AN6077 INTERSIL [Intersil Corporation], AN6077 Datasheet

AN6077

Manufacturer Part Number

AN6077

Description

An IC Operational Transconductance Amplifier With Power Capability

Manufacturer

INTERSIL [Intersil Corporation]

Datasheet

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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.

FIGURE 1. EQUIVALENT DIAGRAM OF THE OTA

ABC

V-

V+

4-1

(mS)

(M )

OTA

19.2 • I

7.5/I

(mA)

Application Note

ABC

An IC Operational Transconductance Amplifier

(mA)

ABC

OUT

(mA)

Max

OUT

= g

(

(mA)

ABC

)

The major controlling factor in the OTA is the input ampliﬁer

bias current I

current and g

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

INVERTING

AMPLIFIER

CURRENT

1-888-INTERSIL or 321-724-7143

FIGURE 2. CURRENT MIRRORS W, X, Y AND Z USED IN

FIGURE 3. THE OUTPUT CURRENT TRANSFER

(OTA) With Power Capability

INPUT

BIAS

-0.2

-0.4

-0.6

-0.8

-1.0

1.0

0.8

0.6

0.4

0.2

-150

October 2000

ABC

THE OTA

CHARACTERISTIC OF THE OTA IS THE SAME AS

THAT OF AN IDEALIZED DIFFERENTIAL

AMPLIFIER

-100

are controlled by this current. In addition, the

; as explained in Figure 1, the total output

-50

NON-INVERTING

INPUT

DIFFERENTIAL AMPLIFIER

TRANSFER CHARACTERISTIC

(mV)

V+

V-

Intersil Corporation 2000

100

AN6077.3

OUTPUT

150

Related parts for AN6077

AN6077 Summary of contents

Page 1

... DIFFERENTIAL AMPLIFIER TRANSFER CHARACTERISTIC 0 -150 -100 - (mV) be CHARACTERISTIC OF THE OTA IS THE SAME AS THAT OF AN IDEALIZED DIFFERENTIAL AMPLIFIER ; as explained in Figure 1, the total output ABC are controlled by this current. In addition, the m © | Copyright Intersil Corporation 2000 AN6077.3 Z OUTPUT 6 X 100 150 ...

Page 2

This is analogous to using an elephant to carry a twig. It may be elegant but it takes a lot to keep it going! When operated in the gain control mode, one input of the standard transconductance multiplier is ...

Page 3

CA3080A Transistors from CA3046 array. AGC System with extended input range. FIGURE 6. A CIRCUIT SHOWING HOW THE SIGNAL ...

Page 4

DIFFERENTIAL - + INPUT DIFFERENTIAL R 2 INPUT DIFFERENTIAL R 2 INPUT - + FIGURE 10. ...

Page 5

E SIG EQUALIZER A = 32dB REF - INPUT = PICKUP =1mV N SIG FIGURE 12. BLOCK DIAGRAM OF A SYSTEM USING A “LOSSER” TYPE TONE CONTROL CIRCUIT ...

Page 6

R C “BOOST” “CUT” 0.01 F (CW) 15K 820 C (CCW TREBLE 0. 1800 0.001 F 5600 0.001 ...

Page 7

Performance Figure plot of the measured response of the complete amplifier at the extremes of tone control rotation. A comparison of Figure 16 with the computed curves of Figure B4 (Appendix B) shows good agreement. The total ...

Page 8

F 680pF 1. CA3080 56K S 5 56K 120K 0.002 F 680pF 1. CA3080 ...

Page 9

Note that the expressions for high frequency gain are identical for both bass circuits, while the expressions for low frequency gain are identical for the treble circuits. Figure B2 shows cut and boost bass and treble controls that have the ...

Page 10

The unaffected portion of the gain (A HIGH control and A for the treble control each case. LOW To make the controls work symmetrically, the low and high frequency break points must be equal for both boost ...

Page 11

ELECTRICAL ROTATION OF BASS CONTROL FIGURE B4 (A). References For Intersil documents available on the internet, see web site http://www.intersil.com/ Intersil AnswerFAX (321) 724-7800. ...

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