Datasheet

I
C
+
V
IN
r
E
) R
E
or R
E
+
V
IN
I
C
* r
E
OPA615
SBOS299E –FEBRUARY 2004–REVISED SEPTEMBER 2009...................................................................................................................................... www.ti.com
DISCUSSION OF PERFORMANCE
The OPA615, which contains a wideband Operational
OPERATIONAL TRANSCONDUCTANCE
Transconductance Amplifier (OTA) and a fast
AMPLIFIER (OTA) SECTION
sampling comparator (SOTA), represents a complete
subsystem for very fast and precise DC restoration,
OVERVIEW
offset clamping and correction to GND or to an
adjustable reference voltage, and low frequency hum
The symbol for the OTA section is similar to that of a
suppression of wideband operational or buffer
bipolar transistor, and the self-biased OTA can be
amplifiers.
viewed as either a quasi-ideal transistor or as a
voltage-controlled current source. Application circuits
Although the IC was designed to improve or stabilize
for the OTA look and operate much like transistor
the performance of complex, wideband video signals,
circuits—the bipolar transistor is also a
it can also be used as a sample-and-hold amplifier,
voltage-controlled current source. Like a transistor, it
high-speed integrator, peak detector for nanosecond
has three terminals: a high-impedance input (base)
pulses, or as part of a correlated double sampling
optimized for a low input bias current of 0.3μA, a
system. A wideband Operational Transconductance
low-impedance input/output (emitter), and the
Amplifier (OTA) with a high-impedance cascode
high-impedance current output (collector).
current source output and a fast and precise
sampling comparator sets a new standard for
The OTA consists of a complementary buffer
high-speed sampling applications.
amplifier and a subsequent complementary current
mirror. The buffer amplifier features a Darlington
Both the OTA and the sampling comparator can be
output stage and the current mirror has a cascoded
used as stand-alone circuits or combined to create
output. The addition of this cascode circuitry
more complex signal processing stages such as
increases the current source output resistance to
sample-and-hold amplifiers. The OPA615 simplifies
1.2MΩ. This feature improves the OTA linearity and
the design of input amplifiers with high hum
drive capabilities. Any bipolar input voltage at the
suppression; clamping or DC-restoration stages in
high impedance base has the same polarity and
professional broadcast equipment, high-resolution
signal level at the low impedance buffer or emitter
CAD monitors and information terminals; and signal
output. For the open-loop diagrams, the emitter is
processing stages for the energy and peak value of
connected to GND; the collector current is then
nanoseconds pulses. This device also eases the
determined by the voltage between base and emitter
design of high-speed data acquisition systems behind
times the transconductance. In application circuits
a CCD sensor or in front of an analog-to-digital
(Figure 36b), a resistor R
E
between the emitter and
converter (ADC).
GND is used to set the OTA transfer characteristics.
An external resistor on the SO-14 package, R
Q
,
The following formulas describe the most important
allows the user to set the quiescent current. R
Q
is
relationships. r
e
is the output impedance of the buffer
connected from Pin 1 (I
Q
adjust) to –V
CC
. It
amplifier (emitter) or the reciprocal of the OTA
determines the operating currents of the OTA section
transconductance. Above ±5mA, the collector current,
and controls the bandwidth and AC behavior as well
I
C
, will be slightly less than indicated by the formula.
as the transconductance of the OTA.
Besides the quiescent current setting feature, a
(1)
Proportional-to-Absolute-Temperature (PTAT) supply
current control will increase the quiescent current
The R
E
resistor may be bypassed by a relatively large
versus temperature. This variation holds the
capacitor to maintain high AC gain. The parallel
transconductance (g
m
) of the OTA and comparator
combination of R
E
and this large capacitor form a
relatively constant versus temperature. The circuit
high-pass filter, enhancing the high frequency gain.
parameters listed in the specification table are
Other cases may require an RC compensation
measured with R
Q
set to 300Ω, giving a nominal
network in parallel to R
E
to optimize the
quiescent current at 13mA. While not always shown
high-frequency response. The large-signal bandwidth
in the application circuits, this R
Q
= 300Ω is required
(V
O
= 1.4V
PP
) measured at the emitter achieves
to get the 13mA quiescent operating current.
770MHz. The frequency response of the collector is
directly related to the resistor value between the
collector and GND; it decreases with increasing
resistor values, because of the low-pass filter formed
with the OTA C-output capacitance.
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