For simplicity, the op amp is modelled as an ideal integrator

with a unity gain frequency of A

(or gain) in the frequency domain is A

equations in the frequency domain, ignoring C

ment, results in an expression for the gain shown in Equation

(1).

(1)

It can be inferred from the denominator of the transfer func-

tion that it has two poles, whose expressions can be ob-

tained by solving for the roots of the denominator and are

shown in Equation (2)

(2)

Equation (2) shows that as the values of R

increased, the magnitude of the poles, and hence the band-

width of the amplifier, is reduced. This theory is verified by

using different values of R

Figure 1 and by comparing their frequency responses. In

Figure 3 the frequency responses for three different values

of R

Ω, the

response is flattest and widest; whereas, it narrows and

peaks significantly when both their values are changed to 10

k

Ω or 30 kΩ. So it is advisable to use lower values of R

R

also help in high sensitivity circuits since they add less noise.

A way of reducing the gain peaking is by adding a feedback

capacitance C

pole in the system and prevents the formation of pairs of

complex conjugate poles which cause the gain to peak.

Figure 4 shows the effect of C

the circuit. Adding a capacitance of 2 pF removes the peak,

while a capacitance of 5 pF creates a much lower pole and

reduces the bandwidth excessively.

AUDIO PRE-AMPLIFIER WITH BANDPASS FILTERING

With low input referred voltage noise, low supply voltage and

low supply current, and a low harmonic distortion, the

LMV791 is ideal for audio applications. Its wide unity gain

bandwidth allows it to provide large gain for a wide range of

frequencies and it can be used to design a pre-amplifier to

drive a load of as low as 600

Ω with less than 0.01% distor-

tion. Two amplifier circuits are shown in Figure 5 and Figure

6. Figure 5 is an inverting amplifier and provides a gain of

−10, while Figure 6 is a non-inverting amplifier and provides

a gain of 11. In either of these circuits, the coupling capacitor

C

providing gain, while the feedback capacitor C

20116864

FIGURE 2.

20116859

FIGURE 3.

20116860

FIGURE 4.

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