can be used to boost output current. The circuit in Figure 10
is capable of supplying output currents up to 1A. Alterna-
tively, the OPA547, OPA548, and OPA549 series power op
amps should be considered for high output current drive,
along with programmable current limit and output disable
capability.
CAPACITIVE LOADS
The dynamic characteristics of the OPA551 and OPA552
have been optimized for commonly encountered gains, loads,
and operating conditions. The combination of low closed-
loop gain and capacitive load will decrease the phase margin
and may lead to gain peaking or oscillations. Figure 7 shows
a circuit that preserves phase margin with capacitive load.
Figure 8 shows the small-signal step response for the circuit
in Figure 7. Consult Application Bulletin AB-028 for more
information.
R1
R2
(1)
RS
“MASTER”
+30V
10Ω
OPA551
VIN
OPA551
(1)
RS
10nF
RG
4kΩ
RF
4kΩ
10Ω
OPA551
VI
CS
1.8nF
CF
220pF
RL
“SLAVE”
–30V
NOTE: (1) RS resistors minimize the circulating
current that can flow between the two devices
due to VOS errors.
FIGURE 7. Driving Large Capacitive Loads.
SMALL-SIGNAL STEP RESPONSE
FIGURE 9. Parallel Amplifers Increase Output Current Ca-
pability.
OPA551, G = –1, CL = 10nF
OPA551
R1
R2
+30V
TIP29C
CF
R4
0.2Ω
(1)
R3
100Ω
Time (2.5µs/div)
VO
OPA551
VIN
R4
0.2Ω
FIGURE 8. Small-Signal Step Response for Figure 7.
LOAD
TIP30C
INCREASING OUTPUT CURRENT
In those applications where the 200mA of output current is
not sufficient to drive the desired load, output current can be
increased by connecting two or more OPA551s or OPA552s
in parallel as shown in Figure 9. Amplifier A1 is the
“master” amplifier and may be configured in virtually an op
amp circuit. Amplifier A2, the “slave”, is configured as a
unity gain buffer. Alternatively, external output transistors
–30V
NOTE: (1) R3 provides current limit and allows the amplifier to
drive the load when the output is between 0.7V and –0.7V.
FIGURE 10. External Output Transistors Boost Output Cur-
rent Up to 1 Amp.
OPA551, OPA552
12
SBOS100A
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