bias point on the amplifier’s input terminal. The resulting

-3dB frequency response due to the combination of C

R

f

πR

(5)

With large values of R

outputs when the inputs are left floating. Decreasing the

value of R

oscillations. If the value of R

C

frequency response.

AVOIDING THERMAL RUNAWAY WHEN USING

BIPOLAR OUTPUT STAGES

When using a bipolar output stage with the LM4702 (as in

Figure 1), the designer must beware of thermal runaway.

Thermal runaway is a result of the temperature dependence

of Vbe (an inherent property of the transistor). As tempera-

ture increases, Vbe decreases. In practice, current flowing

through a bipolar transistor heats up the transistor, which

lowers the Vbe. This in turn increases the current again, and

the cycle repeats. If the system is not designed properly, this

positive feedback mechanism can destroy the bipolar tran-

sistors used in the output stage.

One of the recommended methods of preventing thermal

runaway is to use a heat sink on the bipolar output transis-

tors. This will keep the temperature of the transistors lower.

A second recommended method is to use emitter degenera-

tion resistors (see Re1, Re2, Re3, Re4 in Figure 1). As

current increases, the voltage across the emitter degenera-

tion resistor also increases, which decreases the voltage

across the base and emitter. This mechanism helps to limit

the current and counteracts thermal runaway.

A third recommended method is to use a “Vbe multiplier” to

bias the bipolar output stage (see Figure 1). The Vbe multi-

plier consists of a bipolar transistor (Qmult, see Figure 1)

and two resistors, one from the base to the collector (Rb2,

Rb4, see Figure 1) and one from the base to the emitter

(Rb1, Rb3, see Figure 1). The voltage from the collector to

the emitter (also the bias voltage of the output stage) is

Vbias = Vbe(1+Rb2/Rb1), which is why this circuit is called

the Vbe multiplier. When Vbe multiplier transistor (Qmult,

see Figure 1) is mounted to the same heat sink as the bipolar

output transistors, its temperature will track that of the output

transistors. Its Vbe is dependent upon temperature as well,

and so it will draw more current as the output transistors heat

it up. This will limit the base current into the output transis-

tors, which counteracts thermal runaway.

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