# CHAPTER 12: TRANSISTOR AUDIO POWER AMPLIFIERS¶

## Example 12.1 : Page number 308¶

In :
#Variable declaration
VCC=10.0;                               #Collector supply voltage, V
R1=10.0;                                #Resistor R1, kilo ohm
R2=2.2;                                 #Resistor R2, kilo ohm
RC=3.6;                                 #Collector resistor, kilo ohm
RE=1.1;                                 #Emitter resistor, kilo ohm
VBE=0.7;                                #Base-emitter voltage, V

#Calculation
I1=VCC/(R1+R2);                     #Current through R1 and R2, mA (OHM's LAW)
V2=I1*R2;                           #Voltage across R2 resistor, V (OHM's LAW)
VE=V2-VBE;                          #Emitter voltage, V
IE=VE/RE;                           #Emitter current, mA (OHM's LAW)
IC=IE;                              #Collector current, mA (approximately equal to emitter current)
I_T=I1+IC;                          #Total current drawn from the supply, mA
P_dc=VCC*I_T;                       #Total power drawn from the supply, mW

#Results
print("The total power drawn from the supply=%.1fmW."%P_dc);

The total power drawn from the supply=18.2mW.


## Example 12.2 : Page number 309¶

In :
#Variable declaration
V_L=10.6;                       #Voltage across load, V.(from a.c voltmeter, therfore r.m.s value)

#Calculation
#Since, power =V**2/R,
P_O=(V_L**2/R_L)*1000;                     #A.C output power, mW

#Result
print("The a.c output power = %.1fmW."%P_O);

The a.c output power = 561.8mW.


## Example 12.3 : Page number 309¶

In :
#Variable declaration
V_PP=18.0;                  #Peak-to-peak a.c voltage, V

#Calculation
#Since, V(r.m.s)=(V(peak-to-peak)/2)/sqrt(2)
VL=V_PP/(2*(2**0.5));                      #r.m.s value, V

#Since, power=(square of voltage)/resistance
P_O_max=(VL**2/RL)*1000;                   #Maximum possible a.c load power, mW

#Result
print("The maximum possible a.c load power=%dmW."%P_O_max);

The maximum possible a.c load power=405mW.


## Example 12.4 : Page number 310¶

In :
#Variable declaration
V_battery=12.0;                 #Battery voltage, V
P_out=2.0;                      #Output power, W

#Calculation
#Since, Power=Current*Voltage
IC=(P_out/V_battery)*1000;                 #Maximum collector current , mA

#Result
print("The maximum collector current=%.1fmA."%IC);

The maximum collector current=166.7mA.


## Example 12.5 : Page number 310¶

In :
#Variable declaration
V_battery=12.0;                     #Battery voltage, V

#Calculation
IC_max=V_battery/RL;                #Maximum collector current, mA

#Result
print("The maximum collector current=%dmA."%IC_max);

The maximum collector current=3mA.


## Example 12.6 : Page number 310-311¶

In :
#Variable declaration
P=50.0;                     #Power supplied by power amplifier, W
R=8.0;                      #Resistance of speaker, ohm

#Calculation
#(i)
#Since, Power=Voltage _square/Resistance,
V=(P*R)**0.5;               #a.c output voltage, V

#(ii)
I=V/R;                          #a.c output current, A (OHM's LAW)

#Result
print("(i) The a.c output voltage=%dV."%V);
print("(ii) The a.c output current=%.1fA."%I);

(i) The a.c output voltage=20V.
(ii) The a.c output current=2.5A.


## Example 12.7 : Page number 315¶

In :
#Variable declaration
VCC=20.0;                   #Collector supply voltage, V
ib_peak=10.0;               #Base current(peak), mA
RB=1.0;                     #Base resistance, kilo ohm
RC=20.0;                    #Collector resistance, ohm
beta=25.0;                  #Base current amplification factor
VBE=0.7;                    #Base-emitter voltage, V

#Calculation
IB=round(VCC-VBE/RB,1);                         #Base current, mA (OHM's LAW)
IC=int(beta*IB);                            #Collector current, mA
VCE=VCC-(IC/1000)*RC;                  #Collector emitter voltage, V (KVL)

#(i)
ic_peak=beta*ib_peak;                          #Collector current(peak), mA
P_o_ac=(ic_peak/1000)**2*RC/2;                 #Output power, W

#(ii)
P_dc=VCC*IC/1000;                                    #Input power, W

#(iii)
collector_efficiency=(P_o_ac/P_dc)*100;                   #Collector efficiency of the amplifier circuit,

#Result
print("(i) The output power=%.3fW."%P_o_ac);
print("(ii) The input power=%.1fW."%P_dc);
print("(iii) The collector efficiency=%.1f%%."%collector_efficiency);

(i) The output power=0.625W.
(ii) The input power=9.6W.
(iii) The collector efficiency=6.5%.


