AN6668 Datasheet, PDF (1/18 Pages) Intersil Corporation – Applications of the CA3080 High-Performance Operational Transconductance Ampliflers

Applications of the CA3080 High-Performance
®
Operational Transconductance Amplifiers
Application Note
May 2002
AN6668.2
Author: Hal Wittlinger
Introduction
The CA3080 and CA3080A are similar in generic form to
conventional operational amplifiers, but differ sufficiently to
justify an explanation of their unique characteristics. This
new class of operational amplifier not only includes the usual
differential input terminals, but also contains an additional
control terminal which enhances the device's flexibility for
use in a broad spectrum of applications. The amplifier incor-
porated in these devices is referred to as an Operational
Transconductance Amplifier (OTA), because its output sig-
nal is best described in terms of the output-current that it can
supply:
Transconductance g_M = ^∆---_∆--ⁱ-^O-_e---_I-^U-_N---^T-
The amplifier's output-current is proportional to the voltage
difference at its differential input terminals.
This Application Note describes the operation of the OTA
and features various circuits using the OTA. For example,
communications and industrial applications including modu-
lators, multiplexers, sample-and-hold-circuits, gain control
circuits and micropower comparators are shown and dis-
cussed. In addition, circuits have been included to show the
operation of the OTA being used in conjunction with CMOS
devices as post-amplifiers.
Figure 1 shows the equivalent circuit for the OTA. The output
signal is a current which is proportional to the transconduc-
tance (g_M) of the OTA established by the amplifier bias cur-
rent (I_ABC) and the differential input voltage (e_IN). The OTA
can either source or sink current at the output terminal,
depending on the polarity of the input signal.
V+
7
2-
OTA
R_IN
2R_O
e_IN
g_M x e_IN
2R_O
6 _{IOUT = gM(±eIN)}
3
+
g_M (mS) = 19.2 I_ABC (mA)
4
5 _V-
R_O (MΩ) ≈ 7.5/I_ABC (mA)
I_ABC
FIGURE 1. BASIC EQUIVALENT CIRCUIT OF THE OTA
The availability of the amplifier bias current (I_ABC) terminal
significantly increases the flexibility of the OTA and permits
the circuit designer to exercise his creativity in the utilization
of this device in many unique applications not possible with
the conventional operational amplifier.
Circuit Description
A simplified block diagram of the OTA is shown in Figure 2.
Transistors Q₁ and Q₂ comprise the differential input amplifier
found in most operational amplifiers, while the lettered-circles
(with arrows leading either into or out of the circles) denote
“current-mirrors”. Figure 3A shows the basic type of current-
mirror which is comprised of two transistors, one of which is
diode-connected. In a current-mirror with similar geometries
for Q_A and Q_B, the current I’ establishes a second current I
whose value is essentially equal to that of I’.
V+
7
Y
Z
INVERTING
INPUT
2
Q₁ Q₂
AMPLIFIER
BIAS CURRENT
5
W
I_ABC
NON-INVERTING
3 ^INPUT
OUTPUT
6
X
4
V-
FIGURE 2. SIMPLIFIED DIAGRAM OF THE OTA
This basic current-mirror configuration is sensitive to the
transistor beta (β). The addition of another active transistor,
shown in Figure 3B, greatly diminishes the circuit sensitivity
to transistor beta and increases the current-source output
impedance in direct proportion to the transistor beta. Cur-
rent-mirror W (Figure 2) uses the configuration shown in Fig-
ure 3A, while mirrors X, Y, and Z are basically the version
shown in Figure 3B. Mirrors Y and Z employ PNP transis-
tors, as depicted by the arrows pointing outward from the
mirrors. Appendix 1 describes current-mirrors in more detail.
I’
I I’
I
Q_A
Q_B
Q_B
Q_A
V-
V-
FIGURE 3A. DIODE-CONNECTED TRANSISTOR PAIRED WITH
TRANSISTOR
1
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