®
APPLICATION BULLETIN
POWER AMPLIFIER STRESS
AND POWER HANDLING LIMITATIONS
Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706
Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132
BY BRUCE TRUMP (602) 746-7347
To achieve reliable power amplifier designs you must con-
sider the stress on the amplifier compared to its power
R
I
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 Q
1
and Q
2
provide positive and negative
output current to the load. I
OUT
is shown flowing out of the
amplifier, so Q
1
is supplying the output current. For positive
output current, Q
2
is “off” and can be ignored.
The stress on Q
1
under load is related to the output current
and the voltage across Q
1
(its collector-to-emitter voltage,
V
CE
). The product of these quantities, I
OUT
• V
CE
, is the power
dissipation of Q
1
. This power dissipation is one important
consideration, but the “safe operating area” provides a more
complete description of the amplifier’s limits.
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.
Max Current
SAFE OPERATING AREA
10
T
C
= +25°C
5.0
T
C
= +85°C
T
C
= +125°C
2.0
I
O
(A)
V+
R
F
Q
1
+
V
CE
–
V
CE
= (V+) –V
O
I
OUT
V
O
V
IN
Q
2
R
L
V–
FIGURE 1. Simplified Power Op Amp Circuit.
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.
Thermal
Limits
t=
1m
t=
ms
0.5
s
Second
Breakdown
Region
t=
5m
s
1.0
0.5
Thermal Limitation
(T
J
= 200°C)
0.2
0.1
1
2
5
10
20
50
100
V
CE
|V
S
– V
OUT
| (V)
Second Breakdown
Limited
Voltage
Breakdown
FIGURE 2. Safe Operating Area (SOA)— OPA502. (Figure 2 in PDS-1166)
©
1993 Burr-Brown Corporation
AB-039
Printed in U.S.A. April, 1993
BURR-BROWN [ BURR-BROWN CORPORATION ]
