6 Operational Amplifiers

Chapter 6 Operational Amplifiers

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • Chapter 6Operational AmplifiersThe operational amplifier (op-amp) was designed to perform mathematical operations. Al-though now superseded by the digital computer, op-amps are a common feature of modernanalog electronics.The op-amp is constructed from several transistor stages, which commonly include adifferential-input stage, an intermediate-gain stage and a push-pull output stage. The dif-ferential amplifier consists of a matched pair of bipolar transistors or FETs. The push-pullamplifier transmits a large current to the load and hence has a small output impedance.The op-amp is a linear amplifier with Vout∝ Vin. The DC open-loop voltage gain of atypical op-amp is 103 to 106. The gain is so large that most often feedback is used to obtaina specific transfer function and control the stability.Cheap IC versions of operational amplifiers are readily available, making their use popularin any analog circuit. The cheap models operate from DC to about 20 kHz, while the high-performance models operate up to 50 MHz. A popular device is the 741 op-amp which dropsoff 6 dB/octave above 5 Hz. Op-amps are usually available as an IC in an 8-pin dual, in-linepackage (DIP). Some op-amp ICs have more than one op-amp on the same chip.Before proceeding we define a few terms:linear amplifier – the output is directly proportional to the amplitude of input signal.open-loop gain, A – the voltage gain without feedback (≈ 105).closed-loop gain, G – the voltage gain with negative feedback (approximation to H(jω)).negative feedback – the output is connected to the inverting input forming a feedbackloop (usually through a feedback resistor RF ).6.1Open-Loop AmplifiersFigure 6.1a shows a complete diagram of an operational amplifier. A more common version ofthe diagram is shown in figure 6.1b, where missing parts are assumed to exist. The invertinginput means that the output signal will be 1800 out of phase with the input applied to thisterminal. On the diagram V++≡ +VCC = +15 V (DC) and V−− ≡−VCC =−15 V (DC).105