## Example 12.8 : Page number 317¶

In :
#Variable declaration
P_dc=10.0;                  #zero signal power dissipation, W
P_o=4.0;                    #a.c output power, W

#Calculation
#(i)
Collector_eff=(P_o/P_dc)*100;                 #collector efficiency

#(ii)
#Zero signal power is the maximum power dissipation in a transistor, therefore,
Power_rating=P_dc;                  #Power rating of the transistor, W

#Result
print("(i) The collector efficiency=%d%%."%Collector_eff);
print("(i) The power rating of the transistor=%dW."%Power_rating);

(i) The collector efficiency=40%.
(i) The power rating of the transistor=10W.


## Example 12.9 : Page number 317-318¶

In :
#Variable declaration
n=10.0;                     #Transformer turn ratio
IC=100.0;                   #Zero signal collector current, mA

#Calculation
RL_reflected=n**2*RL;                        #Reflected load as seen by the primary of the transformer, ohm
P_o_ac_max=(IC/1000)**2*RL_reflected/2;             #Maximum a.c power output, W

#Result
print("The maximum a.c power output=%dW."%P_o_ac_max);

The maximum a.c power output=50W.


## Example 12.10 : Page number 318¶

In :
#Variable declaration
VCC=5.0;                    #Collector supply voltage, V
IC=50.0;                    #Zero signal collector current, mA

#Calculation
#(i)
P_o_max=VCC*IC/2;           #Maximum a.c output power, mW

#(ii)
P_dc=VCC*IC;                #D.C input power, mW
#Since, maximum power is dissipated in the zero signal conditions
Power_rating=P_dc;              #Power rating of transistor, mW

#(iii)
Max_collector_eff=(P_o_max/P_dc)*100;             #Maximum collector efficiency

#Result
print("(i) The maximum a.c output power=%dmW"%P_o_max);
print("(ii) The power rating of the transistor=%dmW."%Power_rating);
print("(iii) The maximum collector efficiency =%d%%."%Max_collector_eff);

(i) The maximum a.c output power=125mW
(ii) The power rating of the transistor=250mW.
(iii) The maximum collector efficiency =50%.


## Example 12.11 : Page number 318¶

In :
from math import sqrt

#Variable declaration
ic_max=160.0;               #Maximum a.c collector current, mA
ic_min=10.0;                #Minimum a.c collector current, mA
vce_max=12.0;               #Maximum collector-emitter voltage, V
vce_min=2.0;                #Minimum collector-emitter voltage, V

#Calculation
vce_pp=vce_max-vce_min;                                #peak to peak collector emitter voltage, V
ic_pp=ic_max-ic_min;                                   #peak to peak collector current, V
P_o=(vce_pp/(2*sqrt(2)))*(ic_pp/(2*sqrt(2)));          #a.c output power, mW

#Result
print("The a.c output power=%.1fmW."%P_o);

The a.c output power=187.5mW.


## Example 12.12 : Page number 319-320¶

In :
from math import sqrt

#Variable declaration
VCC=12.0;                           #Battey voltage, V
IC_max_change=100.0;                #maximum collector current change, mA
RL=5.0;                             #Loudspeaker resistance, ohm

#Calculation
VCE_max_change=VCC;                                         #Maximum collector-emitter voltage change
#(i) Loud speaker directly connected in the collector
Vmax_speaker=(IC_max_change/1000)*RL;                              #Maximum voltage across the loudspeaker, V
P_speaker_directly_coupled=Vmax_speaker*IC_max_change;                       #Power developed in the loudspeaker,mW

#(ii) Loudspeaker transformer coupled
Z_out=(VCE_max_change/IC_max_change)*1000;                     #Output impedance of transistor, ohm

#For max power transfer, primary impedance should be Z_out
RL_reflected=Z_out;                                         #Load resistance as seen by primary, ohm
n=sqrt(RL_reflected/RL);                                    #Turns ratio of transformer
Vp=VCC;                                                     #Transformer primary voltage, V
Vs=Vp/n;                                                    #Transformer secondary voltage, V
P_speaker_transformer_coupled=IL**2*RL*1000;                #Power delivered to the speaker, mW

#Result
print("(i)  The power transferred to the speaker when directly coupled=%dmW."%P_speaker_directly_coupled);
print("(ii) The power trasnferred to the speaker when transformer-coupled=%dmW."%P_speaker_transformer_coupled);
print("     The turns ratio=%.1f."%n);

(i)  The power transferred to the speaker when directly coupled=50mW.
(ii) The power trasnferred to the speaker when transformer-coupled=1200mW.
The turns ratio=4.9.


