Handbook for Sound Engineers

340 Chapter 12

In Fig. 12-41A, capacitors C 1 and C 2 block dc from
the previous circuit and provide a 6 dB/octave rolloff
below

(12-49)

(12-50)

The output voltage is

(12-51)

To reduce the common mode rejection ratio
(CMRR),

(12-52)

and

(12-53)

Summing Inverter Amplifiers. In the summing in-
verter, Fig. 12-32G, the virtual ground characteristic of
the amplifier's summing point is used to produce a
scaling adder. In this circuit, Iin is the algebraic sum of
the number of inputs.

(12-54)

and the total input current is

(12-55)

and

(12-56)

Therefore

(12-57)

The output voltage is found with the equation

(12-58)

It is interesting that even though the inputs mix at

one point, all signals are isolated from each other and

one signal does not effect the others and one impedance

does not effect the rest.

Operational Transconductance Amplifiers. The oper-

ational transconductance amplifier (OTA) provides

transconductance gain and current output rather than

voltage gain and output as in an operational amplifier.

The output is the product of the input voltage and

amplifier transconductance, and it can be considered an

infinite impedance current generator.

Varying the bias current on the OTA can completely

control the open-loop gain of the device and can also

control the total power input.

OTAs are useful as multipliers, automatic gain

control (agc) amplifiers, sample and hold circuits,

multiplexers, and multivibrators to name a few.

12.3.4 Dedicated Analog Integrated Circuits for

Audio Applications

By Les Tyler and Wayne Kirkwood, THAT Corp.

The first ICs used in audio applications were

general-purpose op-amps like the famous Fairchild

μA741. Early op-amps like the classic 741 generally

had drawbacks that limited their use in professional

audio, from limited slew rate to poor clipping behavior.

Early on, IC manufacturers recognized that the rela-

tively high-volume consumer audio market would make

good use of dedicated ICs tailored to specific applica-

tions such as phono preamplifiers and companders. The

National LM381 preamplifier and Signetics NE570

compander addressed the needs of consumer equipment

makers producing high-volume products such as phono

preamplifiers and cordless telephones. Operational

Transconductance Amplifiers, such as the RCA

CA3080, were introduced around 1970 to primarily

serve the industrial market. It was not long before

professional audio equipment manufacturers adapted

OTAs for professional audio use as early voltage-

fC

1

1

2 SR 1 C 1

=-------------------

fC 2 1

2 S R 3 +R 4 C 2

=-------------------------------------

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2

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----- -

R 4

R 3

{----- -

fC 1 =fC 2

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1

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1

= ---------

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2

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2

= ---------

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= ---------

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2

}Iin

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= ++

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  Rf

----------=

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  Rf

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1

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Rin

1

---------

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Rin

2

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Rin

2

---------

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