Application Note AN17 - Advanced Linear Devices

GENERAL DESCRIPTION

Low operating voltage linear systems, especially battery

operated systems such as portable instruments or termi-

nals, tend to have a different set of requirements and

constraints placed on them. Usually, for such systems, not

only must the linear circuits operate at low voltages, but

these voltages tend to vary due to limited power supply

regulation or gradual discharge of a battery pack over

time. Furthermore, power drain of the system is nearly

always a major consideration. In order to lower the

operating power dissipation of a circuit, linear active

components such as operational amplifiers not only need

to have very low operating currents, but the selection of

passive components such as gain setting resistors or

network feedback resistors must be at as high values as

possible, and still meet the precision and frequency re-

quirements of the system.

As an example, a simple DC inverting amplifier connected

as a 10X gain amplifier can use a 1kΩ and 10kΩ resistor

gain setting network. The maximum current drain through

the resistors in a ±15V system may be 1.5mA (24.75mW)

and in a 5V system may still be as high as 250µA. Using

a low voltage FET input operational amplifier, on the other

hand, the resistors used may be a 1MegΩ and 10MegΩ

resistor network without sacrificing the gain precision.

However, this requires very high input impedances at the

op amp inputs such that leakage currents at the inputs

would not introduce large errors to the output voltage of

the FET op amp set by the 10MegΩ resistor. The current

drain due to the resistor network in such a 5V system is

drastically reduced to 0.25µA (0.63µW). An added benefit

here is that with very low current flow in the resistors, gain

errors caused by resistor self-heating effects at different

output voltages are minimized or eliminated.

FET operational amplifiers with high input impedance not

only help increase the value of an external resistor net-

work, but also operate as near ideal voltage buffers. This

is useful for a variety of high impedance sensors and

transducers which are essentially charge accumulating

devices that supply a voltage output but cannot sustain

current flow. Often such devices also produce more

current noise effects than voltage noise effects. In a FET

operational amplifier, where usually the current noise

characteristics are far superior to its voltage noise perfor-

mance, the overall system noise may be reduced while the

voltage noise spec alone might have indicated otherwise.

USING LOW VOLTAGE FET INPUT OPERATIONAL AMPLIFIERS

APPLICATION NOTE AN17

ADVANCED

LINEAR

DEVICES, INC.

IMPROVED OPERATING (VOLTAGE) MARGINS

One important factor affecting low voltage applications in

general is the actual input and output operating voltage

ranges available from the operational amplifier. For ex-

ample, an input voltage range of ±11V for a ±15V system

may be perfectly acceptable for a given application, even

though the operational amplifier may need a 4V “overhead

margin” from each supply rail. However, for a single 5V

system, such an overhead voltage is obviously not accept-

able as there would not be any operating voltage left to

supply an output.

For a 5V single supply system or a ±2.5V system, a 1V

overhead from each supply rail of an operational amplifier

would mean only 3V linear operating voltage range, which

is not much room to play with. The same circuit when

operating at 3V and 2V would have 1V and 0V of actual

useful design operating range respectively. From this it

becomes apparent that for low operating voltages, rail-to-

rail operating voltages may be an imperative rather than a

luxury.

In order to implement rai-to-rail voltages (or as wide

operating voltage ranges as possible), FET operational

amplifiers with CMOS input stages have an advantage.

The rail-to-rail operating ranges are usually achieved by

using complementary drive design techniques, best imple-

mented by having complementary N channel and P chan-

nel input stages. Rail-to-rail outputs can be implemented

by class AB output stages using N channel and P channel

MOSFETS for push-pull outputs that span the full supply

voltage range. MOSFET transistor characteristics tend to

assist in accomplishing rail-to-rail outputs because at

small drain voltages, both N channel and P channel

MOSFET have essentially resistive characteristics. With

high impedance loads, such as CMOS data converters,

CMOS comparators or CMOS logic gates, the output

voltages approach within a few hundred microvolts (less

than a mV) of the supply rail voltages. For ratiometric

designs that use the power supply rails, rail-to-rail opera-

tional amplifiers can provide precision all the way to the

rail voltages without introducing significant error.

Another design consideration when using low voltage

precision operational amplifiers comes from analyzing

some of the specifications themselves. The PSRR and the

CMRR (the power supply rejection ratio and the common

mode rejection ratio) of a given operational amplifier are

NOTICE: Advanced Linear Devices (ALD) reserves the right to make changes and to discontinue any product and or services as identified in this publication without notice. Current specifications for any product and or services

should be verified by customer before placing any orders. ALD warrants its products to current specifications in effect at time of manufacture in accordance to its standard warranty. Unless mandated by government

requirements, ALD performs certain, but not necessarily all, specific testing and procedures as ALD deems necessary to support this warranty.

