Wednesday, 22 August 2012

Special Devices

PROGRAMMABLE OP AMPS

One class of specialized op amp is the micropower programmable op amp. Such devices utilize an external resistor to establish the quiescent operating current for the internal stages. That is, the internal stages are biased at a particular operating current by the selection of an external resistor. Several characteristics of the amplifier are altered by changes in the programming current (JP):

1. DC supply current
2. Open-loop voltage gain
3. Input bias current
4. Slew rate
5. Unity gain frequency
6. Input noise voltage

Typical values of DC supply current range from less than 1 microampere to as high as 1 milliampere, and they are proportional to the value of programming current. The ability to operate at very low currents makes these devices especially attractive for battery-powered applications. Additionally, the DC voltage requirements are generally quite flexible, with ±1.2 to ±18 volte being a representative range. Here again, the programmable op amp is well suited to battery-powered applications. The DC supply current remains fairly constant with changes in supply voltage, provided the programming current is held constant.
The open-loop voltage gain increases as the programming current increases. It is reasonable to expect as much as a 100:1 change as the programming current is varied
over its operating range. This characteristic can be considered an advantage (e.g., programmable voltage gain) or a disadvantage (e.g., unstable voltage gain)
depending on the nature of the application. Input bias current also increases as programming current increases. In this case, variations as great as 200:1 are reasonable over the range of programming currents. The minimum input bias current is often in the fractional nanoampere
range.
The slew rate of a programmable op amp increases as programming current increases. It can also be increased by using higher DC supply voltages. As the programming current is varied over its operating range, the slew rate can be expected to vary as much as 1500:1. The upper limit is typically greater than that for the standard 741 op amp (i.e., greater than 0.5 \^juS).
The unity gain frequency (or gain bandwidth product) increases as the programming
current increases. Ranges as great as 500:1 are reasonable changes to expect as the programming current is varied over its operating range. The highest unity gain frequency is typically higher than the standard 741 op amp rating (i.e, higher than 1.0 MHz).

The input noise voltage of a programmable op amp decreases as the programming current increases. Ranges of as much as 200:1 are reasonable as the programming current is varied throughout its control range. In general, the programmable amplifier can be used for most of the applications previously discussed for general-purpose op amps, provided the appropriate specifications are adequate for a given application. The data sheets are interpreted in the same manner as for other amplifiers, with the exception that the effects of
programming current are included. These effects may be shown by including multiple data sheets for different values of programming current and/or by presenting graphs that show the effects of programming current. These devices are often selected for low-power applications or for applications that require a controllable parameter. Representative devices include the MC1776 and MC3476, manufactured by Motorola, In

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