Circuit Description
(Continued)
Q
3
and Q
13
, all transistors and diodes are identical in size.
Transistors Q
1
and Q
2
with Diode D
1
form a current mirror
which forces the sum of currents I
4
and I
5
to equal I
ABC
:
(2)
I
4
+ I
5
= I
ABC
where I
ABC
is the amplifier bias current applied to the gain
pin.
For small differential input voltages the ratio of I
4
and I
5
ap-
proaches unity and the Taylor series of the In function can be
approximated as:
amplifier. For convenience assume the diodes are biased
with current sources and the input signal is in the form of cur-
rent I
S
. Since the sum of I
4
and I
5
is I
ABC
and the difference
is I
OUT
, currents I
4
and I
5
can be written as follows:
Since the diodes and the input transistors have identical ge-
ometries and are subject to similar voltages and tempera-
tures, the following is true:
(3)
(6)
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
D
/2
and that the diodes be biased with currents. In practice, re-
placing the current sources with resistors will generate insig-
nificant errors.
(4)
Collector currents I
4
and I
5
are not very useful by themselves
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
5
minus I
4
thus:
(5)
The term in brackets is then the transconductance of the am-
plifier and is proportional to I
ABC
.
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 kΩ resistor as a current source
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.
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
DS007981-8
FIGURE 1. Linearizing Diodes
For optimum signal-to-noise performance, I
ABC
should be as
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
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6
the input signal via R
IN
(Figure
2)
until the output distortion is
below some desired level. The output voltage swing can
then be set at any level by selecting R
L
.
Although the noise contribution of the linearizing diodes is
negligible relative to the contribution of the amplifier’s inter-
nal transistors, I
D
should be as large as possible. This mini-
mizes the dynamic junction resistance of the diodes (r
e
) and
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