ALD assumes no liability for any circuitry described herein. Applications for any circuits contained herein is for illustrative purposes only. No representation of continued operation of said circuits under any operating conditions

are implied. Any use of such circuits are the responsibility of the user. No circuit licenses, copyrights or patents of any kind is implied or granted. ALD does not authorize or warrant any of its products or designs for use in life

APPLICATION NOTE AN17 Advanced Linear Devices 2

usually specified in dB. However, the effect of these

parameters when applied to a low signal voltage applica-

tion is of interest. Use of large supply voltages, with a

large common mode voltage supply, will introduce a larger

error signal voltage into an already small signal to be

amplified.

As an illustration of this point, first let us take the case of

PSRR. A circuit operating at ±15V and using an opera-

tional amplifier with PSRR of 80dB would have introduced

an error voltage of 100µV for each 1V change of the supply

voltage. A 10% ±15 V power supply would have total

supply voltage variation of 3V. The equivalent input offset

error voltage introduced by PSRR would therefore be 300

µV. Now consider an operational amplifier having the

same PSRR spec but operating at 2V. A 10% variation of

a 2V supply now introduces 0.2V change to the supply

voltage for the part. The operational amplifier with the

same 80dB PSRR spec would now only introduce a 20µV

error.

REDUCED POWER SUPPLY AND COMMON MODE

ERRORS

Now consider CMRR which is also usually specified in dB.

An operational amplifier with 80 dB CMRR at ±15V with a

voltage signal equal to 50% of the supply range would see

±7.5V voltage excursion and a 1.5mV equivalent offset

voltage error introduced into the signal. In comparison,

the error voltage for a low voltage operational amplifier at

2V supply, with 80 dB CMRR and 50% signal range would

be 100 µV instead. In this case the percentage error

introduced into the signal may be the same, but a 1V peak

signal compared to a ±7.5V peak signal, both having 50%

signal voltage range, would in actual fact have quite

different absolute error voltages introduced.

OUTPUT CURRENT DRIVE

Another anomaly to consider in adjusting the intuitive

process to low voltage circuit design, especially for an

experienced analog designer accustomed to using ±15V

operational amplifiers, is the output current drive spec.

For a 2kΩ load in a ±15V (30V) system, to drive the output

to a voltage close to full scale would require 7.5mA output

drive current. The same 2kΩ load in a ±2.5V system would

only require 1.25mA from the output stage. Of course high

power output drive is not quite compatible with low voltage

operational amplifier applications, but the point is that one

can be fooled with the intuition that low voltage operational

amplifier applications require the same high output cur-

rents as their higher voltage cousins when driving a similar

impedance load.

Using FET operational amplifiers, circuit designs are often

simplified due to design considerations such as input bias

and offset current effects, where these effects are consid-

ered to have negligible effect upon the performance of the

design task at hand. Variation of a very high input

impedance would not impact a design if the worst case

minimum input impedance under all circumstances is

acceptable. This reduces the need for current compensa-

tion with bias balance resistors and noise bypass resis-

tors. The high impedances inside a circuit network also

tend to reduce current spikes in the power supply line,

minimizing any signal and load induced variation of the

power supply. This is especially true for high impedance

power supply sources such as batteries. Lowered current

spikes in a circuit reduce the need for local supply bypass

capacitors, and frequently, even when a bypass capacitor

is deemed necessary, a single one can be used to serve

many operational amplifier circuits.

IDEAL FOR HIGH PERFORMANCE SIGNALS

Low voltage FET operational amplifier circuits excel when

high source impedance device applications are consid-

ered. These applications include a class of devices such

as high impedance bridge networks, capacitive sensors,

pH probes, humidity sensors, diode detector arrays and

pressure sensors. A single supply FET operational ampli-

fier is often a suitable choice for interface circuits that

perform the tasks of buffering, amplification, signal condi-

tioning and linearization, and temperature compensation.

A low voltage power source may, in this case, bring

significant additional cost savings to the system due to

reduced power rating requirements for the system. High

in-circuit element impedances can also mean easier single

supply to dual supply circuit conversion. For example, in

single supply linear systems there is often a need to

generate a virtual ground which acts as a reference point

for some of the positive and negative transition analog

signals. This reference can be generated simply and very

inexpensively by using two large resistors in a voltage

divider with a bypass capacitor, providing that only a

voltage with no current supplying capability is required. In

a FET operational amplifier circuit, conditions are fre-

quently right for just such a reference.

