Amplificadores operacionales

Operational Amplifiers
Table of contents

1. Design
1.1. The Differential Amplifier 1.2. Level Shifter 1.3. Power Amplifier

2. Characteristics 3. The Opamp without NFB 4. Linear Amplifiers
4.1. The Non-Inverting Amplifier 4.2. The Voltage Follower 4.3. The Inverting Amplifier

5. Frequency Characteristics
5.1. Band width 5.2. Slew Rate

6. Applications
6.1. Non-Inverting Amplifier6.2. Inverting Amplifier 6.3. With push-pull output 6.4. Summing Amplifier 6.5. Logarithmizing Amplifier 6.6. Signal Rectification 6.7. Voltage Regulator 6.8. Comparator 6.9. Schmitt Trigger 6.10. Astable Multivibrator 6.11. Phase Shifter

Operational Amplifiers
The theory of electrical signal processing requires amplifiers to perform, with electrical signals, mathematical operations such asaddition, subtraction, multiplication, division, differentiation, integration, etc. These amplifiers must fulfil the following requirements:
• • • • •

Differential inputs D.C. amplification Very high voltage gain Very high input resistance Very low output resistance

They are then called "operational amplifiers" (opamps) because they are able to perform mathematical operations. With opamps,even analog computers are constructed which surpass any digital computer when high speed of signal processing is required. The first opamps were built using discreet transistors, but it a was difficult and expensive process because of temperature drift problems. The big breakthrough came with integrated circuits. Having all circuit elements on one monolithic silicon chip solved most of thetemperature drift problems and allowed for cheap mass production. Today we have to consider the opamp as a circuit element. We will study its characteristics but not dwell on how it works internally.

1. Design
The basic form of an opamp is a high gain dc-amplifier with a differential input port and a single output port. A differential input has two terminals, which are both independent of ground orcommon. The signal between these two terminals is the input signal, which will be amplified. The terminals are called non-inverting input and inverting input. The two inputs can be used in three different ways: 1. Non-Inverting Amplifier: The input signal is applied between the non-inverting input and ground. The inverting input is connected to ground. The output signal will be in phase with theinput signal 2. Inverting Amplifier: The input signal is applied between the inverting input and ground. The non-inverting input is connected to ground. The output signal will be 180° out of phase with the input signal. 3. Differential Amplifier: Two input signals are each connected to the non-inverting and the

inverting input, using both common as second terminal. The output signal will be theamplified difference between the two. Uo = (Ui+ - Ui-) • g

Fig. 1.1. The three basic ways of applying input signals to the opamp. When there is no voltage difference between the input terminals, the output voltage should be 0. The internal circuit of opamps consists basically of three main parts:

1.1. The Differential Amplifier:
A differential amplifier stage consists of two transistors incommon emitter configuration which are supplied with a common emitter current.

Fig. 1.1.1. The basic design of a differential amplifier stage. As long as there is no voltage difference between the two bases of the transistors, the two transistors will draw the same collector currents and a certain voltage will appear at the output. If the base of T1 becomes more positive than of T2, T1 will drawmore current, the voltage across RC1 will increase. As the total current is constant, the current through T2 will decrease by the same amount. The voltage across RC2 will decrease and the output voltage becomes more positive. So the base of T1 is the non-inverting input. If the base of T2 becomes more positive than that of T1, T2 will draw more current. The voltage across RC2 increases and the...