## Example 12.13 : Page number 320-321¶

In :
from math import sqrt

#Variable declaration
beta=100.0;                                 #Base current amplification factor
VCE_peak=30.0;                               #Peak value of collector voltage, V
IC_peak=35.0;                               #Peak value of collector current, mA
VCE_min=5.0;                                 #Minimum value of collector voltage, V
IC_min=1.0;                                 #Minimum value of collector current, mA

#Calculation
#(i)
IC_zero_signal=(IC_peak-IC_min)/2 +1;                  #Zero signal collector current, mA

#(ii)
IB_zero_signal=IC_zero_signal/beta;                 #Zero signal base current, mA

#(iii)
VCE_zero_signal=(VCE_peak-VCE_min)/2 +5;                    #Zero signal collector-emitter voltage, V
VCC=VCE_zero_signal;                                    #Collector supply voltage,V (due to transformer coupling, aproximately equal to zero signal VCE)
P_dc=VCC*IC_zero_signal;                                #d.c input power, mW
VCE_ac=(VCE_peak-VCE_min)/(2*sqrt(2));                         #a.c output voltage, V
IC_ac=(IC_peak-IC_min)/(2*sqrt(2));                           #a.c output  current, mA
P_ac=VCE_ac*IC_ac;                                      #a.c output power, mW

#(iv)
collector_eff=(P_ac/P_dc)*100;                          #Collector efficiency

#(v)
#a.c resistance RL'=negative inverse of slope of the d.c load line
slope=(IC_peak-IC_min)/(VCE_min-VCE_peak);                                  #Slope of he d.c load line, kilo mho
RL_ac=-(1/slope)*1000;                                                     #a.c resistance, ohm
n=sqrt(RL_ac/RL);                                                           #Transformer turn ratio

#Result
print("(i)   The approximate value of zero signal collector current=%dmA."%IC_zero_signal);
print("(ii)  The zero signal base current=%.2fmA."%IB_zero_signal);
print("(iii) The d.c input power= %dmW and a.c output power =%dmW."%(P_dc,P_ac));
print("(iv)  The collector efficiency=%.1f%%."%collector_eff);
print("(v)   The turn ratio of the transformer=%d."%n);

(i)   The approximate value of zero signal collector current=18mA.
(ii)  The zero signal base current=0.18mA.
(iii) The d.c input power= 315mW and a.c output power =106mW.
(iv)  The collector efficiency=33.7%.
(v)   The turn ratio of the transformer=3.


## Example 12.14 : Page number 321-322¶

In :
from math import sqrt

#Variable declaration
RL_reflected=325.0;             #Load resistance, when referred to primary, ohm
VCC=20.0;                       #Supply voltage, V
IC=58.0;                      #Quiscent value of collector current, mA

#Calculation
#(i)
n=sqrt(RL_reflected/RL);                    #Transformer turn ratio

#(ii)
P_ac=(((IC/1000)**2)*RL_reflected/2)*1000;                   #A.C output power, mW

#(iii)
P_dc=VCC*IC;                                    #d.c input power, mW
collector_eff=(P_ac/P_dc)*100;                        #Collector efficiency

#Result
print("(i)   Transformer turn ratio=%d."%n);
print("(ii)  The a.c output power=%dmW."%P_ac);
print("(iii) The collector efficiency=%d%%."%collector_eff);

(i)   Transformer turn ratio=5.
(ii)  The a.c output power=546mW.
(iii) The collector efficiency=47%.


## Example 12.15 : Page number 323¶

In :
#Variable declaration
P_total=4.0;                #Total power dissipated by the power transistor, W
T_j_max=90.0;               #Maximum junction temperature, degree celsius
theta=10.0;                 #Thermal resistance, degree celsius per watt

#Calculation
#Since, Total power dissipation=half of(max. junc. temp. - ambient temp.)
T_amb=T_j_max-(P_total*theta);                      #Ambient temperature, degree celsius

#Result
print("The ambient temperature=%d degree celsius."%T_amb);

The ambient temperature=50 degree celsius.


