forward-biasing of parasitic p-n junctions This occurs for input voltages within Sht where amplifier offset voltages and bias currents increase sharply due to the sligh approximately 1v of either supply rail When both differential pairs are active throughout the entire input common-mode range, amplifier transient response is faster through the middle of the common- mode range by as much as a factor of 2 for bipolar input stages and by a factor of the square root of 2 for FEt input stages. Input stage transconductance determines the slew rate and the unity-gain crossover frequency of the amplifier, hence response time degrades slightly at the extremes of the input common-mode range when either the PNP stage(signals approaching VpoS) or the NPN stage(signals approaching GND) are forced into cutoff. The thresholds at which the transconductance changes occur approximately within 1v of either supply rail, and the behavior is similar to that of the input bias current Applications which initially appear to require true rail-rail inputs should be carefully evaluated, and the amplifier chosen to ensure that its input offset voltage, input bias current, common-mode rejection, and noise(voltage and current)are suitable. a true rail-to-rail input amplifier should not generally be used if an input range which includes only one rail is satisfactory. SINGLE-SUPPLY/RAIL-TO-RAIL OP AMP OUTPUT STAGES The earliest IC op amp output stages were NPN emitter followers with NPn current sources or resistive pull-downs, as shown in Figure 1.6. Naturally, the slew rates were greater for positive-going than for negative- going signals. while all modern op amps have push-pull output stages of some sort, many are still asymmetrical, and have a greater slew rate in one direction than the other. This asymmetry, which generally results from the use of IC processes with better npn than PNP transistors, may also result in the ability of the output to approach one supply more closely than the other. In many applications, the output is required to swing only to one rail, usually the negative rail (i.e, ground in single-supply systems). A pulldown resistor to the negative rail will allow the output to approach that rail (provided the load impedance is high enough, or is also grounded to that rail), but only slowly. Using an FEt current source instead of a resistor can speed things up, but this adds complexity.8 where amplifier offset voltages and bias currents increase sharply due to the slight forward-biasing of parasitic p-n junctions. This occurs for input voltages within approximately 1V of either supply rail. When both differential pairs are active throughout the entire input common-mode range, amplifier transient response is faster through the middle of the common￾mode range by as much as a factor of 2 for bipolar input stages and by a factor of the square root of 2 for FET input stages. Input stage transconductance determines the slew rate and the unity-gain crossover frequency of the amplifier, hence response time degrades slightly at the extremes of the input common-mode range when either the PNP stage (signals approaching VPOS) or the NPN stage (signals approaching GND) are forced into cutoff. The thresholds at which the transconductance changes occur approximately within 1V of either supply rail, and the behavior is similar to that of the input bias currents. Applications which initially appear to require true rail-rail inputs should be carefully evaluated, and the amplifier chosen to ensure that its input offset voltage, input bias current, common-mode rejection, and noise (voltage and current) are suitable. A true rail-to-rail input amplifier should not generally be used if an input range which includes only one rail is satisfactory. SINGLE-SUPPLY/RAIL-TO-RAIL OP AMP OUTPUT STAGES The earliest IC op amp output stages were NPN emitter followers with NPN current sources or resistive pull-downs, as shown in Figure 1.6. Naturally, the slew rates were greater for positive-going than for negative-going signals. While all modern op amps have push-pull output stages of some sort, many are still asymmetrical, and have a greater slew rate in one direction than the other. This asymmetry, which generally results from the use of IC processes with better NPN than PNP transistors, may also result in the ability of the output to approach one supply more closely than the other. In many applications, the output is required to swing only to one rail, usually the negative rail (i.e., ground in single-supply systems). A pulldown resistor to the negative rail will allow the output to approach that rail (provided the load impedance is high enough, or is also grounded to that rail), but only slowly. Using an FET current source instead of a resistor can speed things up, but this adds complexity