(Note 7) T

Parameter

Conditions

LM2900

LM3900

LM3301

Units

Min Typ

Max Min Typ

Max Min Typ

Max

Mirror Gain

0.90

1.0

1.1 0.90

1.0

1.1 0.90

1

1.10 µA/µA

0.90

1.0

1.1 0.90

1.0

1.1 0.90

1

1.10

∆Mirror Gain

2

5

2

5

2

5

%

Mirror Current

(Note 5)

10

500

10

500

10

500

µA

DC

Negative Input Current

T

1.0

1.0

1.0

mA

DC

Input Bias Current

Inverting Input

300

300

nA

Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

functional, but do not guarantee specific performance limits.

Note 2: For operating at high temperatures, the device must be derated based on a 125˚C maximum junction temperature and a thermal resistance of 92˚C/W which

applies for the device soldered in a printed circuit board, operating in a still air ambient. Thermal resistance for the S.O. package is 131˚C/W.

Note 3: The output current sink capability can be increased for large signal conditions by overdriving the inverting input. This is shown in the section on Typical Char-

acteristics.

Note 4: This spec indicates the current gain of the current mirror which is used as the non-inverting input.

therefore a typical design center for many of the application circuits.

currents which may result from large signal overdrive with capacitance input coupling need to be externally limited to values of approximately 1 mA. Negative input

currents in excess of 4 mA will cause the output voltage to drop to a low voltage. This maximum current applies to any one of the input terminals. If more than one

of the input terminals are simultaneously driven negative smaller maximum currents are allowed. Common-mode current biasing can be used to prevent negative in-

put voltages; see for example, the “Differentiator Circuit” in the applications section.

Note 7: These specs apply for −40˚C

Note 8: Human body model, 1.5 k

Ω in series with 100 pF.

Application Hints

When driving either input from a low-impedance source, a

limiting resistor should be placed in series with the input lead

to limit the peak input current. Currents as large as 20 mA

will not damage the device, but the current mirror on the

non-inverting input will saturate and cause a loss of mirror

gain at mA current levels — especially at high operating tem-

peratures.

Precautions should be taken to insure that the power supply

for the integrated circuit never becomes reversed in polarity

or that the unit is not inadvertently installed backwards in a

test socket as an unlimited current surge through the result-

ing forward diode within the IC could cause fusing of the in-

ternal conductors and result in a destroyed unit.

Output short circuits either to ground or to the positive power

supply should be of short time duration. Units can be de-

stroyed, not as a result of the short circuit current causing

metal fusing, but rather due to the large increase in IC chip

dissipation which will cause eventual failure due to exces-

sive junction temperatures. For example, when operating

from a well-regulated +5 V

a 100 k

Ω shunt-feedback resistor (from the output to the in-

verting input) a short directly to the power supply will not

cause catastrophic failure but the current magnitude will be

approximately 50 mA and the junction temperature will be

above T

rent, 11 M

Ω provides approximately 30 mA, an open circuit

provides 1.3 mA, and a direct connection from the output to

the non-inverting input will result in catastrophic failure when

the output is shorted to V

+

as this then places the

base-emitter junction of the input transistor directly across

the power supply. Short-circuits to ground will have magni-

tudes of approximately 30 mA and will not cause cata-

strophic failure at T

Unintentional signal coupling from the output to the

non-inverting input can cause oscillations. This is likely only

in breadboard hook-ups with long component leads and can

be prevented by a more careful lead dress or by locating the

non-inverting input biasing resistor close to the IC. A quick

check of this condition is to bypass the non-inverting input to

ground with a capacitor. High impedance biasing resistors

used in the non-inverting input circuit make this input lead

highly susceptible to unintentional AC signal pickup.

Operation of this amplifier can be best understood by notic-

ing that input currents are differenced at the inverting-input

terminal and this difference current then flows through the

external feedback resistor to produce the output voltage.

Common-mode current biasing is generally useful to allow

operating with signal levels near ground or even negative as

this maintains the inputs biased at +V

sistors (Note 6) catch-negative input voltages at approxi-

mately −0.3 V

limited by the external input network. For operation at high

temperature, this limit should be approximately 100 µA.

This new “Norton” current-differencing amplifier can be used

in most of the applications of a standard IC op amp. Perfor-

mance as a DC amplifier using only a single supply is not as

precise as a standard IC op amp operating with split supplies

but is adequate in many less critical applications. New func-

tions are made possible with this amplifier which are useful

in single power supply systems. For example, biasing can be

designed separately from the AC gain as was shown in the

“inverting amplifier,” the “difference integrator” allows con-

trolling the charging and the discharging of the integrating

capacitor with positive voltages, and the “frequency doubling

tachometer” provides a simple circuit which reduces the

ripple voltage on a tachometer output DC voltage.

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