EKT 204ANALOGUE ELECTRONICS CIRCUITS 1

Power AmplifiersSyllabusPower amplifier classification; class A, class B, class AB and class C, amplifier distortion, transistor power dissipation, thermal management.

Part IPower 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:

ParameterSmall-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 – 70fT (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 TRANSISTORThe 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 TRANSISTOREXAMPLE 8.1

Determine the required ratings (current, voltage and power) of the BJT.

– BJTPOWER TRANSISTOREXAMPLE 8.1 – Solution

For the maximum collector current;

0CEV

A 3824

max L

CCC R

VI

For the maximum collector-emitter voltage;

0CI

V 24max CCCE VV

– BJTPOWER TRANSISTOREXAMPLE 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 TRANSISTOREXAMPLE 8.1 – Solution

The maximum power occurs when

02 LCCC RIV

0C

T

dIdP

i.e. when

or when A 5.1CI

At this point; V 12 LCCCCE RIVV

and; W18 CCET IVP

Differentiating

– BJTPOWER TRANSISTOREXAMPLE 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.

Top view

Cross-sectional view