AB-039 Datasheet, PDF (1/6 Pages) Burr-Brown (TI) – POWER AMPLIFIER STRESS AND POWER HANDLING LIMITATIONS

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POWER AMPLIFIER STRESS
AND POWER HANDLING LIMITATIONS
BY BRUCE TRUMP [phone]
To achieve reliable power amplifier designs you must con-
sider the stress on the amplifier compared to its power
handling limitations. Power handling limits are specified by
the Safe Operating Area (SOA) curves of the power amp.
Stress on the amplifier depends on amplifier load and signal
conditions which can be evaluated with straightforward
techniques.
Consider the simplified power op amp shown in Figure 1.
Output transistors Q₁ and Q₂ provide positive and negative
output current to the load. I_OUT is shown flowing out of the
amplifier, so Q₁ is supplying the output current. For positive
output current, Q₂ is “off” and can be ignored.
V+
R_I
R_F
Q₁
+
V_CE
–
V_CE = (V+) –V_O
I_OUT
V_IN
Q₂
V_O
R_L
The stress on Q₁ under load is related to the output current
and the voltage across Q₁ (its collector-to-emitter voltage,
V_CE). The product of these quantities, I_OUT • V_CE, is the power
dissipation of Q₁. This power dissipation is one important
consideration, but the “safe operating area” provides a more
complete description of the amplifier’s limits.
V–
FIGURE 1. Simplified Power Op Amp Circuit.
SAFE OPERATING AREA
The power handling ability of a power transistor is charac-
terized by its Safe Operating Area (SOA), Figure 2. The
SOA curve shows permissible voltage, (V_CE) and current,
(I_OUT). The maximum safe current is a function of V_CE. The
characteristic shape of this curve has four distinct regions.
At low V_CE, maximum output current can be safely delivered
to the load. Exceeding the maximum current in this region
can overstress wire bonds or metallization on the die and
destroy the device.
As V_CE is increased, the power dissipation of the transistor
increases until self-heating raises the junction temperature to
its maximum safe value. All points along this thermally
limited region (dotted lines) produce the same power dissi-
pation. V_CE • I_O is a constant 120W (at 25°C) in Figure 2. All
points on this region of the curve produce the same maxi-
mum junction temperature. Exceeding the safe output cur-
rent in this region may damage the transistor junction.
Max Current
Thermal
SAFE OPERATING AREA
Limits
10
T_C = +25°C
5.0
T_C = +85°C
T_C = +125°C
2.0
1.0
Second
Breakdown
Region
0.5
0.2
0.1
1
Thermal Limitation
(T_J = 200°C)
Second Breakdown
Limited
2
5
10
20
V_CE |V_S – V_OUT| (V)
FIGURE 2. Safe Operating Area (SOA)— OPA502. (Figure 2 in PDS-1166)
Voltage
Breakdown
50
100
©_{1993 Burr-Brown Corporation}
AB-039
Printed in U.S.A. April, 1993