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Design of power amplifiers

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Design of class A power amplifier

Step 1: Selection of transistor

Power transmitted to load, PL' = PL/nt                  [nt = efficiency of transformer]
[If nt is not given assume nT = 90% or 0.9]

Q = (Pq max)/PL') = 2
[calculate Pq max]

Select transistor with Pd > 2 * Pq max
[Usually select ECN 149]
 

Step 2: Selection of operating point

Vceq = Vcc - Vre
[calculate Vceq]

Vce peak = Vceq - Vce sat
[calculate Vce peak]

Ic peak = (2 * PL')/Vce peak
[calculate Ic peak]

Icq = Ic peak + Icmin
Assume Ic min = 0
[calculate Icq]
 

Step 3: Selection of Re & Ce

Pre = sqr(Vre)/Re
[Select power rating of Re > (2 * Pre). Specify power rating as multiple of 0.25 W]

Ce = 1/(2 * pi * FL * RL)
[If FL is not given assume FL = 50 Hz]

Since Ce is very high  we leave Re unbypassed
 

Step 4: Selection of R1 & R2

s = (1 + hfe max)/(1 + ((hfe max * Re )/(Rb + Re))

Find Rb[Do not standardise]

Vr2 = Vbe + (Icq * Re)
Vr1 = Vcc - Vr2

Assume Vbe = 0.6V [for Si, 0.3 for Ge if not specified]

R1/R2 = Vr1/Vr2  .............(A)
[Get R1 in terms of R2 & substitute in Rb]

Rb = R1 parallel R2 = (R1 * R2)/(R1 + R2)
Find R2
Select lower standard value to make circuit indepent of beta

Substitute in (A) to find R1
Select higher standard value so that circuit draws minimum current from supply
 

Step 5: Selection of  output transformer

RL' = (sqr(N1/N2)) * RL
[calculate  (N1/N2)]

Select audio frequency transformer with turns ratio 1:(N1/N2)
[Draw the circuit diagram with calculated values]
 

step 6: Calculation of o/p power[If required]

step 7: Calculation of  efficiency

PL' FL = (Vce peak * Ic peak)/2

Pi dc = (Vcc * Icq) + (Vcc ^ 2)/(R1 + R2)
 
 

Half load efficiency, n HL = (PL' HL)/(Pi dc)

PL' HL  = PL' FL / 2

Max power dissipation = power dissipation at no signal = Pd max of transistor

Design of class B power amplifier

Step 1: Selection of transistor

Q = (Pq max)/PL') = 1/5
[calculate Pq max]

Select transistor with Pd > 2 * Pq max
[Usually select ECN 149]
 

Step 2: Selection of operating point

Vre = Vcc/10
[calculate Vre]

Vceq = Vcc - Vre
[calculate Vceq]

Vce peak = Vceq - Vce sat
[calculate Vce peak]

Ic peak = (2 * PL')/Vce peak
[calculate Ic peak]

Icq = Ic peak + Icmin
Assume Ic min = 0
[calculate Icq]

Idc full wave = (2 * Idc peak)/pi

Idc half wave = Idc peak/pi

Step 3: Selection of Re & Ce

Pre = sqr(Vre)/Re
[Select power rating of Re > (2 * Pre). Specify power rating as multiple of 0.25 W]

Ce = 1/(2 * pi * FL * RL)
[If FL is not given assume FL = 50 Hz. Select higher std value]
 

Step 4: Selection of R1 & R2

s = (1 + hfe max)/(1 + ((hfe max * Re )/(Rb + Re))

Find Rb[Do not standardise]

Vr2 = Vbe + (Idc half wave * Re)

Vr1 = Vcc - Vr2

  Assume Vbe = 0.6V [for Si]

R1/R2 = Vr1/Vr2  .............(A)
[Get R1 in terms of R2 & substitute in Rb]

Rb = R1 parallel R2 = (R1 * R2)/(R1 + R2)
Find R2
Select lower standard value to make circuit indepent of beta

Substitute in (A) to find R1
Select higher standard value so that circuit draws minimum current from supply
 

Step 5: Selection of  output transformer

RL' = (sqr(N1/N2)) * RL
[calculate  (N1/N2)]

Power rating of primary  > PL'

Select audio frequency transformer with turns ratio 1:(N1/N2)
[Draw the circuit diagram with calculated values]
 

step 6: Calculation of o/p power[If required]

step 7: Calculation of  efficiency

PL' FL = (Vce peak * Ic peak)/2

Pi dc FL = (Vcc * Idc FW) + (Vcc ^ 2)/(R1 + R2)
 
 

Half load efficiency, n HL = (PL' HL)/(Pi dc HW)

Pi dc HW = (Vcc * Idc HW) + (Vcc ^ 2)/(R1 + R2)

PL' HL  = PL' FL / 2
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Standard values