Electrical Characteristics
(Note 6)
Parameter
Power Supply Rejection
Mirror Gain
DMirror
Gain
Mirror Current
Negative Input Current
Input Bias Current
Conditions
V
a
e
15 V
DC
unless otherwise stated (Continued)
LM2900
Min
T
A
e
25 C f
e
100 Hz
20
mA
(Note 3)
200
mA
(Note 3)
20
mA
to 200
mA
(Note 3)
(Note 4)
T
A
e
25 C (Note 5)
Inverting Input
0 90
0 90
Typ
70
10
10
2
10
10
300
11
11
5
500
0 90
0 90
Max
Min
LM3900
Typ
70
10
10
2
10
10
300
11
11
5
500
0 90
0 90
Max
Min
LM3301
Units
Typ
70
1
1
2
10
10
1 10
1 10
5
500
Max
dB
mA mA
%
mA
DC
mA
DC
nA
Note 1
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 2
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
Characteristics
Note 3
This spec indicates the current gain of the current mirror which is used as the non-inverting input
Note 4
Input V
BE
match between the non-inverting and the inverting inputs occurs for a mirror current (non-inverting input current) of approximately 10
mA
This is
therefore a typical design center for many of the application circuits
Note 5
Clamp transistors are included on the IC to prevent the input voltages from swinging below ground more than approximately
b
0 3 V
DC
The negative input
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
input voltages see for example the ‘‘Differentiator Circuit’’ in the applications section
Note 6
These specs apply for
b
40 C
s
T
A
s
a
85 C unless otherwise stated
Note 7
Human body model 1 5 kX 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 mir-
ror gain at mA current levels especially at high operating
temperatures
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
internal conductors and result in a destroyed unit
Output short circuits either to ground or to the positive pow-
er supply should be of short time duration Units can be
destroyed 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
a
5 V
DC
power supply at T
A
e
25 C
with a 100 kX shunt-feedback resistor (from the output to
the inverting 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
J
max Larger feedback resistors will reduce the
current 11 MX provides approximately 30 mA an open cir-
cuit provides 1 3 mA and a direct connection from the out-
put to the non-inverting input will result in catastrophic fail-
a
ure 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
A
e
25 C
Unintentional signal coupling from the output to the non-in-
verting 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 resis-
tors 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
a
V
BE
Internal clamp
transistors (see note 5) catch-negative input voltages at ap-
proximately
b
0 3 V
DC
but the magnitude of current flow has
to be limited by the external input network For operation at
high temperature this limit should be approximately 100
mA
This new ‘‘Norton’’ current-differencing amplifier can be
used in most of the applications of a standard IC op amp
Performance 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 functions 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 integra-
tor’’ allows controlling the charging and the discharging of
the integrating capacitor with positive voltages and the ‘‘fre-
quency doubling tachometer’’ provides a simple circuit
which reduces the ripple voltage on a tachometer output DC
voltage
3
NSC [ NATIONAL SEMICONDUCTOR ]
