Datasheet
1
2
3
4
8
7
6
5
+1
R
S
(25
Ω
to 200
Ω
)
R
S
(25
Ω
to 200
Ω
)
49.9
Ω
0.1
µ
F
R
ADJ
250
Ω
−
5V
(1)
+
2.2
µ
F
0.1
µ
F
Solid
Tantalum
+5V
(1)
+
2.2
µ
F
Solid Tantalum
R
Q
= 250
Ω
, roughly sets I
Q
= 11.2mA.
NOTE: (1) V
S
=
±
6.5V absolute maximum.
+V
S
−
V
S
49.9
Ω
V
I
V
O
Common-E Amplifier or Forward Amplifier
BASIC APPLICATIONS CIRCUITS
G +
R
L
1
g
m
) R
E
(1)
OPA860
www.ti.com
....................................................................................................................................................... SBOS331C – JUNE 2005 – REVISED AUGUST 2008
Figure 46. Basic Connections
With this control loop, quiescent current will be nearly
constant with temperature. Since this differs from the
temperature-dependent behavior of the internal
Figure 47 compares the common-emitter
current source, other temperature-dependent
configuration for a BJT with the common-E amplifier
behavior may differ from that shown in the Typical
for the OTA section. There are several advantages in
Characteristics. The circuit of Figure 45 will control
using the OTA section in place of a BJT in this
the I
Q
of the OTA section of the OPA860 somewhat
configuration. Notably, the OTA does not require any
more accurately than with a fixed external resistor,
biasing, and the transconductance gain remains
R
Q
. Otherwise, there is no fundamental advantage to
constant over temperature. The output offset voltage
using this more complex biasing circuitry. It does,
is close to 0, compared with several volts for the
however, demonstrate the possibility of
common-emitter amplifier.
signal-controlled quiescent current. This capability
The gain is set in a similar manner as for the BJT
may suggest other possibilities such as AGC,
equivalent with Equation 1 :
dynamic control of AC behavior, or VCO.
Most applications circuits for the OTA section consist
Just as transistor circuits often use emitter
of a few basic types, which are best understood by
degeneration, OTA circuits may also use
analogy to a transistor. Used in voltage-mode, the
degeneration. This option can be used to reduce the
OTA section can operate in three basic operating
effects that offset voltage and offset current might
states — common emitter, common base, and
otherwise have on the DC operating point of the OTA.
common collector. In the current-mode, the OTA can
The E-degeneration resistor may be bypassed with a
be useful for analog computation such as current
large capacitor to maintain high AC gain. Other
amplifier, current differentiator, current integrator, and
circumstances may suggest a smaller value capacitor
current summer.
used to extend or optimize high-frequency
performance.
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