INPUT STAGE OVERVOLTAGE INPUT SHOULD NOT EXCEED ABSOLUTE MAXIMUM RATINGS (Usually Specified With Respect to Supply Voltages) A Common Specification Requires the Input Signal <Vs+0.3v Input voltage should be held Near Zero in the absence of Input Stage Conduction Current Needs to be Limited( Rule of Thumb: s 5mA) Avoid Reverse Bias Junction Breakdown in Input Stage Base Emitter junctions Differential and Common-Mode Ratings may Differ No Two Amplifiers are exactly the Same Some Op Amps Contain Input Protection(Voltage Clamps Current Limits, or Both), but Absolute Maximum Ratings Must Still be observed Figure 7.1 Although more recent vintage operational amplifiers designed for single-supply rail-to-rail operation are now including information with regard to input stage overvoltage effects, there are very many amplifiers available today without such the device in question before steps can be taken to protect it. All amplifiers il or a information provided by the manufacturer. In those cases, the circuit designer usir these components must ascertain the input stage current-voltage characteristic conduct current to the positive/negative supply, provided the applied input voltage exceeds some internal threshold. This threshold is device dependent, and can rang from 0. 7V to 30V, depending on the internal construction of the input stage Regardless of the threshold level, externally generated fault currents should be limited to no more than + 5mA Many factors contribute to the current-voltage characteristic of an amplifiers input stage: internal differential clamping diodes, current-limiting series resistances substrate potential connections, and differential input stage topologiesBjTs or s) Input protection diodes used as differential input clamps are typically constructed from the base-emitter junctions of NPn transistors. These diodes usually form a parasitic p-n-junction to the negative supply when the applied input voltage exceeds the negative supply Current-limiting series resistances used in the input stages of operational amplifiers can be fabricated from three types of material thin-film resistors are fabricated over thin layers of oxide which provide ap on and n-or p-type diffusions, polysilicon, or thin-films(SiCr, for example). polysilic junctions to either supply. Diffused resistors, on the other hand exhibit p-l o-n insulating barrier to the substrate; as such, they do not exhibit any parasitic3 INPUT STAGE OVERVOLTAGE INPUT SHOULD NOT EXCEED ABSOLUTE MAXIMUM RATINGS (Usually Specified With Respect to Supply Voltages) A Common Specification Requires the Input Signal <|Vs | 0.3V Input Voltage Should be Held Near Zero in the Absence of Supplies Input Stage Conduction Current Needs to be Limited (Rule of Thumb: 5mA) Avoid Reverse Bias Junction Breakdown in Input Stage Base￾Emitter Junctions Differential and Common-Mode Ratings may Differ No Two Amplifiers are exactly the Same Some Op Amps Contain Input Protection (Voltage Clamps, Current Limits, or Both), but Absolute Maximum Ratings Must Still be Observed Figure 7.1 Although more recent vintage operational amplifiers designed for single-supply or rail-to-rail operation are now including information with regard to input stage overvoltage effects, there are very many amplifiers available today without such information provided by the manufacturer. In those cases, the circuit designer using these components must ascertain the input stage current-voltage characteristic of the device in question before steps can be taken to protect it. All amplifiers will conduct current to the positive/negative supply, provided the applied input voltage exceeds some internal threshold. This threshold is device dependent, and can range from 0.7V to 30V, depending on the internal construction of the input stage. Regardless of the threshold level, externally generated fault currents should be limited to no more than ±5mA. Many factors contribute to the current-voltage characteristic of an amplifier’s input stage: internal differential clamping diodes, current-limiting series resistances, substrate potential connections, and differential input stage topologies (BJTs or FETs). Input protection diodes used as differential input clamps are typically constructed from the base-emitter junctions of NPN transistors. These diodes usually form a parasitic p-n-junction to the negative supply when the applied input voltage exceeds the negative supply. Current-limiting series resistances used in the input stages of operational amplifiers can be fabricated from three types of material: n- or p-type diffusions, polysilicon, or thin-films (SiCr, for example). Polysilicon and thin-film resistors are fabricated over thin layers of oxide which provide an insulating barrier to the substrate; as such, they do not exhibit any parasitic p-n junctions to either supply. Diffused resistors, on the other hand, exhibit p-n