Two-op-amp Instrumentation Amplifier

A two-op-amp instrumentation amplifier can also be used to

make a high-input-impedance DC differential amplifier (Fig-

ure 7). As in the three-op-amp circuit, this instrumentation

amplifier requires precise resistor matching for good CMRR.

R

Single-Supply Inverting Amplifier

There may be cases where the input signal going into the

amplifier is negative. Because the amplifier is operating in

single supply voltage, a voltage divider using R

implemented to bias the amplifier so the input signal is within

the input common-common voltage range of the amplifier.

The capacitor C

resistor R

source, V

quency, fc = 1/2

π R

As a result, the output signal is centered around mid-supply

(if the voltage divider provides V

input). The output can swing to both rails, maximizing the

signal-to-noise ratio in a low voltage system.

ACTIVE FILTER

Simple Low-Pass Active Filter

The simple low-pass filter is shown in Figure 9. Its low-

frequency gain(

ω → o) is defined by −R

low-frequency gains other than unity to be obtained. The

filter has a −20 dB/decade roll-off after its corner frequency

fc. R

R

frequency response of the filter is shown in Figure 10

Note that the single-op-amp active filters are used in to the

applications that require low quality factor, Q (

≤ 10), low

frequency (

≤ 5 kHz), and low gain (≤ 10), or a small value for

the product of gain times Q (

≤ 100). The op amp should have

an open loop voltage gain at the highest frequency of inter-

est at least 50 times larger than the gain of the filter at this

frequency. In addition, the selected op amp should have a

slew rate that meets the following requirement:

Slew Rate

≥ 0.5x(ω

Where

ω

the output peak-to-peak voltage.

10092011

FIGURE 7. Two-op-amp Instrumentation Amplifier

10092013

FIGURE 8. Single-Supply Inverting Amplifier

10092014

FIGURE 9. Simple Low-Pass Active Filter

10092015

FIGURE 10. Frequency Response of Simple Low-pass

Active Filter in Figure 9

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