ample, we assume a lubricated mica insulator between the

LM675 and the heat sink. The heat sink thermal resistance

must then be less than

4.2˚C/W − 2˚C/W − 1.6˚C/W = 0.6˚C/W.

This is a rather large heat sink and may not be practical in

some applications. If a smaller heat sink is required for rea-

sons of size or cost, there are two alternatives. The maxi-

mum ambient operating temperature can be restricted to

50˚C (122˚F), resulting in a 1.6˚C/W heat sink, or the heat

sink can be isolated from the chassis so the mica washer is

not needed. This will change the required heat sink to a

1.2˚C/W unit if the case-to-heat-sink interface is lubricated.

The thermal requirements can become more difficult when

an amplifier is driving a reactive load. For a given magnitude

of load impedance, a higher degree of reactance will cause

a higher level of power dissipation within the amplifier. As a

general rule, the power dissipation of an amplifier driving a

60˚ reactive load will be roughly that of the same amplifier

driving the resistive part of that load. For example, some re-

active loads may at some frequency have an impedance

with a magnitude of 8

Ω and a phase angle of 60˚. The real

part of this load will then be 8

Ω x cos 60˚ or 4Ω, and the am-

plifier power dissipation will roughly follow the curve of power

dissipation with a 4

Ω load.

Typical Applications

Non-Inverting Unity Gain Operation

DS006739-6

Inverting Unity Gain Operation

DS006739-7

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