SECTION 1 SINGLE-SUPPLY AMPLIFIERS - Analog Devices

OP AMP OUTPUT STAGES USINGCOMPLEMENTARY DEVICES ALLOW PUSH-PULL DRIVEFigure 1.6An IC process with relatively well-matched (AC and DC) PNP and NPN transistorsallows both the output voltage swing and slew rate to be reasonably well matched.However, an output stage using BJTs cannot swing completely to the rails, but onlyto within the transistor saturation voltage (V CESAT ) of the rails (see Figure 1.7).For small amounts of load current (less than 100µA), the saturation voltage may beas low as 5 to 10mV, but for higher load currents, the saturation voltage canincrease to several hundred mV (for example, 500mV at 50mA).On the other hand, an output stage constructed of CMOS FETs can provide truerail-to-rail performance, but only under no-load conditions. If the output mustsource or sink current, the output swing is reduced by the voltage dropped acrossthe FETs internal "on" resistance (typically, 100ohms).9