Chapter 10: Operational Amplifiers

An operational amplifier, or op-amp, is a very high gain differential amplifier with high input impedance and low output impedance. Typical uses of the operational amplifier are to provide voltage amplitude changes (amplitude and polarity), oscillators, filter circuits, and many types of instrumentation circuits. An op-amp contains a number of differential amplifier stages to achieve a very high voltage gain.



Figure 10.1 shows a basic op-amp with two inputs and one output as would result using a differential amplifier input stage. Each input results in either the same or an opposite polarity (or phase) output, depending on whether the signal is applied to the plus (+) or the minus (–) input, respectively.





Single-Ended Input
Single-ended input operation results when the input signal is connected to one input with the other input connected to ground. Figure 10.2 shows the signals connected for this operation. In Fig. 10.2a, the input is applied to the plus input (with minus input at ground), which results in an output having the same polarity as the applied input signal. Figure 10.2b shows an input signal applied to the minus input, the output then being opposite in phase to the applied signal.






Figure 10.1
Basic op-amp



Figure 10.2
Single-ended operation




Double-Ended (Differential) Input
In addition to using only one input, it is possible to apply signals at each input—this being a double-ended operation. Figure 10.3a shows an input, Vd applied between the two input terminals (recall that neither input is at ground), with the resulting amplified output in phase with that applied between the plus and minus inputs. Figure 10.3b shows the same action resulting when two separate signals are applied to the inputs, the difference signal being Vi1 – Vi2.






Figure 10.3
Double-ended (differential) operation




Double-Ended Output
Whereas the operation discussed so far has a single output, the op-amp can also be operated with opposite outputs, as shown in Fig. 10.4. An input applied to either input will result in outputs from both output terminals, these outputs always being opposite in polarity. Figure 10.5 shows a single-ended input with a double-ended output. As shown, the signal applied to the plus input results in two amplified outputs of opposite polarity. Figure 10.6 shows the same operation with a single output measured between output terminals (not with respect to ground). This difference output signal is Vo1 – Vo2. The difference output is also referred to as a floating signal since neither output terminal is the ground (reference) terminal.



Notice that the difference output is twice as large as either Vo1 or Vo2 because they are of opposite polarity and subtracting them results in twice their amplitude [e.g., 10 V – (–10 V) = 20 V]. Figure 10.7 shows a differential input, differential output operation. The input is applied between the two input terminals and the output taken from between the two output terminals. This is fully differential operation.






Figure 10.4
Double-ended output



Figure 10.5
Double-ended output with single-ended input




Figure 10.6
Double-ended output





Figure 10.7
Differential-input, differential-output operation




Common-Mode Operation
When the same input signals are applied to both inputs, common-mode operation results, as shown in Fig. 10.8. Ideally, the two inputs are equally amplified, and since they result in opposite-polarity signals at the output, these signals cancel, resulting in 0-V output. Practically, a small output signal will result.




Figure 10.8
Common-mode operation

Common-Mode Rejection
A significant feature of a differential connection is that the signals that are opposite at the inputs are highly amplified, whereas those that are common to the two inputs are only slightly amplified—the overall operation being to amplify the difference signal while rejecting the common signal at the two inputs. Since noise (any unwanted input signal) is generally common to both inputs, the differential connection tends to provide attenuation of this unwanted input while providing an amplified output of the difference signal applied to the inputs. This operating feature is referred to as common-mode rejection.

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