Datasheet
R
1
1.25k
Ω
TLV2262
OPA861
1/2 REF200
100
µ
A
R
2
425
Ω
V+
I
Q
Adjust
1 I
1
G +
R
L
1
g
m
) R
E
OPA861
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SBOS338G –AUGUST 2005–REVISED MAY 2013
QUIESCENT CURRENT CONTROL PIN With this control loop, quiescent current will be nearly
constant with temperature. Since this method differs
The quiescent current of the transconductance
from the temperature-dependent behavior of the
portion of the OPA861 is set with a resistor, R
ADJ
,
internal current source, other temperature-dependent
connected from pin 1 to –V
S
. The maximum
behavior may differ from that shown in the Typical
quiescent current is 6mA. R
ADJ
should be set
Characteristics. The circuit of Figure 31 will control
between 50Ω and 1kΩ for optimal performance of the
the I
Q
of the OPA861 somewhat more accurately than
OTA section. This range corresponds to the 5mA
with a fixed external resistor, R
Q
. Otherwise, there is
quiescent current for R
ADJ
= 50Ω, and 1mA for R
ADJ
=
no fundamental advantage to using this more
1kΩ. If the I
Q
adjust pin is connected to the negative
complex biasing circuitry. It does, however,
supply, the quiescent current will be set by the 250Ω
demonstrate the possibility of signal-controlled
internal resistor.
quiescent current. This capability may suggest other
possibilities such as AGC, dynamic control of AC
Reducing or increasing the quiescent current for the
behavior, or VCO.
OTA section controls the bandwidth and AC behavior
as well as the transconductance. With R
ADJ
= 250Ω,
this sets approximately 5.4mA total quiescent current BASIC APPLICATIONS CIRCUITS
at 25°C. It may be appropriate in some applications to
Most applications circuits for the OTA section consist
trim this resistor to achieve the desired quiescent
of a few basic types, which are best understood by
current or AC performance.
analogy to a transistor. Used in voltage-mode, the
Applications circuits generally do not show the OTA section can operate in three basic operating
resistor R
Q
, but it is required for proper operation. states—common emitter, common base, and
common collector. In the current-mode, the OTA can
With a fixed R
ADJ
resistor, quiescent current
be useful for analog computation such as current
increases with temperature (see Figure 12 in the
amplifier, current differentiator, current integrator, and
Typical Characteristics section). This variation of
current summer.
current with temperature holds the transconductance,
g
m
, of the OTA relatively constant with temperature
Common-E Amplifier or Forward Amplifier
(another advantage over a transistor).
Figure 32 compares the common-emitter
It is also possible to vary the quiescent current with a
configuration for a BJT with the common-E amplifier
control signal. The control loop in Figure 31 shows
for the OTA section. There are several advantages in
1/2 of a REF200 current source used to develop
using the OTA section in place of a BJT in this
100mV on R
1
. The loop forces 125mV to appear on
configuration. Notably, the OTA does not require any
R
2
. Total quiescent current of the OPA861 is
biasing, and the transconductance gain remains
approximately 37 × I
1
, where I
1
is the current made to
constant over temperature. The output offset voltage
flow out of pin 1.
is close to 0, compared with several volts for the
common-emitter amplifier.
The gain is set in a similar manner as for the BJT
equivalent with Equation 1:
(1)
Just as transistor circuits often use emitter
degeneration, OTA circuits may also use
degeneration. This option can be used to reduce the
effects that offset voltage and offset current might
otherwise have on the DC operating point of the OTA.
The E-degeneration resistor may be bypassed with a
large capacitor to maintain high AC gain. Other
circumstances may suggest a smaller value capacitor
used to extend or optimize high-frequency
Figure 31. Optional Control Loop for Setting
performance.
Quiescent Current
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