Datasheet

+V
CC
(13)
+V
CC
(5)
C
(12)
B
(3)
E
(2)
+1
R
B
R
L
R
B
R
E
V
−
Single Transistor
V+
V
I
V
O
(a) Common Emitter Amplifier
V
O
100
Ω
OTAV
I
B
E
R
L
R
E
NoninvertingGain
(b) Common−E Amplifier for OTA
Inverting Gain
V several volts
OS
≈
3
2
C
12
Transconductance varies over temperature. Transconductance remains constant over temperature.
V
OS
≈ 0
OPA615
www.ti.com ...................................................................................................................................... SBOS299E –FEBRUARY 2004–REVISED SEPTEMBER 2009
Figure 35 shows a simplified block diagram of the While the OTA function and labeling appear similar to
OPA615 OTA. Both the emitter and the collector those of a transistor, it offers essential distinctive
outputs offer a drive capability of ±20mA for driving differences and improvements: 1) The collector
low impedance loads. The emitter output is not current flows out of the C terminal for a positive
current-limited or protected. Momentary shorts to B-to-E input voltage and into it for negative voltages;
GND should be avoided, but are unlikely to cause 2) A common emitter amplifier operates in
permanent damage. non-inverting mode while the common base operates
in inverting mode; 3) The OTA is far more linear than
a bipolar transistor; 4) The transconductance can be
adjusted with an external resistor; 5) As a result of
the PTAT biasing characteristic, the quiescent current
increases as shown in the typical performance curve
vs temperature and keeps the AC performance
constant; 6) The OTA is self-biased and bipolar; and
7) The output current is approximately zero for zero
differential input voltages. AC inputs centered on zero
produce an output current centered on zero.
BASIC APPLICATION CIRCUITS
Most application circuits for the OTA section consist
of a few basic types which are best understood by
analogy to discrete transistor circuits. Just as the
transistor has three basic operating modes—common
emitter, common base, and common collector—the
OTA has three equivalent operating modes;
common-E, common-B, and common-C (see
Figure 36, Figure 37 and Figure 38). Figure 36 shows
the OTA connected as a Common-E amplifier, which
is equivalent to a common emitter transistor amplifier.
Input and output can be ground-referenced without
any biasing. The amplifier is noninverting because a
Figure 35. Simplified OTA Block Diagram
current flowing out of the emitter will also flow out of
the collector as a result of the current mirror shown in
Figure 35.
Figure 36. a) Common Emitter Amplifier Using a Discrete Transistor; b) Common-E Amplifier Using the
OTA Portion of the OPA615
Copyright © 2004–2009, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): OPA615