NE5517, NE5517A, AU5517
Voltage-Controlled Resistor (VCR)
Voltage-Controlled Oscillators
Because an OTA is capable of producing an output current
proportional to the input voltage, a voltage variable resistor
can be made. Figure 26 shows how this is done. A voltage
presented at the R
X
terminals forces a voltage at the input.
This voltage is multiplied by g
M
and thereby forces a current
through the R
X
terminals:
R
x
+
R
)
R
A
g
M
)
R
A
Figure 32 shows a voltage-controlled triangle-square
wave generator. With the indicated values a range from
2.0 Hz to 200 kHz is possible by varying I
ABC
from 1.0 mA
to 10
mA.
The output amplitude is determined by I
OUT
×
R
OUT
.
Please notice the differential input voltage is not allowed
to be above 5.0 V.
With a slight modification of this circuit you can get the
sawtooth pulse generator, as shown in Figure 33.
APPLICATION HINTS
where g
M
is approximately 19.21
mMHOs
at room
temperature. Figure 27 shows a Voltage Controlled Resistor
using linearizing diodes. This improves the noise
performance of the resistor.
Voltage-Controlled Filters
Figure 28 shows a Voltage Controlled Low-Pass Filter.
The circuit is a unity gain buffer until X
C
/g
M
is equal to
R/R
A
. Then, the frequency response rolls off at a 6dB per
octave with the −3 dB point being defined by the given
equations. Operating in the same manner, a Voltage
Controlled High-Pass Filter is shown in Figure 29. Higher
order filters can be made using additional amplifiers as
shown in Figures 30 and 31.
To hold the transconductance g
M
within the linear range,
I
ABC
should be chosen not greater than 1.0 mA. The current
mirror ratio should be as accurate as possible over the entire
current range. A current mirror with only two transistors is
not recommended. A suitable current mirror can be built
with a PNP transistor array which causes excellent matching
and thermal coupling among the transistors. The output
current range of the DAC normally reaches from 0 to
−2.0 mA. In this application, however, the current range is
set through R
REF
(10 kW) to 0 to −1.0 mA.
I
DACMAX
+
2
@
V
REF
+
2
@
5V
+
1mA
R
REF
10kW
+
R
)
RA
gM
@
RA
R
+V
CC
3
2
NE5517/A
5
−
4
200W
200W
−V
CC
R
X
C
8
R
100kW
10kW
−V
CC
INT
V
OUT
7
+
11
I
O
30kW
V
C
INT
+V
CC
X
Figure 26. VCR
+V
CC
+V
CC
3
V
OS
R
P
1kW
6
4
−V
CC
2
NE5517/A
11
1
30kW
V
C
I
D
INT
+V
CC
5
C
7
8
R
X
R
100kW
10kW
−V
CC
INT
Figure 27. VCR with Linearizing Diodes
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