NE5517, NE5517A, AU5517
Linearizing Diodes
Impedance Buffer
For V
IN
greater than a few millivolts, Equation 3 becomes
invalid and the transconductance increases non-linearly.
Figure 22 shows how the internal diodes can linearize the
transfer function of the operational amplifier. Assume D
2
and D
3
are biased with current sources and the input signal
current is I
S
. Since I
4
+ I
5
= I
B
and I
5
− I
4
= I
0
,
that is: I
4
= (I
B
− I
0
), I
5
= (I
B
+ I
0
)
+VS
The upper limit of transconductance is defined by the
maximum value of I
B
(2.0 mA). The lowest value of I
B
for
which the amplifier will function therefore determines the
overall dynamic range. At low values of I
B
, a buffer with
very low input bias current is desired. A Darlington
amplifier with constant-current source (Q
14
, Q
15
, Q
16
, D
7
,
D
8
, and R
1
) suits the need.
APPLICATIONS
Voltage-Controlled Amplifier
I
D
I
ID
2
ID
S
2
I0
+
2 I
)
I
S
I0
+
I5
*
I4
I
4
D
3
1/2I
D
Q
4
I
S
I
S
1/2I
D
I
B
−VS
I
5
D
2
I
5
S
I
B
D
*
I
In Figure 23, the voltage divider R
2
, R
3
divides the
input-voltage into small values (mV range) so the amplifier
operates in a linear manner.
It is:
I
OUT
+ *V
IN
@
R
3
@
g
M
;
R
2
)
R
3
V
OUT
+
I
OUT
@
R
L
;
A
+
V
OUT
R
3
+
@
g
M
@
R
L
V
IN
R
2
)
R
3
(3) g
M
= 19.2 I
ABC
(g
M
in
mmhos
for I
ABC
in mA)
Figure 22. Linearizing Diode
For the diodes and the input transistors that have identical
geometries and are subject to similar voltages and
temperatures, the following equation is true:
T In
q
ID
2
I
D
2
Since g
M
is directly proportional to I
ABC
, the amplification
is controlled by the voltage V
C
in a simple way.
When V
C
is taken relative to −V
CC
the following formula
is valid:
I
ABC
+
(V
C
*
1.2V)
R
1
)
I
S
*
I
S
+
KT In
q
1 2(I
B
)
I
O
)
1 2(I
B
*
I
O
)
(eq. 6)
I
I
I
O
+
I
S
2 B for |I
S
|
t
D
2
I
D
The 1.2 V is the voltage across two base-emitter baths in
the current mirrors. This circuit is the base for many
applications of the AU5517/NE5517.
The only limitation is that the signal current should not
exceed I
D
.
V
C
+V
CC
R
1
R
4
= R
2
/ /R
3
3
+
11
NE5517
R
2
V
IN
4
R
3
−
6
I
OUT
R
L
R
S
INT
−V
CC
TYPICAL VALUES: R
1
= 47kW
R
2
= 10kW
R
3
= 200W
R
4
= 200W
R
L
= 100kW
R
S
= 47kW
8
V
OUT
1
5
7
I
ABC
INT
+V
CC
Figure 23.
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