Datasheet

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SBOS156B − MARCH 1987 − REVISED APRIL 2008
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9
CAPACITIVE LOADS
The dynamic characteristics of the OPA445 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 3
shows a circuit which preserves phase margin with
capacitive load. The circuit does not suffer a voltage drop
due to load current; however, input impedance is reduced
at high frequencies. Consult Application Bulletin
SBOA015, available for download at www.ti.com, for
details of analysis techniques and application circuits.
V
O
C
L
5000pF
R
2
R
1
G=1+
R
2
2k
Ω
R
1
2k
Ω
V
IN
NOTE: Design equations and component values are approximate.
User adjustment is required for optimum performance.
R
C
20Ω
C
C
0.22
µ
F
R
C
=
R
2
2C
L
×
10
10
−
(1 + R
2
/R
1
)
OPA445
C
C
=
C
L
×
10
3
R
C
Figure 3. Driving Large Capacitive Loads
INCREASING OUTPUT CURRENT
In those applications where the 15mA of output current is
not sufficient to drive the required load, output current can
be increased by connecting two or more OPA445s in
parallel as shown in Figure 4. Amplifier A1 is the master
amplifier and may be configured in virtually any op amp
circuit. Amplifier A2, the slave, is configured as a unity gain
buffer. Alternatively, external output transistors can be
used to boost output current. The circuit in Figure 5 is
capable of supplying output currents up to 1A.
R
1
R
2
OPA445
OPA445
Slave
Master
V
IN
R
S
(1)
10Ω
R
S
(1)
10
Ω
R
L
NOTE: (1) R
S
resistors minimize the circulating
current that will always flow between the two devices
due to V
OS
errors.
Figure 4. Parallel Amplifiers Increase Output
Current Capability
R
1
R
2
OPA445
TIP30C
TIP29C
V
IN
+45V
−
45V
V
O
R
3
(1)
100
Ω
NOTE: (1) Provides current limit for OPA445 and allows the amplifier to
drive the load when the output is between +0.7V and
−
0.7V.
R
4
0.2
Ω
R
4
0.2
Ω
LOAD
C
F
Figure 5. External Output Transistors Boost Output Current Up to 1 Amp