AN6077 Datasheet, PDF (1/11 Pages) Intersil Corporation – An IC Operational Transconductance Amplifier With Power Capability

An IC Operational Transconductance Amplifier
CaNllOCoRerEnetCmOraOBalMiSAl:[e-mail]
(OTA) With Power Capability
October 2000
AN6077.3
[ /Title
(AN60
77)
/Sub-
ject
(An IC
Opera-
tional
Transc
onduc-
tance
Ampli-
fier
(OTA)
With
Power
Capa-
bility)
/Autho
r ()
/Key-
words
(Inter-
sil
Corpo-
ration,
power
switch,
power
ampli-
fier,
pro-
gram-
mable
power
switch)
/Cre-
ator ()
In 1969, the first triple operational transconductance
amplifier or OTA was introduced. The wide acceptance of
this new circuit concept prompted the development of the
single, highly linear operational transconductance amplifier,
the CA3080. Because of its extremely linear
transconductance characteristics with respect to amplifier
bias current, the CA3080 gained wide acceptance as a gain
control block. The CA3094 improved on the performance of
the CA3080 through the addition of a pair of transistors;
these transistors extended the current carrying capability to
300mA, peak. This new device, the CA3094, is useful in an
extremely broad range of circuits in consumer and industrial
applications; this paper describes only a few of the many
consumer applications.
What Is an OTA?
The OTA, operational transconductance amplifier, concept is
as basic as the transistor; once understood, it will broaden the
designer's horizons to new boundaries and make realizable
designs that were previously unobtainable. Figure 1 shows an
equivalent diagram of the OTA. The differential input circuit is
the same as that found on many modern operational
amplifiers. The remainder of the OTA is composed of current
mirrors as shown in Figure 2. The geometry of these mirrors is
such that the current gain is unity. Thus, by highly
degenerating the current mirrors, the output current is
precisely defined by the differential input amplifier. Figure 3
shows the output current transfer characteristic of the
amplifier. The shape of this characteristic remains constant
and is independent of supply voltage. Only the maximum
current is modified by the bias current.
7 V+
-
2
OTA
R_IN
e_in
2R_O
g_m e_in
6
I_OUT = g_m (±e_in)
2R_O
3
+
4 V-
5 I_ABC
g_m = 19.2 • I_ABC
(mS)
(mA)
R_O ≈ 7.5/I_ABC
(MΩ) (mA)
±I_OUT ≈ I_ABC
Max (mA)
(mA)
FIGURE 1. EQUIVALENT DIAGRAM OF THE OTA
7 V+
Y
Z
INVERTING
INPUT
2
Q₁ Q₂
3
NON-INVERTING
INPUT
OUTPUT
6
AMPLIFIER
BIAS ₅
W
X
CURRENT _IABC
4 V-
FIGURE 2. CURRENT MIRRORS W, X, Y AND Z USED IN
THE OTA
1.0
0.8
DIFFERENTIAL AMPLIFIER
TRANSFER CHARACTERISTIC
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1.0
-150 -100
-50
0
50
∆V_be (mV)
100 150
FIGURE 3. THE OUTPUT CURRENT TRANSFER
CHARACTERISTIC OF THE OTA IS THE SAME AS
THAT OF AN IDEALIZED DIFFERENTIAL
AMPLIFIER
The major controlling factor in the OTA is the input amplifier
bias current I_ABC; as explained in Figure 1, the total output
current and g_m are controlled by this current. In addition, the
input bias current, input resistance, total supply current, and
output resistance are all proportional to this amplifier bias
current. These factors provide the key to the performance of
this most flexible device, an idealized differential amplifier,
i.e., a circuit in which differential input to single ended output
conversion can be realized. With this knowledge of the
basics of the OTA, it is possible to explore some of the
applications of the device.
DC Gain Control
The methods of providing DC gain control functions are
numerous. Each has its advantage: simplicity, low cost, high
level control, low distortion. Many manufacturers who have
nothing better to offer propose the use of a four quadrant
4-1
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