Handbook for Sound Engineers

338 Chapter 12

packages—the 8 pin TO-99 metal can, the 8 pin
dual-in-line package (MINI DIP), and the 14 pin DIP.

Inverting Amplifiers. In the inverting amplifier the +
input is grounded and the signal is applied to the minus
(input, Fig. 12-37. The output of the circuit is deter-
mined by the input resistor R 1 and the feedback resistor
Rf.

(12-43)

where,
Ein is the signal input voltage in volts,
Rf is the feedback resistor in ohms,
R 1 is the input resistor in ohms.

The low frequency rolloff is

(12-44)

Noninverting Amplifier. In the noninverting amplifier,
Fig. 12-38. the signal is applied to the plus input, while
the minus input is part of the feedback loop. The output
is

(12-45)

The low-frequency rolloff is in two steps.

(12-46)

(12-47)

To keep low-frequency noise gain at a minimum,

keep

Power Supply Compensation. The power supply for

wideband op-amp circuits should be bypassed with

capacitors, Fig. 12-39A, between the plus and minus pin

and common. The leads should be as short as possible

and as close to the IC as possible. If this is not possible,

bypass capacitors should be on each printed circuit

board.

Input Capacitance Compensation. Stray input capaci-

tance can lead to oscillation in feedback op-amps

because it represents a potential phase shift at the

frequency of

(12-48)

where,

Rf is the feedback resistor,

Cs is the stray capacitance.

One way to reduce this problem is to keep the value

of Rf low. The most useful way, however, is to add a

compensation capacitor, Cf, across Rf as shown in Fig.

12-39B. This makes Cf /Rf and Cs/Rin a frequency

compensated divider.

Output Capacitance Compensation. Output capaci-

tance greater than 100 pF can cause problems, requiring

a series resistor Ro being installed between the output of

the IC and the load and stray capacitance as shown in

Fig. 12-39C. The feedback resistor (Rf) is connected

after Ro to compensate for the loss in signal caused by

Ro. A compensating capacitor (Cf) bypasses Rf to reduce

gain at high frequencies.

Gain and Bandwidth. A perfect op-amp would have

infinite gain and infinite bandwidth. In real life

however, the dc open loop voltage gain is around

100,000 or 100 dB and the bandwidth where gain is 0 is

1 MHz, Fig. 12-40.

To determine the gain possible in an op-amp, for a

particular bandwidth, determine the bandwidth, follow

vertically up to the open loop gain response curve and

horizontally to the voltage gain. This, of course, is with

no feedback at the upper frequency. For example, for a

Figure 12-37. A simple inverting amplifier.

Figure 12-38. A simple noninverting amplifier.

EO Ein

• Rf
  R 1

= ©¹§·--------

fC^1

2 SR 1 C 1

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

+

C 1 R 1 Rf

Ein

2

3

6

Eo

EO Iin

1 +Rf

R 1

--------------

©¹

= §·

C 1

C 2 R 4

R 3

R 1 Rf

Ein

Eo

2

(^3) +
(^6)
fC 1 1
2 SR 1 C 1
= -------------------
fC
2
1
2 SR 3 C 2
= -------------------
fC 1 !fC 2.
f^1
2 SRfCs
------------------=