OPA660 Burr-Brown, OPA660 Datasheet - Page 9

• Manufacturer Part Number: OPA660
• Description: Wide Bandwidth OPERATIONAL TRANSCONDUCTANCE AMPLIFIER AND BUFFER
• Manufacturer: Burr-Brown

BUFFER SECTION—AN OVERVIEW
The buffer section of the OPA660 is an open-loop buffer
consisting of complementary emitter-followers. It uses no
feedback, so its low frequency gain is slightly less than unity
and somewhat dependent on loading. It is designed prima-
rily for interstage buffering. It is not designed for driving
long cables or low impedance loads (although with small
signals, it may be satisfactory for these applications).
TRANSCONDUCTANCE
(OTA) SECTION—AN OVERVIEW
The symbol for the OTA section is similar to a transistor.
Applications circuits for the OTA look and operate much
like transistor circuits—the transistor, too, is a voltage-
controlled current source. Not only does this simplify the
understanding of applications circuits, but it aids the circuit
optimization process. Many of the same intuitive techniques
used with transistor designs apply to OTA circuits as well.
The three terminals of the OTA are labeled B, E, and C. This
calls attention to its similarity to a transistor, yet draws
distinction for clarity.
While it is similar to a transistor, one essential difference is
the sense of the C output current. It flows out the C terminal
for positive B-to-E input voltage and in the C terminal for
negative B-to-E input voltage. The OTA offers many advan-
tages over a discrete transistor. The OTA is self-biased,
simplifying the design process and reducing component
count. The OTA is far more linear than a transistor.
Transconductance of the OTA is constant over a wide range
of collector currents—this implies a fundamental improve-
ment of linearity.
BASIC CONNECTIONS
Figure 2 shows basic connections required for operation.
These connections are not shown in subsequent circuit
diagrams. Power supply bypass capacitors should be located
as close as possible to the device pins. Solid tantalum
capacitors are generally best. See “Circuit Layout” at the end
of the applications discussion and Figure 26 for further
suggestions on layout.
FIGURE 2. Basic Connections.
–5V
470pF
R = 250
I
Q
(1)
Q
20mA
(25 to
200
250
R
R
B
Q
sets roughly
Tantalum
Solid
10nF
+
2.2µF
1
2
3
4
NOTE: (1) V
1
S
8
7
6
5
= ±6V absolute max.
(25 to 200
+
+5V
Tantalum
R
2.2µF
10nF
Solid
B
(1)
470pF
9
QUIESCENT CURRENT CONTROL PIN
The quiescent current of the OPA660 is set with a resistor,
R
currents of both the buffer and OTA sections. This controls
the bandwidth and AC behavior as well as the
transconductance of the OTA section.
R
25 C. With a fixed 250
cause this current to vary from approximately 16mA to 26mA.
It may be appropriate in some applications to trim this resistor
to achieve the desired quiescent current or AC performance.
Applications circuits generally do not show resistor, R
but it is required for proper operation.
With a fixed R
temperature (see typical performance curve, Quiescent Current
vs Temperature). This variation of current with temperature
holds the transconductance, gm, of the OTA relatively con-
stant with temperature (another advantage over a transistor).
It is also possible to vary the quiescent current with a control
signal. The control loop in Figure 3 shows a 1/2 of a REF200
current source used to develop 100mV on R
forces 100mV to appear on R
OPA660 is approximately 85 • I
made to flow out of pin 1.
FIGURE 3. Optional Control Loop for Setting Quiescent
With this control loop, quiescent current will be nearly
constant with temperature. Since this differs from the tem-
perature-dependent behavior of the internal current source,
other temperature-dependent behavior may differ from that
shown in typical performance curves.
The circuit of Figure 3 will control the I
somewhat more accurately than with a fixed external resis-
tor, R
Q
Q
, connected from pin 1 to V–. It affects the operating
= 250 sets approximately 20mA total quiescent current at
V+
NOTE: (1) Requires input common-mode range and
output swing close to V–, thus the choice of OPA1013.
Q
1/2 REF200
. Otherwise, there is no fundamental advantage to
100µA
Current.
Q
1k
OPA1013
R
1
resistor, quiescent current increases with
1/2
(1)
resistor, process variations could
100
2
. Total quiescent current of the
1
OPA660
1
Current Source
, where I
425
R
Circuitry
2
I
Internal
1
50k
I
Q
Q
–V
1
of the OPA660
= 85 • (100µA)
= 20mA
85 • I
is the current
CC
4
1
. The loop
1
OPA660
R
R
1
2
Q
,
®