High in-circuit impedances do make the system more

vulnerable to noises, both generated internally within the

system components and coupled from outside. Internal to

the system higher impedance means less di/dt caused

coupling noise. However, higher internal circuit imped-

ances may require careful grounding of the circuit board

and perhaps some degree of shielding. The amount of

grounding and shielding is a function of a given application

and its level of susceptibility to the environment in which

it is expected to operate.

HIGH RELIABILITY

Often, low voltage FET operational amplifier designs not

only imply low power operation but also provide improved

reliability. For example, a circuit with 5 V and 1 mA power

drain uses 5 mW whereas a circuit with 30 V (+/- 15 V) and

1 mA burns 30 mW of power. The difference in power

dissipation means less self-heating for a low voltage

circuit and therefore lowered operating chip junction tem-

perature.

APPLICATION NOTE AN17 Advanced Linear Devices 3

The unit, when first powered up, reaches stable operating

temperature virtually instantly and introduces less self-

heating induced errors into the signal. Cooler operating

temperature also implies better reliability and longer term

drift as almost all components operate more reliably at

reduced temperature. These factors can make a low

voltage FET operational amplifier implementation of a

circuit an attractive choice even if lowered power drain is

not a primary objective.

APPLICATIONS

As previously mentioned, the primary advantages of using

low voltage FET operational amplifiers are high input

impedances, low power dissipation and relative ease of

using large resistors and small capacitors in the circuit

networks. As a result, many applications can be simpli-

fied. Figure 1 illustrates some of these advantages in a

single supply amplifier circuit with a self generated signal

reference. The signal reference is accomplished simply by

adding a resistor R3 to the normal gain feedback resistor

network consisting of R1 and R2. The input is amplified in

the non-inverting mode and provides high gain referenced

to the DC voltage set by the resistor R3. Vin range of 0 to

20 mV would be amplified with a gain of 100X and give

outputs from 0V to 2V.

Other typical FET operational amplifier applications in-

clude micropower inverting amplifiers, two stage single

supply non-inverting amplifiers and charge integrators as

shown in Figure 2 to Figure 4b. These circuits can provide

precision set by the external components with very low

error introduced by the input bias currents and input offset

currents. Two circuits that take full advantage of low input

bias currents of FET operational amplifiers are the non-

inverting amplifier, Figure 5, and the unity gain non-

inverting amplifier, Figure 6. In both cases, the input

impedance is greater than 1.0 x E+13 Ω.

Classic applications that require FET operational amplifi-

ers are those that depend on low input bias currents, such

as current measurement circuits as well as charge mea-

surement circuits. Current to voltage converter, as shown

in Figure 7, and current to voltage inverter with amplified

gain ( Figure 8 ) are circuits that deal directly with very low

level currents as signals. The outputs are low impedance

voltage outputs with precision directly proportional to the

precision of the resistors R1, R2 and R3. Circuits that

depend on charge preservation, such as the Capacitance

Multiplier circuit in Figure 9 and the Precision Sample and

Hold circuit in Figure 10, have operating frequency range,

capacitance range and hold time specifications that de-

pend directly on the input bias current specification.

Another example of a low voltage FET operational ampli-

fier application is a micropower buffered rail-to-rail adjust-

able voltage source. In the circuit the voltage setting

potentiometer can have a large value using very little

direct current drain across the power supply rails. This

voltage is then buffered with a FET input CMOS opera-

tional amplifier such as an ALD1701 that swings from rail-

to-rail, as shown in Fig. 11. Although the circuit appears

straightforward and simple enough, actual implementa-

tion of this function would have been complex and rela-

tively difficult without a rail-to-rail FET input operational

amplifier that handles both high input impedance and

complementary FET input voltage range requirements

simultaneously.

A more esoteric group of circuit function is the logarithmic

and anti-logarithmic circuits, Figure 15 to Figure 16 By

using these circuits, first by converting two input voltages

into log values, then a summing function in which an

addition of the two converted voltages are performed,

followed by an antilog conversion of the result of the

summation, one effectively creates a divide circuit func-

tion. Figure 17 shows a circuit function that divides input

A by input B and provides an output that is the result of the

division.

