To be within a linear safe operating area, an amplifier is required to either limit the maximum linear output current drive or the maximum internal power dissipation (P_D(MAX)) to prevent the device from exceeding the maximum junction temperature (T_J(MAX)) of 150°C. This limit is usually represented by a linear safe operating area (SOA) graph that defines the operational boundary by plotting the linear output current drive (I_OUT) on the y-axis and the voltage developed across the output transistor (V_cc – V_OUT) on the x-axis.

As Figure 2-14 shows, the upper bound of the safe operating area is defined by the maximum linear output current drive of the amplifier (420 mA) starting from the left of the x-axis. When the voltage developed across the output transistor (V_cc – V_OUT) increases towards the right of the x-axis, the linear output current reduces to maintain a fixed internal power dissipation across the output transistors. The fixed internal power dissipation depends on the heat sink used, and is derived by Equation 25 to keep the T_J(MAX) below 150°C for a given ambient temperature (T_A). The linear output current is derived from Equation 26 by sweeping (V_cc – V_OUT) from right to left on the x-axis until the I_OUT equals the maximum linear output current drive of 420 mA. The (V_cc – V_OUT) on the x-axis is equally applicable for (V_ee – V_OUT), depending upon the output source or sink current cycle for a sinusoidal signal (see Figure 2-8).