Chapter 8Chapter 8

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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; theone we select to use at this stage consists of

1.Computing the noninverting- and inverting-terminal 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.