CONCLUSION

In summary, low voltage FET input operational amplifiers

face a different set of requirements and challenges, but

also provide a unique set of characteristics to meet those

challenges. The analog designer may have to look at

some of the requirements in a different way and may also

need to interpret things differently. In a world where

everything is increasingly mobile, portable and transport-

able, low power drain, low voltage circuits certainly play an

increasing role. Low voltage FET input circuits, such as

CMOS operational amplifiers, are becoming increasingly

important participant in this trend.

APPLICATION NOTE AN17 Advanced Linear Devices 4

APPLICATIONS

Note: Gain of 10 amplifier

Input impedance is limited to R1.

Total typical current drain of 20µA

Figure 2 Micropower Inverting Amplifier

100K

+2.5V

-2.5V

10MΩ

OUT

ALD1706

1MΩ

OUT

= R

Figure 3 Two Stage Single Supply Non-Inverting Amplifier

R 2

100K

+5 V 250K

R 3

R 1

VIN

+5V

250K 100K

R3R2

VIN

VOUT

1/2 ALD2701

Figure 1 Single Supply Amplifier with Self-Generated Signal Reference

+5V

or GND +5V

25K 10K

1K V

OUT

1/2 ALD2701

+5V

or GND

250K 100K

100Ω

Gain = 10,000

APPLICATION NOTE AN17 Advanced Linear Devices 5

Figure 5 Micropower Non-Inverting Amplifier

-+2.5V

900K

100K

VIN

VOUT

-2.5V

Gain of

10 amplifier

VOUT R2

= { 1+ } VIN

900K

100K

= { 1+ } VIN

=10VIN

Figure 4b Charge Amplifier

OUT

Charge Generator

Q= CV

dt = C

= dV

dt =

Vdc

Figure 4a Integrator

Note: This circuit forces a current i1 on the summing junction and causes the output to ramp at a rate of

∆Vout/∆t equal to i1. It is set by input resistor R and input voltage Vin. It can be seen that any

leakage current on the summing junction directly subtracts from i1 and affect the ramp rate. For

long integration time, dt is large and imply small i2 or large C since voltage swing ∆Vout is usually

limited. For precision applications, amplifier input leakage directly affects the value of i2 and may

be a significant contributor to overall error of integrator.

OUT

= 1

= V

= C dVc dV

OUT

dt dt

= C = i

∫V

APPLICATION NOTE AN17 Advanced Linear Devices 6

Figure 7 Simple I-V Converter

+5V

OUT

= I

range from 1nA to 1mA

Figure 8 High Gain I-V Converter

OUT

= I

{ 1+R

}

Figure 6 Unity Gain Non-Inverting Buffer

This is extremely effective in providing isolation between input and output. Essentially the

source impedance can be open circuit (i.e. capacitive source) while providing drive to

resistive load as low as 1kΩ amd capacitive load of 400pF.

+5V

OUT

= V

APPLICATION NOTE AN17 Advanced Linear Devices 7

Figure 9 Capacitance Multiplier

OUT

100pF

= 99 C

= C

{ 1+ R

} = 100 X C

Figure 10 Precision Sample/Hold

OUT

0.1µF

Figure 11 Micropower Buffered Rail to Rail Adjustable Voltage Source

OUT

V+ = +5V

APP 11 EPS WK

V+ = +5V

1MΩALD1701

VIN can be set to within 1mV of either power supply rail. The total power dissipation of the

circuit is essentially that of the operational amplifier power dissipation of approximately 0.5mW.

APPLICATION NOTE AN17 Advanced Linear Devices 8

Figure 12 Voltage to Current Converter, Inverting Type

OUT

= V

OUT

= -V

Figure 13 Voltage to Current Converter, Non-Inverting Type

OUT

V+

< V

< V+

Rail-to-Rail, Hi Z Input i

Independent of R

Figure 14 Voltage to Current Converter

100Ω

10K

OUT

10K

= R

{ 1+R

} V

APPLICATION NOTE AN17 Advanced Linear Devices 9

OUT

0.1µF

IN914

2N2222A

ALD1701

For Log Amp circuit, see Fig. 15.

For Antilog Amp circuit, see Fig. 16

Figure 15 Logarithmic Amplifier

Figure 16 Antilogarithmic Amplifier

0.1µF

IN914

2N2222A ALD1701

OUT

Figure 17 Division of Two Input Signals

LOG

AMP LOG B

LOG B

LOG A

LOG A/B

LOG

AMP

LOG

AMP

ANTI-LOG

AMP

A/B

APPLICATION NOTE AN17 Advanced Linear Devices 10