Datasheet

Common-C Amplifier
G +
R
L
R
E
)
1
g
m
[ *
R
L
R
E
(4)
Current-Mode Analog Computations
G +
1
1 )
1
g
m
R
E
[ 1
(3)
OPA860 APPLICATIONS
G +
1
1 )
1
g
m
R
E
+ 1
R
O
+
1
g
m
100
Ω
V
I
3 B
2
E
C
8
G = 1
V
O
= 0V
G = 1
V
OS
= 0.7V
OTA
R
E
V
O
(b) Common−C Amplifier
(Buffer)
(a) Common−Collector Amplifier
(Emitter Follower)
V
O
V
I
R
E
V
−
V+
Common-B Amplifier
(b)Common-BAmplifier
(a)Common-BaseAmplifier
NoninvertingGain
V olts
OS
=SeveralV
R
E
V-
V+
V
O
V
IN
R
L
R
1
100W
3 B
2
E
C
8
OTA
R
E
V
IN
R
L
InvertingGain
V =0V
OS
V
O
G=
R
L
R +
E
1
g
m
= -
R
L
R
E
OPA860
www.ti.com ....................................................................................................................................................... SBOS331C – JUNE 2005 – REVISED AUGUST 2008
Figure 50 b shows the OTA connected as an
E-follower — a voltage buffer. It is interesting to notice
This low impedance can be converted to a high
that the larger the R
E
resistor, the closer to unity gain
impedance by inserting the buffer amplifier in series.
the buffer will be. If the OTA section is to be used as
a buffer, use R
E
≥ 500 Ω for best results. For the OTA
section used as a buffer, the gain is given by
Equation 3 :
As mentioned earlier, the OTA section of the OPA860
can be used advantageously for analog computation.
Among the application possibilities are functionality
as a current amplifier, current differentiator, current
integrator, current summer, and weighted current
summer. Table 1 lists these different uses with the
associated 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.
The OPA860 is comprised of both the OTA section
and the Buffer section. This applications information
focuses more on using both sections together to form
various useful amplifiers. A more thorough description
of the OTA section in filter applications can be found
in the OPA861 data sheet, available for download at
www.ti.com .
Figure 50. Common-Collector vs Common-C
Amplifier
A low value resistor in series with the B OTA and
buffer inputs is recommended. This resistor helps
isolate trace parasitic from the inputs, reduces any
tendency to oscillate, and controls frequency
response peaking. Typical resistor values are from
25 Ω to 200 Ω .
Figure 51 shows the Common-B amplifier. This
configuration produces an inverting gain and a low
impedance input. Equation 4 shows the gain for this
configuration.
Figure 51. Common-Base Transistor vs
Common-B OTA
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