feedback
Ri=10           #in kohm
Ro=20           #in kohm
beta=-0.5       #no unit(feedback)
p=beta*A        #no unit
Zi=Ri           #in kohm
Zo=Ro           #in kohm

#for Voltage-Series feedback circuit:
Af=A/(1+p)
Zif=Zi*(1+p)
Zof=Zo/(1+p)

print "Voltage gain with feedback=",round(Af,2)
print "Input Impedance=",round(Zif,2),"kohm"
print "Output Impedance=",round(Zof*1000,2),"ohm"

Voltage gain with feedback= -1.96
Input Impedance= 510.0 kohm
Output Impedance= 392.16 ohm

Example-14.2 Page Number-747

In [14]:
A=-1000.0        #gain without feedback
beta=-0.1        #feedback
ChangeA= 20      #in %(change in gain)

#We know,Change in feedback gain (ChangeAf)=(1/(beta*A))*ChangeA:
ChangeAf=(1/(beta*A))*ChangeA

print "Change in feedback gain=",ChangeAf,"%"

Change in feedback gain= 0.2 %

Example-14.3 Page Number-748

In [1]:
R1=80.0            #in kohm
R2=20.0            #in kohm
Ro=10.0            #in kohm
Rd=10.0            #in kohm
gm=4000*(10**(-6))          #in S

Rl=(Ro*Rd)/(Ro+Rd)  #in kohm
#neglecting 100kohm of R1 and R2 in series,we get
A=-(gm*Rl*1000)
#feedback factor B:
B=-R2/(R1+R2)
#gain with feedback Af:
Af=A/(1+(B*A))

print "Voltage gain without feedback=",round(A,2)
print "Voltage gain with feedback=",round(Af,2)

Voltage gain without feedback= -20.0
Voltage gain with feedback= -4.0

Example-14.4 Page Number-749

In [33]:
A=100000       #gain of Op-amp
R1=1.8         #in kohm
R2=0.2         #in kohm

#feedback factor B:
B=R2/(R1+R2)
#feedback gain of Op-amp Af:
Af=A/(1+(B*A))

print "Amplifier Gain of the given circuit=",round(Af,2)

Amplifier Gain of the given circuit= 10.0

Example-14.5 Page Number-750

In [38]:
hfe=120.0            #no unit
hie=900.0            #in ohm
Vs=10              #in mV(rms value)
Re=510.0             #in ohm(emitter resistor)
Rc=2200.0           #in ohm(collector resistor)
re=7.5             #in ohm
#Wihout feedback:
A=-hfe/(hie+Re)
B=-Re
#Gain With feedback:
Af=A/(1+(B*A))
#Voltage gain with feedback:
Avf=Af*Rc
#Voltage gain without feedback:(Re=0)
Av=-Rc/re
print "Voltage gain without feedback=",round(Av,2)
print "Voltage gain with feedback=",round(Avf,2)

Voltage gain without feedback= -293.33
Voltage gain with feedback= -4.22

Example-14.6 Page Number-752

In [39]:
gm=5*(10**(-3))         #converting gm in mS to gm with unit S
Rs=1000.0               #in ohm(source resistor)
Rd=5100.0               #in ohm(drain resistor)
Rf=20000.0              #in ohm(feedback resistor)
#gain without feedback:
A=-gm*Rd
#gain with feedback:
Af=(-gm*Rd*Rf)/(Rf+(gm*Rd*Rs))

print "Voltage gain without feedback=",round(A,2)
print "Voltage gain with feedback=",round(Af,2)

Voltage gain without feedback= -25.5
Voltage gain with feedback= -11.21

Example-14.7 Page Number-758

In [46]:
gm=5000*(10**(-6))         #converting gm in microS to gm with unit S     
rd=40.0                    #in kohm
R=10000.0                  #in ohm(feedback circuit value)
f=1000                     #frequency in hertz

#calculating the required value of capacitance to ensure A>29
C=1/(6.28*R*f*2.45)
C=C/(10**(-9))              #converting F to nF
#Calculating required value of Rl:
A=40                       #let(A>29)
Rl=(A/gm)/1000
print "Required value of Capacitance=",round(C,2),"nF"
print "Required value of Rd=",Rl,"kohm"

Required value of Capacitance= 6.5 nF
Required value of Rd= 8.0 kohm

Example-14.8 Page Number-759

In [48]:
R=51*(10**3)               #in ohm
C=0.001*(10**-6)           #in Farad

#for a Wein-bridge oscillator,Resonant fequency fo:
fo=1/(6.28*R*C)

print "Resonant frequency=",round(fo,2),"Hz"

Resonant frequency= 3122.27 Hz

Example-14.9 Page Number-760

In [51]:
fo=10*(10**3)            #in Hz(Resonant frequency)
R=100*(10**3)            #in ohm(let)

#calculating the value of capacitane for Wein bridge oscillator:
C=1/(2*3.14*R*fo)
C=C/(10**(-12))              #converting F to pF
print "Required value of Capacitance=",round(C,1),"pF"

Required value of Capacitance= 159.2 pF