An operational amplifier is a direct-coupled high-gain amplifier usually consisting of one or more differential amplifiers and usually followed by a level translator and an output stage. The output stage is generally a push-pull or push-pull complementary-symmetry pair. An operational amplifier is available as a single integrated circuit package.
The operational amplifier is a versatile device that can be used to amplify dc as well as ac input signals and was originally designed for computing such mathematical functions as addition, subtraction, multiplication, and integration. Thus the name operational amplifier stems from its original use for these mathematical operations and is abbreviated to op-amp. With the addition of suitable external feedback components, the modern day op-amp can be used for a variety of applications, such as ac and dc signal amplification, active filters, oscillators, comparators, regulators, and others.
Block Diagram Representation of a Typical Op-Amp
Since an op-amp is a multistage amplifier, it can be represented by a block diagram.
The input stage is the dual-input, balanced-output differential amplifier. This stage generally provides most of the voltage gain of the amplifier and also establishes the input resistance of the op-amp. The intermediate stage is usually another differential, which is driven by the output of the first stage. In most amplifiers the intermediate stage is dual input, unbalanced (single-ended) output. Because direct coupling is used, the dc voltage at the output of the intermediate stage is well above ground potential. therefore, generally, the level translator (shifting) circuit is used after the intermediate stage to shift the dc level at the output of the intermediate stage downward to zero volts with respect to ground. The final stage is usually a push-pull complementary amplifier output stage. The output stage increases the output voltage swing and raises the current supplying capability of the op-amp. A well-designed output stage also provides low output resistance.
Schematic Symbol
Figure shows the most widely used symbols for an op-amp with two inputs and one output. For simplicity, power supply and other pin connections are omitted. Since the input differential amplifier stage of the op-amp is designed to be operated in the differential mode, the differential inputs are designated by the (+) and (-) notations. The (+) input is the non-inverting input. An ac signal (or dc voltage) applied to this input produces and in phase (or same polarity) signal at the output. On the other hand, the (-) input is the inverting input because an ac signal (or dc voltage) applied to this input produces an 180o out-of-phase (or opposite polarity) signal at the output.
V1 = Voltage at the non-inverting input (volts)
V2 = Voltage at the inverting inputs (volts)
V = output voltage (volts)
All these are voltages are measured with respect to ground
A = large-signal voltage gain, which is specified on the data sheet for an op-amp
Level Translator
Because of the direct coupling the dc level at the emitter rises from stages to stage. This increase in dc level tends to shift the operating point of the succeeding stages and therefore limits the output voltage swing and may even distort the output signal.
To shift the output dc level to zero, level translator circuits are used. An emitter follower with voltage divider is the simplest form of level translator as shown in Figure 3.
Figure 3
Thus a dc voltage at the base of Q produces 0 V dc at the output. It is decided by R1 and R2. Instead of voltage divider emitter follower either with diode current bias or current mirror bias as shown in Figure 4 may be used to get better results.
Figure 4
In this case, level shifter, which is common collector amplifier, shifts the level by 0.7 V. If this shift is not sufficient, the output may be taken at the junction of two resistors in the emitter leg.
Figure 5, shows a complete OP-AMP circuit having input different amplifiers with balanced output, intermediate stage with unbalanced output, level shifter and an output amplifier.
Figure 5
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