NE5517, NE5517A, AU5517
http://onsemi.com
10
Voltage-Controlled Resistor (VCR)
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
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.
Voltage-Controlled Oscillators
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
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
R
REF
+ 2 @
5V
10kW
+ 1mA
−V
CC
4
3
−
+
NE5517/A
8
V
OUT
−V
CC
11
+V
CC
R
X
5
I
O
2
R
7
INT
INT
C
+V
CC
V
C
R
X
+
R ) R
A
g
M
@ R
A
Figure 26. VCR
30kW
200W 200W
100kW
10kW
−V
CC
4
3
NE5517/A
8
−V
CC
11
+V
CC
R
X
5
I
D
2
R
7
INT
INT
C
+V
CC
V
C
+V
CC
V
OS
R
P
1
6
Figure 27. VCR with Linearizing Diodes
30kW
1kW
100kW
10kW
