EKT 204

ANALOGUE ELECTRONICS CIRCUITS 1

Power Amplifiers

SyllabusPower amplifier classification; class A, class B, class

AB and class C, amplifier distortion, transistor power

dissipation, thermal management.

Part I

Power Transistor –

BJT & MOSFET

POWER AMPLIFIERS

POWER TRANSISTOR

Transistor limitations

• Maximum rated current,

• Maximum rated voltage,

• Maximum rated power.

The maximum rated power is related to the maximum

allowable temperature of the transistor.

– BJT

Large-area devices – the geometry and doping

concentration are different from those of small-signal

transistors

Examples of BJT rating:

Parameter

Small-signal

BJT

(2N2222A)

Power BJT

(2N3055)

Power BJT

(2N6078)

VCE (max) (V) 40 60 250

IC (max) (A) 0.8 15 7

PD (max) (W) 1.2 115 45

35 – 100 5 – 20 12 – 70

fT (MHz) 300 0.8 1

POWER TRANSISTOR

Current gain depends on IC and is smaller in power BJT.

The maximum rated collector current, IC(rated) may be

related to the following:

1. maximum current that the wires connecting the

semiconductor to the external terminals can handle

2. The collector current at which the gain falls below a

minimum specified value

3. current which leads to maximum power dissipation

when the transistor is in saturation.

– BJTPOWER TRANSISTOR

Typical dc beta

characteristics

( hFE versus IC)

for 2N3055

– BJTPOWER TRANSISTOR

The maximum voltage limitation:

• Avalanche breakdown in the reverse-biased base-

collector junction (involves gain and breakdown at the

p-n junction)

• Second breakdown – nonuniformities in current

density which inreases temperature in local regions in

semiconductor.

– BJTPOWER TRANSISTOR

Avalanche Breakdown (Figure 1)

• In Figure 1, the breakdown voltage when the base

terminal is open-circuited (IB=0) is VCEO, approx. 130V

(Figure 1).

• All the curves tend to merge to the same collector-

emitter voltage, denoted as VCE(sus) once breakdown

has occurred.

• VCE(sus) is the voltage necessary to sustain the

transistor in breakdown.

• In Figure 1, VCE(sus) is approx. 115V

– BJTPOWER TRANSISTOR

– BJTPOWER TRANSISTOR

IC–VCE characteristics

showing breakdown

effect

Figure 1

– BJTPOWER TRANSISTOR

BBECCEQ ivivp

The second term is usually small, hence;

CCEQ ivp

The average power over ONE CYCLE of the signal:

T

CCEQ dtivT

P0

1

The total instantaneous power dissipation in transistor

– BJTPOWER TRANSISTOR

The average power dissipated in a BJT must be kept below

a specified maximum value to ensure that the temperature

of the device does not exceed the maximum allowable

value.

If collector current and collector-emitter voltage are dc

quantities, the maximum rated power, PT

CCET IVP

The power handling ability of a BJT is limited by two factors,

i.e. junction temperature, TJ and second breakdown. Safe

Operating Area (SOA) must be observed, i.e. do not exceed

BJT power dissipation.

– BJTPOWER TRANSISTOR

The safe operating area (SOA) is bounded by IC(max); VCE(sus)

and maximum rated power curve, PT and the transistor’s

second breakdown characteristics curve (Figure 2)

SOA of a BJT

(linear scale)

Figure 2

– BJTPOWER TRANSISTOR

SOA of a BJT

(log scale)

Figure 3

– BJTPOWER TRANSISTOR

EXAMPLE 8.1

Determine the required ratings

(current, voltage and power) of

the BJT.

– BJTPOWER TRANSISTOR

EXAMPLE 8.1 – Solution

For the maximum

collector current;0CEV

A 38

24max

L

CCC

R

VI

For the maximum collector-

emitter voltage;0CI

V 24max CCCE VV

– BJTPOWER TRANSISTOR

EXAMPLE 8.1 – Solution

The load line equation

is;

The load line must lie

within the SOA

LCCCCE RIVV

The transistor power

dissipation;

LCCCCCLCCCCCET RIIVIRIVIVP 2

– BJTPOWER TRANSISTOR

EXAMPLE 8.1 – Solution

The maximum power occurs when

02 LCCC RIV

0C

T

dI

dP

i.e. when

or when A 5.1CI

At this point; V 12 LCCCCE RIVV

and; W18 CCET IVP

Differentiating

– BJTPOWER TRANSISTOR

EXAMPLE 8.1 – Solution

Thus the transistor ratings are;

W18

V 24

A 3

max

max

T

CE

C

P

V

I

In practice, to find a suitable transistor for a given

application, safety factors are normally used. The

transistor with

will be required.

W18 V, 24 A, 3 maxmax TCEC PVI

– BJTPOWER TRANSISTOR

Physical structure;

• Large emitter area to

handle large current

densities

• Narrow emitter width to

minimize parasitic base

resistance

• May include small

resistors (ballast resistor)

in emitter leg to help

maintain equal currents

in each B–E junction.

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