## Example 12.16 : Page number 323-324¶

In :
#Variable declaration
theta=300.0;                #Thermal resistance, degree celsius per watt
T_j_max=90.0;               #Maximum junction temperature, degree celsius
T_amb=30.0;                 #Ambient temperature, degree celsius

#Calculation
#(i) Without heat sink
P_total=((T_j_max-T_amb)/theta)*1000;              #Maximum permissible power dissipation without sink, mW

print("(i)The maximum permissible power dissipation without heat sink=%dmW."%P_total);

#(ii) With heat sink
theta=60.0;                                    #reduced thermal resistance, degree celsius per watt
P_total=((T_j_max-T_amb)/theta)*1000;          #Maximum permissible power dissipation with heat sink, mW

print("(ii)The maximum permissible power dissipation with heat sink=%dmW."%P_total);

(i)The maximum permissible power dissipation without heat sink=200mW.
(ii)The maximum permissible power dissipation with heat sink=1000mW.


## Example 12.17 : Page number 324¶

In :
#Variable declaration
theta=20.0;                         #Thermal resistance, degree celsius per watt
T_j_max=200.0;                      #Maximum junction temperature, degree celsius
T_amb=25.0;                         #Ambient temperature, degree celsius
VCE=4.0;                            #Collector-emitter voltage, V

#Calculation
P_total=(T_j_max-T_amb)/theta;          #Maximum permissible power dissipation, W

#since, the max. power dissipation=VCE_max*IC_max,therefore
IC_max=P_total/VCE;                                             #Maximum collector current, A

print("The maximum collector current that the transistor can carry without destruction=%.2fA."%IC_max);

#The ambient temperature rises
T_amb=75.0;                     #The risen ambibent temperature, degree celsius
P_total=(T_j_max-T_amb)/theta;          #Maximum permissible power dissipation, W
IC_max=P_total/VCE;                                             #Maximum collector current, A

print("The maximum collector current for the risen ambient temperature=%.2fA."%IC_max);

The maximum collector current that the transistor can carry without destruction=2.19A.
The maximum collector current for the risen ambient temperature=1.56A.


## Example 12.18 : Page number 328-329¶

In :
from math import pi

#Variable declaration
VCC=12.0;                   #Supply voltage, V

#Calculation
#(i)
IC_sat=VCC/(2*RL);                  #Collector saturation current, A

#(ii)
P_dc=round(VCC*IC_sat/round(pi,2),2);                 #d.c input power, W

#(iii)
Collector_eff=(P_o_max/P_dc)*100;       #Collector efficiency

#Result
print("(i)   The maximum load power =%.2fW."%P_o_max);
print("(ii)  The d.c input power=%.2fW."%P_dc);
print("(iii) The collector efficiency=%.1f%%."%Collector_eff);

(i)   The maximum load power =2.25W.
(ii)  The d.c input power=2.87W.
(iii) The collector efficiency=78.4%.


## Example 12.19 : Page number 329¶

In :
#Variable declaration
P_T=10.0;                   #Power rating of each transistor, W
max_eff=0.785;              #Maximum collector effciency

#Calculation
#Since, input power=max. a.c power + Power rating of transistor
#And, max. efficiency=max. a.c power/input d.c power
P_2T=2*P_T;                                   #Total power dissipation by two transistors
P_o_max=(max_eff*P_2T)/(1-max_eff);           #Maximum output a.c power, W

#result
print("The maximum output power that can be obtained=%.2fW."%P_o_max);

The maximum output power that can be obtained=73.02W.


## Example 12.20 : Page number 329¶

In :
#Variable declaration
eff=60.0/100;                   #Efficiency of the amplifier
P_T=2.5;                        #Power dissipated by each transistor, W

#Calculation
#Since, input power=max. a.c power + Power rating of transistor
#And, max. efficiency=max. a.c power/input d.c power
P_2T=2*P_T;                        #Total power dissipated by both transistors, W
P_ac=(eff*P_2T)/(1-eff);           #Output a.c power, W
P_dc=P_ac+P_2T;                    #Input d.c power, W

#Result
print("The a.c output power= %.1fW."%P_ac);
print("The d.c input power= %.1fW."%P_dc);

The a.c output power= 7.5W.
The d.c input power= 12.5W.


## Example 12.21 : Page number 329-330¶

In :
#Variable declaration
VCC=10.0;               #Supply voltage, V

#Calculation
IC_sat=(VCC/(2*RL))*1000;          #Saturated collector current, mA
VCE_off=VCC/2;                     #Collector-emitter voltage in off state, V

#Result
print("1st end point of a.c load line, IC(sat)=%dmA."%IC_sat);
print("2nd end point of a.c load line, VCE(off)=%dV."%VCE_off);

1st end point of a.c load line, IC(sat)=500mA.
2nd end point of a.c load line, VCE(off)=5V.

In [ ]: