AN1512 Philips, AN1512 Datasheet - Page 2

• Manufacturer Part Number: AN1512
• Description: All in one NE5230
• Manufacturer: Philips

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Philips Semiconductors
ABSTRACT
Recent improvements in low voltage operational amplifier design
have resulted in novel applications formerly thought impossible.
Design improvements include rail-to-rail operation at both the input
and output, programmable bandwidth, full swing capability to 0.8V
with 9V V
unique features combine to make possible applications such as
voltage-to-current conversion down to zero; a low voltage,
full-swing, instrumentation amplifier; a solar-powered, gated function
generator; active filters and many more applications. A circuit for
increased output power is also presented.
INTRODUCTION
The name of the game in low voltage op amps is: MAX
HEADROOM. For a given range of voltage, how much of that
voltage can be utilized by the op amp’s output? Traditionally, not
very much, especially on the positive side. Among other important
concerns are input voltage range, drive capability, speed, input bias
current and power supply current. Trade-offs are part of every
design. But with each new design, fewer trade-offs are tolerated. For
example, drive capability and speed were not available in the same
amplifier. Now, they are not only offered together but expected in
precision designs. And so it is with low voltage and input and output
rail-to-rail operation. The NE5230 is the first to offer this
combination. Moreover, the device offers power supply current
programmability. This feature allows variable bandwidth, slew rate
and, to some degree, input bias current. This unique combination of
features makes the device useful in a great many applications. What
this means to the user is outlined below.
INPUT AND OUTPUT RAIL-TO-RAIL OPERATION
Figure 1 illustrates input and output rail-to-rail operation. The
amplifier’s common-mode voltage includes both input rails, and its
output swings up to both rails. The NE5230 is unique in this regard.
Figure 1 demonstrates the usefulness of this feature. The
voltage-to-current converter shown in Figure 2 can deliver current
down to zero for a zero input voltage. With minor changes, this
circuit can also be configured to sink as well as source current by
simply using an NPN connected to the negative supply instead of
the PNP connected to the positive supply. The output current is a
function of the input voltage divided by the value of the load resistor.
Selection of the resistor value is limited, on the one hand, by the
maximum power capability of the amplifier, which is outlined in the
data sheet. While many amplifiers can swing to the negative rail, no
other can accomplish this at both output rails under load.
Applications range from transducer interfaces to level-shift circuits.
1991 Dec
All in one: NE5230
CC
and availability in surface mount packages. These
Figure 1. Rail-to-Rail Operation
V
IN
NE5230
V
O
SL00904
2
INSTRUMENTATION AMPLIFIER
Another useful application for rail-to-rail operation is the
instrumentation amplifier shown in Figure 3. Along with its other
practical features of high common-mode rejection ratio, high input
impedance, and low offset drift, this familiar amplifier’s capability has
now been enhanced. Signal swing beyond both input rails and swing
up to both output rails is now possible. With equal value resistors,
the gain configuration is a non-inverting gain of three. With the
power supply voltage of 2.00V, with no load at room temperature,
the performance was predictable. The photographs in Figure 4
show the input and output of the waveforms of the circuit used.
Among its other applications, remote transducers, portable precision
instrumentation and any low voltage, low power application are
included in this amplifier’s repertoire.
LOW VOLTAGE, GATED FUNCTION GENERATOR
Another useful application is as solar-powered, gated function
generator. Although it can be powered conventionally, circuit
performance is not significantly different with solar cells. This circuit
uses a Wien Bridge sine wave oscillator. The Wien Bridge has been
used since the vacuum tube era. It is simple, stable, and requires
few external components. The circuit utilizes both positive and
negative feedback to achieve balanced operation. The oscillator will
stop working if too much negative feedback is used and will saturate
in both states if too much positive feedback is used. In a practical
implementation, some non-linear element must be employed to
realize this stable condition. The gain of the amplifier must be large
enough at the frequency of oscillation to make the input excursions
small enough to be compensatable by this non-linear element.
Diodes and FETs have beenused to accomplish this. One of the
most popular is the lamp; small, inexpensive and readily available,
its voltage variable resistance makes it an ideal candidate for this
application. It works like this: as the negative feedback voltage
increases across the lamp, its resistance increases, and thereby
reduces the output voltage. When the output voltage decreases, the
amount of negative feedback voltage across the lamp decreases,
thereby decreasing the resistance of the lamp. This balancing act
continues until a stable oscillation is achieved. It is important to note
that the lamp resistance is changing due to the thermal effects
caused by the changing voltage across it. The frequency of
oscillation is determined by
f
OSC
2
1
RC
Application note
AN1512