AN6077 INTERSIL [Intersil Corporation], AN6077 Datasheet - Page 2

• Manufacturer Part Number: AN6077
• Description: An IC Operational Transconductance Amplifier With Power Capability
• Manufacturer: INTERSIL [Intersil Corporation]

multiplier. This is analogous to using an elephant to carry a
twig. It may be elegant but it takes a lot to keep it going!
When operated in the gain control mode, one input of the
standard transconductance multiplier is offset so that only
one half of the differential input is used; thus, one half of the
multiplier is being thrown away.
The OTA, while providing excellent linear amplifier
characteristics, does provide a simple means of gain control.
For this application the OTA may be considered the
realization of the ideal differential amplifier in which the full
differential amplifier g
output. Because the differential amplifier is ideal, its g
directly proportional to the operating current of the
differential amplifier; in the OTA the maximum output current
is equal to the amplifier bias current I
the amplifier bias current, the amplifier gain may be varied:
shows the basic configuration of the OTA DC gain control
circuit.
As long as the differential input signal to the OTA remains
under 50mV peak-to-peak, the deviation from a linear
transfer will remain under 5 percent. Of course, the total
harmonic distortion will be considerably less than this value.
Signal excursions beyond this point only result in an
undesired “compressed” output. The reason for this
compression can be seen in the transfer characteristic of the
differential amplifier in Figure 3. Also shown in Figure 3 is a
curve depicting the departure from a linear line of this
transfer characteristic.
The actual performance of the circuit shown in Figure 4 is
plotted in Figure 5. Both signal to noise ratio and total
harmonic distortion are shown as a function of signal input.
Figures 5B and 5C show how the signal handling capability of
the circuit is extended through the connection of diodes on
the input as shown in Figure 6 [2]. Figure 7 shows total
system gain as a function of amplifier bias current for several
values of diode current. Figure 8 shows an oscilloscope
reproduction of the CA3080 transfer characteristic as applied
to the circuit of Figure 4. The oscilloscope reproduction of
Figure 9 was obtained with the circuit shown in Figure 6. Note
A = g
CONTROL
SIGNAL
FIGURE 4. BASIC CONFIGURATION OF THE OTA DC GAIN
INPUT
GAIN
m
R
L
V
V
M
X
where R
CONTROL CIRCUIT
51
51
L
m
is the output load resistance. Figure 4
is converted to a single ended
2
3
I
4-2
ABC
OTA
CA3080A
+
-
5
R
M
7
4
ABC
+6V
-6V
. Thus, by varying
6
10K
I
O
MODULATED
Application Note 6077
AMPLITUDE
I
O
OUTPUT
= g
m
m
is
V
X
the improvement in linearity of the transfer characteristic.
Reduced input impedance does result from this shunt
connection. Similar techniques could be used on the OTA
output, but then the output signal would be reduced and the
correction circuitry further removed from the source of non
linearity. It must be emphasized that the input circuitry is
differential.
FIGURE 5. PERFORMANCE CURVES FOR THE CIRCUIT OF
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
0.1
1
1
DIODE CURRENT = 0.5mA
CA3080A
DIODE CURRENT = 0mA
DIODE CURRENT = 1mA
DISTORTION IS PRIMARILY
A FUNCTION OF SIGNAL INPUT
RATIO
I
ABC
S/N
FIGURES 4 AND 6
1.0
10
10
INPUT VOLTAGE (mV)
INPUT VOLTAGE (mV)
INPUT VOLTAGE (mV)
FIGURE 5A.
FIGURE 5B.
FIGURE 5C.
10 A
500 A
10 A
500 A
100
10
500 A
100
10 A
THD
1V
100
1V
I
CA3080A
S/N RATIO
ABC
THD
I
CA3080A
S/N RATIO
ABC
THD
10V
1.0V
10V
100
80
60
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0