OPA861
www.ti.com
SBOS338–AUGUST 2005
6
8
5
BUF602
VOUT
3
8
2
Ω
Ω
180
2
10pF
10pF
VREF
3
Ω
Ω
33
Ω
Ω
33
10
10
180
6
VIN
Figure 34. Control-Loop Amplifier Using Three OPA861s
C1
100pF
Ω
20
JFET−Input, Wideband
VOUT
VIN
OPA656
R2
D1
D2
Ω
100k
Ω
20
D1, D2 = 1N4148
Ω
RQ = 1k
R1
CCII
Ω
40.2
E
2
8
C
The OTA amplifier works as a current conveyor (CCII) in this circuit, with a current gain of 1.
R1 and C1 set the DC restoration time constant.
B
3
D1 adds a propagation delay to the DC restoration.
R2 and C1 set the decay time constant.
R2
Ω
100
VREF
Figure 35. DC Restorer Circuit
Negative Impedance Converter Filter: Low-Pass
The transfer function is shown in Equation 5:
Filter
VOUT
VIN
1
+
The OPA861 can be used as a negative impedance
converter to realize the low-pass filer shown in
Figure 36.
1 ) sC2R ) s2C1C2R2
(5)
with:
1
w0 +
R
Ǹ
C1C2 R
C1
R
C1
C2
Q + Ǹ
VIN
VOUT
C2
Figure 36. Low-Pass Negative Impedance
Converter Filter
17
TI [ TEXAS INSTRUMENTS ]
