Datasheet LT1006 (Analog Devices) - 9

LT1006
U U W U APPLICATIO S I FOR ATIO
The LT1006 is fully specified for single supply operation, Q4) and phase reversal occurs at the output. This can (i.e., when the negative supply is 0V). Input common cause lock-up in servo systems. Due to a unique phase mode range includes ground; the output swings within a reversal protection circuitry (Q21, Q22, Q27, Q28), the few millivolts of ground. Single supply operation, how- LT1006’s output does not reverse, as illustrated below, ever, can create special difficulties, both at the input and even when the inputs are at –1.5V. at the output. The LT1006 has specific circuitry which At the output, the aforementioned single supply designs addresses these problems. either cannot swing to within 600mV of ground (OP-20) At the input, the driving signal can fall below 0V— or cannot sink more than a few microamperes while inadvertently or on a transient basis. If the input is more swinging to ground (LM124, LM158). The LT1006’s than a few hundred millivolts below ground, two distinct all-NPN output stage maintains its low output resistance problems can occur on previous single supply designs, and high gain characteristics until the output is saturated. such as the LM124, LM158, OP-20, OP-21, OP-220, In dual supply operations, the output stage is crossover OP-221, OP-420: distortion free. a) When the input is more than a diode drop below ground, Since the output cannot go exactly to ground, but can only unlimited current will flow from the substrate (V– termi- approach ground to within a few millivolts, care should be nal) to the input. This can destroy the unit. On the LT1006, exercised to ensure that the output is not saturated. For the 400Ω resistors, in series with the input (see Schematic example, a 1mV input signal will cause the amplifier to set Diagram), protect the devices even when the input is 5V up in its linear region in the gain 100 configuration shown below ground. below, but is not enough to make the amplifier function b) When the input is more than 400mV below ground properly in the voltage follower mode. (at 25°C), the input stage saturates (transistors Q3 and
Voltage Follower with Input Exceeding the Negative Common Mode Range (VS = 5V, 0V)
4V 4V 4V 2V 2V 2V 0V 0V 0V 6VP-P INPUT, –1.5V TO 4.5V 1006 TA11a LM324, LM358, OP-20, OP-21 1006 TA11b LT1006 1006 TA11c EXHIBIT OUTPUT PHASE NO PHASE REVERSAL REVERSAL
Gain 100 Amplifier Voltage Follower
5V 5V R 99R – – OUTPUT LT1006 100mV LT1006 SATURATED ≈ 5mV 1mV + 1mV + 600Ω 600Ω LT1006 • TA02 LT1006 • TA03 1006fa 9