Datasheet

G +
R
L
R
E
)
1
g
m
[ *
R
L
R
E
(b)OTACommon-BAmplifier
(a)TransistorCommon-BaseAmplifier
100W
3 B
2
E
C
8
OPA861
R
E
V
IN
R
L
InvertingGain
V =0V
OS
V
O
NoninvertingGain
V =SeveralVolts
OS
R
E
V-
V+
R
L
V
O
V
IN
R
1
G=
R
L
R +
E
1
g
m
= -
R
L
R
E
G +
1
1 )
1
g
m
R
E
+ 1
R
O
+
ǒ
1
g
m
ø R
E
Ǔ
100
Ω
V
I
3 B
2
E
C
8
G = 1
V
OS
= 0V
G = 1
V
OS
= 0.7V
OPA861
R
E
V
O
(b) OTA Common−C Amplifier
(Buffer)
(a) Transistor Common−Collector Amplifier
(Emitter Follower)
V
O
V
I
R
E
V
−
V+
G +
1
1 )
1
g
m
R
E
[ 1
OPA861
www.ti.com SBOS338G –AUGUST 2005–REVISED MAY 2013
Common-C Amplifier
This low impedance can be converted to a high
impedance by inserting the buffer amplifier in series.
Figure 35b shows the OPA861 connected as an E-
follower—a voltage buffer. It is interesting to notice
Current-Mode Analog Computations
that the larger the R
E
resistor, the closer to unity gain
the buffer will be. If the OPA861 is to be used as a
As mentioned earlier, the OPA861 can be used
buffer, use R
E
≥ 500Ω for best results. For the
advantageously for analog computation. Among the
OPA861 used as a buffer, the gain is given by
application possibilities are functionality as a current
Equation 3:
amplifier, current differentiator, current integrator,
current summer, and weighted current summer.
Table 1 lists these different uses with the associated
(3)
transfer functions.
These functions can easily be combined to form
active filters. Some examples using these current-
mode functions are shown later in this document.
Figure 35. Common-Collector vs Common-C
Amplifier
A low value resistor in series with the B-input is
recommended. This resistor helps isolate trace
parasitic from the inputs, reduces any tendency to
oscillate, and controls frequency response peaking.
Figure 36. Common-Base Transistor vs
Typical resistor values are from 25Ω to 200Ω.
Common-B OTA
Common-B Amplifier
Figure 36 shows the Common-B amplifier. This
configuration produces an inverting gain and a low
impedance input. Equation 4 shows the gain for this
configuration.
(4)
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