OPA861
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SBOS338–AUGUST 2005
QUIESCENT CURRENT CONTROL PIN
With this control loop, quiescent current will be nearly
constant with temperature. Since this method differs
from the temperature-dependent behavior of the
internal current source, other temperature-dependent
behavior may differ from that shown in the Typical
Characteristics. The circuit of Figure 28 will control
the IQ of the OPA861 somewhat more accurately than
with a fixed external resistor, RQ. Otherwise, there is
no fundamental advantage to using this more com-
plex biasing circuitry. It does, however, demonstrate
the possibility of signal-controlled quiescent current.
This capability may suggest other possibilities such
as AGC, dynamic control of AC behavior, or VCO.
The quiescent current of the transconductance
portion of the OPA861 is set with a resistor, RADJ
,
connected from pin 1 to –VS. The maximum quiesc-
ent current is 6mA. RADJ should be set between 50Ω
and 1kΩ for optimal performance of the OTA section.
This range corresponds to the 5mA quiescent current
for RADJ = 50Ω, and 1mA for RADJ = 1kΩ. If the IQ
adjust pin is connected to the negative supply, the
quiescent current will be set by the 250Ω internal
resistor.
Reducing or increasing the quiescent current for the
OTA section controls the bandwidth and AC behavior
as well as the transconductance. With RADJ = 250Ω,
this sets approximately 5.4mA total quiescent current
at 25°C. It may be appropriate in some applications to
trim this resistor to achieve the desired quiescent
current or AC performance.
BASIC APPLICATIONS CIRCUITS
Most applications circuits for the OTA section consist
of a few basic types, which are best understood by
analogy to a transistor. Used in voltage-mode, the
OTA section can operate in three basic operating
states—common emitter, common base, and com-
mon collector. In the current-mode, the OTA can be
useful for analog computation such as current ampli-
fier, current differentiator, current integrator, and cur-
rent summer.
Applications circuits generally do not show the
resistor RQ, but it is required for proper operation.
With a fixed RADJ resistor, quiescent current in-
creases with temperature (see Figure 11 in the
Typical Characteristics section). This variation of
current with temperature holds the transconductance,
gm, of the OTA relatively constant with temperature
(another advantage over a transistor).
Common-E Amplifier or Forward Amplifier
Figure 29 compares the common-emitter configur-
ation for a BJT with the common-E amplifier for the
OTA section. There are several advantages in using
the OTA section in place of a BJT in this configur-
ation. Notably, the OTA does not require any biasing,
and the transconductance gain remains constant over
temperature. The output offset voltage is close to 0,
compared with several volts for the common-emitter
amplifier.
It is also possible to vary the quiescent current with a
control signal. The control loop in Figure 28 shows
1/2 of a REF200 current source used to develop
100mV on R1. The loop forces 125mV to appear on
R2. Total quiescent current of the OPA861 is approxi-
mately 37 × I1, where I1 is the current made to flow
out of pin 1.
The gain is set in a similar manner as for the BJT
equivalent with Equation 1:
V+
RL
OPA861
G +
1
g
1/2 REF200
m ) RE
(1)
µ
100
A
IQ Adjust
R1
1.25k
Just as transistor circuits often use emitter degener-
ation, 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 perform-
ance.
1
I1
Ω
R2
425
Ω
TLV2262
Figure 28. Optional Control Loop for Setting
Quiescent Current
13
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