Electronics Module by Sam Kinyera OBWOYA - HTML preview

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Figure 5.5 Clipping of an Amplifier’s Output

The bandwidth is limited in the same way as dynamic range of the amplifier. The

gain-bandwidth of an Op-amps is fixed. When an op-amp gain-bandwidth 3MHz is

connected to have a gain of 100, then the bandwidth of the amplifier will be limited

to 30kHz 100

( × 30kHz = 3MHz). All op-amps introduce noise to the signal and

this is a major limitation of the amplifier circuit. Resistors also introduce noise in

the circuit. The equation for this thermal noise is

2

V

= 4kTBR

5.2

( )

noise

where k is Boltzmann’s constant, T is the temperature, B is the bandwidth of the measurement device, and R is the value of the resistance.

Anothere limitation of the op-amp is offset voltage. All op-amps have a small amount

of voltage present between the inverting and non-inverting terminals. This DC po-

tential is then amplified just as if it was part of the signal from the sensor.

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(iii)Instrumentation amplifier

Possibly the most important circuit configuration for amplifying sensor output is the

instrumentation amplifier (IA). An IA should have:

1. Finite, accurate and stable gain, usually between 1 and 1000.

2. Extremely high input impedance.

3. Extremely low output impedance

4. Extremely high CMRR.

Note that CMRR (common mode rejection ratio) is defined as:

A

CMMR = vd

5.3

( )

A

vc

Where:

V

A =

out

= differential-mode gain

5.4

( )

vd V + − V −

V

A =

out

= common-mode gain

5.5

( )

vc

V + + V −

2

The difference amplifier described here, does not satisfy the second requirement of

high input impedance. This problem is solved by placing a non-inverting amplifier

at each one of the inputs to the difference amplifier as shown in Fig. 5.6. Remember

that a non-inverting amplifier has a nearly infinite input impedance. In Fig. 5.6, the

two resistors are connected together to create one common resistor, R instead of

G

grounding the resistors. The overall differential gain of the circuit is given by Eq.

5.6:

⎛

R ⎞ ⎛ R ⎞

A = 1+ 2 3

2

5.6

( )

vd

⎝⎜

R

⎝⎜ R

G ⎠

⎟

1 ⎠

⎟