LM13700N National Semiconductor, LM13700N Datasheet - Page 6

• Manufacturer Part Number: LM13700N
• Description: Operational Amplifier (Op-Amp) IC
• Manufacturer: National Semiconductor

Specifications of LM13700N

• No. Of Amplifiers: 2
• Slew Rate: 50V/µs
• No. Of Pins: 16
• Peak Reflow Compatible (260 C): No
• Input Bias Current: 1µA
• Supply Voltage: 36V
• Output Current: 20mA
• Input Offset Voltage Max: 4mV
• Gain: 2dB
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Circuit Description
Q
Transistors Q
which forces the sum of currents I
where I
pin.
For small differential input voltages the ratio of I
proaches unity and the Taylor series of the In function can be
approximated as:
Collector currents I
and it is necessary to subtract one current from the other.
The remaining transistors and diodes form three current mir-
rors that produce an output current equal to I
The term in brackets is then the transconductance of the am-
plifier and is proportional to I
Linearizing Diodes
For differential voltages greater than a few millivolts, Equa-
tion (3) becomes less valid and the transconductance be-
comes increasingly nonlinear. Figure 1 demonstrates how
the internal diodes can linearize the transfer function of the
For optimum signal-to-noise performance, I
large as possible as shown by the Output Voltage vs. Ampli-
fier Bias Current graph. Larger amplitudes of input signal
also improve the S/N ratio. The linearizing diodes help here
by allowing larger input signals for the same output distortion
as shown by the Distortion vs. Differential Input Voltage
graph. S/N may be optimized by adjusting the magnitude of
3
and Q
ABC
13
, all transistors and diodes are identical in size.
is the amplifier bias current applied to the gain
1
and Q
4
and I
2
I
with Diode D
4
5
+ I
are not very useful by themselves
5
ABC
= I
.
ABC
(Continued)
4
and I
1
form a current mirror
5
to equal I
ABC
5
minus I
should be as
FIGURE 1. Linearizing Diodes
4
and I
ABC
4
5
thus:
:
ap-
(2)
(3)
(4)
(5)
6
amplifier. For convenience assume the diodes are biased
with current sources and the input signal is in the form of cur-
rent I
is I
Since the diodes and the input transistors have identical ge-
ometries and are subject to similar voltages and tempera-
tures, the following is true:
Notice that in deriving Equation (6) no approximations have
been made and there are no temperature-dependent terms.
The limitations are that the signal current not exceed I
and that the diodes be biased with currents. In practice, re-
placing the current sources with resistors will generate insig-
nificant errors.
Applications:
Voltage Controlled Amplifiers
Figure 2 shows how the linearizing diodes can be used in a
voltage-controlled amplifier. To understand the input biasing,
it is best to consider the 13 k
and use a Thevenin equivalent circuit as shown in Figure 3 .
This circuit is similar to Figure 1 and operates the same. The
potentiometer in Figure 2 is adjusted to minimize the effects
of the control signal at the output.
the input signal via R
below some desired level. The output voltage swing can
then be set at any level by selecting R
Although the noise contribution of the linearizing diodes is
negligible relative to the contribution of the amplifier’s inter-
nal transistors, I
mizes the dynamic junction resistance of the diodes (r
OUT
S
. Since the sum of I
, currents I
D
4
should be as large as possible. This mini-
and I
IN
( Figure 2 ) until the output distortion is
5
4
can be written as follows:
and I
resistor as a current source
5
DS007981-8
is I
ABC
L
.
and the difference
e
) and
D
(6)
/2