OPA660 DRIVES MAGNETIC RECORDING HEAD
By Jeff Huehn
The OPA660 is a unique solution to many high-speed design
problems. It offers the high-speed and wide-bandwidth of
discrete transistors necessary for RF, IF, and video circuitry
while maintaining the simplicity, reliability, and ease-of-use
of a monolithic, integrated circuit. For applications where
neither an op amp nor a discrete transistor quite fit, the
OPA660 may be exactly what you鈥檙e looking for.
The OPA660 consists of two distinct building blocks, a
buffer and an Operational Transconductance Amplifier
(OTA), that can be used independently or in combination to
achieve a variety of amplification stages. Quiescent current
for both blocks is set using an external resistor allowing the
user to program quiescent current depending on perfor-
mance needs. This resistor is omitted from the drawings for
clarity but is required for proper operation. The voltage
buffer is an open-loop buffer with a gain slightly less than
unity and is ideal for interstage buffering. The OTA is a
voltage controlled current source much like a bipolar transis-
tor with the very important difference that the sense of the
output current of the OTA is non-inverting. The B input is
a high impedance input similar to the base, the E input is a
low impedance input (or output in an E-follower configura-
tion) similar to an emitter and C is an output whose current
follows the equation:
I
C
= g
M
(V
B
鈥?V
E
)
The OTA can be used in most familiar transistor configura-
tions such as those shown in Figure 1, but remember that
there is no inversion between input voltage and output
current. The most commonly used is the Common-E ampli-
fier. Gains up to 20V/V can be achieved. There are two
phenomena that limit bandwidth in this configuration. The
first is a capacitance that increases with the transconductance
and reacts with the input impedance. This dominates when
g
M
is large. The second is a lowpass filter created by the
1
V
OUT
8
C
R
IN
V
IN
3 B
OTA
R
L
R
IN
V
IN
3 B
8
C
OTA
E
2
R
E
E
2
V
OUT
R
E
G
鈮?/div>
R
L
1
+ R
E
g
M
V
OUT
8
C
3 B
OTA
R
L
G
鈮?
Common-C Amplifier
Common-E Amplifier
E
2
R
E
V
IN
Common-B Amplifier
G
鈮?/div>
鈥揜
L
1
+ R
E
g
M
FIGURE 1. OTA Configurations.
漏
1993 Burr-Brown Corporation
AB-052
Printed in U.S.A. April, 1993
1
2
3
