OP191/OP291/OP491

Rev. D | Page 18 of 24

INPUT OVERVOLTAGE PROTECTION

As with any semiconductor device, whenever the condition

exists for the input to exceed either supply voltage, check the

input overvoltage characteristic. When an overvoltage occurs,

the amplifier could be damaged depending on the voltage level

and the magnitude of the fault current. Figure 62 shows the

characteristics for the OP191 family. This graph was generated

with the power supplies at ground and a curve tracer connected

to the input. When the input voltage exceeds either supply by

more than 0.6 V, internal PN junctions energize, allowing

current to flow from the input to the supplies. As described, the

OP291/OP491 do have 5 kΩ resistors in series with each input

to help limit the current. Calculating the slope of the current vs.

voltage in the graph confirms the 5 kΩ resistor.

+2mA

+1mA

–1mA

–2mA

–5V

+10V

–10V

+5V

Figure 62. Input Overvoltage Characteristics

This input current is not inherently damaging to the device as

long as it is limited to 5 mA or less. For an input of 10 V over

the supply, the current is limited to 1.8 mA. If the voltage is

large enough to cause more than 5 mA of current to flow, then

an external series resistor should be added. The size of this

resistor is calculated by dividing the maximum overvoltage by

5 mA and subtracting the internal 5 kΩ resistor. For example, if

the input voltage could reach 100 V, the external resistor should

be (100 V/5 mA) − 5 kΩ = 15 kΩ. This resistance should be

placed in series with either or both inputs if they are subjected

to the overvoltages.

OUTPUT VOLTAGE PHASE REVERSAL

Some operational amplifiers designed for single-supply

operation exhibit an output voltage phase reversal when their

inputs are driven beyond their useful common-mode range.

Typically, for single-supply bipolar op amps, the negative supply

determines the lower limit of their common-mode range.

With these devices, external clamping diodes with the anode

connected to ground and the cathode to the inputs prevent

input signal excursions from exceeding the device’s negative

supply (that is, GND), preventing a condition that could cause

the output voltage to change phase. JFET input amplifiers can

also exhibit phase reversal, and, if so, a series input resistor is

usually required to prevent it.

The OP191 is free from reasonable input voltage range

restrictions due to its novel input structure. In fact, the input

signal can exceed the supply voltage by a significant amount

without causing damage to the device. As shown in Figure 64,

the OP191 family can safely handle a 20 V p-p input signal on

±5 V supplies without exhibiting any sign of output voltage

phase reversal or other anomalous behavior. Thus, no external

clamping diodes are required.

OVERDRIVE RECOVERY

The overdrive recovery time of an operational amplifier is the

time required for the output voltage to recover to its linear

region from a saturated condition. This recovery time is

important in applications where the amplifier must recover

quickly after a large transient event, such as a comparator. The

circuit shown in Figure 63 was used to evaluate the OPx91

overdrive recovery time. The OPx91 takes approximately 8 μs to

recover from positive saturation and approximately 6.5 μs to

recover from negative saturation.

+

–

1/2

OP291

3

2

1

R1

9kΩ

R2

10kΩ

R3

10kΩ

10V STEP

Figure 63. Overdrive Recovery Time Test Circuit

10

90

100

0%

TIME (200µs/DIV)

10

90

100

0%

TIME (200µs/DIV)

20mV

20mV

5µs

5µs

+5V

–5V

20V p-p

8

1

3

2

4

1/2

OP291

+

–

Figure 64. Output Voltage Phase Reversal Behavior