OPA445SM Datasheet, PDF (9/25 Pages) Texas Instruments – High Voltage FET-Input OPERATIONAL AMPLIFIER

OPA445
www.ti.com
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.
R₁
R₂
2kΩ
2kΩ
R_C
20Ω
C_C
0.22µF
V_IN
OPA445
R_C =
R₂
2C_L × 10¹⁰ − (1 + R₂/R₁)
G = 1 + ^R2
R₁
V_O
C_L
5000pF
C_C =
C_L × 10³
R_C
NOTE: Design equations and component values are approximate.
User adjustment is required for optimum performance.
Figure 3. Driving Large Capacitive Loads
SBOS156B − MARCH 1987 − REVISED APRIL 2008
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.
R₁
V_IN
R₂
Master
OPA445
R_S(1)
10Ω
OPA445
Slave
R_S(1)
10Ω
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
R₁
R₂
+45V
TIP29C
C_F
R₄
R₃₍₁₎
0.2Ω
100Ω
OPA445
V_IN
R₄
0.2Ω
V_O
LOAD
TIP30C
−45V
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.
Figure 5. External Output Transistors Boost Output Current Up to 1 Amp
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