 These pages are meant  mainly for engineering students studying in second year( Mumbai university only). If you are from some other university then confirm with your professors whether these formulae/methods are valid for your university.

# Design of power amplifiers

Class A amplifier
Class B amplifier

Do not write text included in square bracket

## 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

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]

### Step 3: Selection of Re & Ce

Re = Vre/Icq
[calculate Re]

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

[If stability factor not given] assume s = 10

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' = Vce peak/Ic peak
[calculate RL']

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]

Max o/p power = (Vce peak * Ic peak)/2

### step 7: Calculation of  efficiency

Full load efficiency, n FL = (PL' FL)/(Pi dc)

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

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') = 1/5
[calculate Pq max]

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

### Step 2: Selection of operating point

Select Vcc such that
(Vceo/2) <= Vcc <= Vceo
[select Vcc = 25 V for ECN 149]

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

Re = Vre/Icq
[Calculate Re. Select nearest std value]

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

[If stability factor not given] assume s = 10

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' = Vce peak/Ic peak
[calculate RL']

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]

Max o/p power = (Vce peak * Ic peak)/2

### step 7: Calculation of  efficiency

Full load efficiency, n FL = (PL' FL)/(Pi dc FW)

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