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Chapter 8
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A voltage amplifier
Simple voltage amplifier model
If the input resistance of the amplifier Rin
were very large, the source voltage vS and
the input voltage vin would be approximately
equal:
By an analogous argument, it can also be
seen that the desired output resistance for
the amplifier Rout should be very small, since
for an amplifier with Rout = 0, the load voltage
would be
We can see that as Rin approaches infinity and
Rout approaches zero, the ideal amplifier
magnifies the source voltage by a factor A
vL = AvS
Thus, two desirable characteristics for a
general-purpose voltage amplifier are a very
large input impedance and a very small output
impedance.
The ideal operational amplifier behaves very
much as an ideal difference amplifier, that is,
a device that amplifies the difference between
two input voltages. Operational amplifiers are
characterized by near-infinite input resistance
and very small output resistance. As shown in
Figure 8.4, the output of the op-amp is an
amplified version of the difference between the
voltages present at the two inputs.
The input denoted by a plus sign is called the
noninverting input (or terminal), while that
represented with a minus sign is termed the
inverting input (or terminal).
The current flowing into the input circuit of the
amplifier is zero, or:
The input signal to be amplified is connected to the
inverting terminal, while the noninverting terminal is
grounded.
Inverting amplifier
The voltage at the noninverting input v+ is easily identified as
zero, since it is directly connected to ground: v+ = 0.
The effect of the feedback connection from output to inverting
input is to force the voltage at the inverting input to be equal to
that at the noninverting input.
Summing amplifier
Noninverting amplifier
Voltage Follower
Differential amplifier
The analysis of the differential amplifier
may be approached by various methods; the
one we select to use at this stage consists of
1. Computing the noninverting- and invertingterminal voltages v+ and v−.
2. Equating the inverting and noninverting
input voltages: v− = v+.
3. Applying KCL at the inverting node, where
i2 = −i1.
The differential amplifier provides the ability to
reject common-mode signal components (such as
noise or undesired DC offsets) while amplifying the
differential-mode components. To provide
impedance isolation between bridge transducers
and the differential amplifier stage, the signals v1
and v2 are amplified separately.
Instrumentation amplifier
The class of filters one can obtain by means
of op-amp designs is called active filters.
Active low-pass filter
Normalized response of active low-pass filter
Active high-pass filter
Normalized response of active high-pass filter
Active bandpass filter
Normalized amplitude response of active bandpass filter
Op-amp integrator
Op-amp differentiator
The effect of limiting supply voltages is that
amplifiers are capable of amplifying signals
only within the range of their supply voltages.
Another property of all amplifiers that may pose
severe limitations to the op-amp is their finite
bandwidth.
Open-loop gain of practical op-amp
The finite bandwidth of the practical op-amp
results in a fixed gain-bandwidth product for
any given amplifier.
Another limitation of practical op-amps results
because even in the absence of any external
inputs, it is possible that an offset voltage will be
present at the input of an op-amp.
Another nonideal characteristic of op-amps
results from the presence of small input
bias currents at the inverting and
noninverting terminals.