OPA861
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SBOS338–AUGUST 2005
Common-C Amplifier
Current-Mode Analog Computations
Figure 32b shows the OPA861 connected as an
E-follower—a voltage buffer. It is interesting to notice
that the larger the RE resistor, the closer to unity gain
the buffer will be. If the OPA861 is to be used as a
buffer, use RE ≥ 500Ω for best results. For the
OPA861 used as a buffer, the gain is given by
Equation 3:
As mentioned earlier, the OPA861 can be used
advantageously for analog computation. Among the
application possibilities are functionality as a current
amplifier, current differentiator, current integrator, cur-
rent summer, and weighted current summer. Table 1
lists these different uses with the associated transfer
functions.
1
These functions can easily be combined to form
active filters. Some examples using these cur-
rent-mode functions are shown later in this document.
G +
[ 1
1
R
1 ) gm
E
(3)
V+
V+
VO
RL
G = 1
V
OS = 0.7V
VI
VO
RE
Noninverting Gain
VOS = Several Volts
−
V
(a) Transistor Common−Collector Amplifier
(Emitter Follower)
RE
1
G +
+ 1
1
R
1 ) gm
−
V
E
(a) Transistor Common−Base Amplifier
1
8
ǒ Ǔ
RO +
ø RE
gm
C
RL
RL
RE
Ω
100
G +
+ *
3
B
G = 1
VOS = 0V
1
OPA861
VI
RE ) gm
E
2
RE
8
VO
VO
C
Ω
100
3
B
Inverting Gain
VOS = 0V
OPA861
(b) OTA Common−C Amplifier
(Buffer)
E
2
RL
RE
Figure 32. Common-Collector vs Common-C
Amplifier
−
V
A low value resistor in series with the B-input is
recommended. This resistor helps isolate trace para-
sitic from the inputs, reduces any tendency to oscil-
late, and controls frequency response peaking. Typi-
cal resistor values are from 25Ω to 200Ω.
(b) OTA Common−B Amplifier
Figure 33. Common-Base Transistor vs
Common-B OTA
Common-B Amplifier
Figure 33 shows the Common-B amplifier. This con-
figuration produces an inverting gain and a low
impedance input. Equation 4 shows the gain for this
configuration.
RL
RL
RE
G +
[ *
1
RE ) gm
(4)
This low impedance can be converted to a high
impedance by inserting the buffer amplifier in series.
15
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