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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 ⎠
⎟
