OPA452 Datasheet, PDF (12/17 Pages) Texas Instruments – 80V, 50mA OPERATIONAL AMPLIFIERS

INCREASING OUTPUT CURRENT
In those applications where the 50mA of output current is not
sufficient to drive the desired load, output current can be
increased by connecting two or more OPA452s or OPA453s
in parallel, as shown in Figure 7. 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 8 is capable of
supplying output currents up to 1A. Alternatively, the OPA547,
OPA548, and OPA549 series power op amps should be
considered for high output current drive, along with program-
mable current limit and output disable capability.
R₁
V_IN
R₂
“MASTER”
OPA452
R_S(1)
10Ω
OPA452
R_S(1)
10Ω
“SLAVE”
R_L
NOTE: (1) R_S resistors minimize the circulating
current that can flow between the two devices
due to V_OS errors.
FIGURE 7. Parallel Amplifiers Increase Output Current Ca-
pability.
R₁
R₂
+40V
TIP29C
C_F
R₄
R₃₍₁₎
0.2Ω
100Ω
OPA452
V_IN
V_O
R₄
0.2Ω
LOAD
TIP30C
–40V
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 8. External Output Transistors Boost Output Cur-
rent Up to 1 Amp.
INPUT PROTECTION
The OPA452 and OPA453 feature internal clamp diodes to
protect the inputs when voltages beyond the supply rails are
encountered. However, input current should be limited to
5mA. In some cases, an external series resistor may be
required. Many input signals are inherently current-limited,
therefore, a limiting resistor may not be required. Please
consider that a large series resistor, in conjunction with the
input capacitance, can affect stability.
USING THE OPA453 IN LOW GAINS
The OPA453 is intended for applications with signal gains of
5 or greater, but it is possible to take advantage of its high
slew rate in lower gains using an external compensation
technique in an inverting configuration. This technique main-
tains low noise characteristics of the OPA453 architecture at
low frequencies. Depending on the application, a small in-
crease in high-frequency noise may result. This technique
shapes the loop gain for good stability while giving an easily
controlled 2nd-order low-pass frequency response.
Considering only the noise gain (noninverting signal gain) for
the circuit of Figure 9, the low-frequency noise gain (NG₁) will
be set by the resistor ratios, whereas the high-frequency
noise gain (NG₂) will be set by the capacitor ratios. The
capacitor values set both the transition frequencies and the
high-frequency noise gain. If this noise gain, determined by
NG₂ = 1 + C_S/C_F, is set to a value greater than the recom-
mended minimum stable gain for the op amp and the noise
gain pole, set by 1/R_FC_F, is placed correctly, a very well
controlled, 2nd-order low-pass frequency response will result.
To choose the values for both C_S and C_F, two parameters
and only three equations need to be solved. First, the target
for the high-frequency noise gain (NG₂) should be greater
than the minimum stable gain for the OPA453. In the circuit
in Figure 9, a target NG₂ of 10 is used. Second, the signal
gain of –1 in Figure 10 sets the low-frequency noise gain to
NG₁ = 1 + R_F/R_G (= 2 in this example). Using these two gains,
knowing the Gain Bandwidth Product (GBP) for the OPA453
(7.5MHz), and targeting a maximally flat 2nd-order, low-pass
Butterworth frequency response (Q = 0.707), the key fre-
quency in the compensation can be found.
For the values in Figure 9, the f_–3dB will be approximately
180kHz. This is less than that predicted by simply dividing the
GBP by NG₁. The compensation network controls the band-
width to a lower value while providing good slew rate at the
output and an exceptional distortion performance due to
increased loop gain at frequencies below NG₁ • Z₀. The
capacitor values in Figure 10 are calculated for NG₁ = 2 and
NG₂ = 10 with no adjustment for parasitics.
Actual circuit values can be optimized by checking the small-
signal step response with actual load conditions. See Figure 9
for the small-signal step response of this OPA453, G = –1
circuit with a 1000pF load. It is well-behaved with no tendency
to oscillate. If C_S and C_F were removed, the circuit would be
unstable.
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OPA452, 453
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