Introduction to Basic Electronics Lecture - 6

Basic Electronics Signal Amplifier - Introduction - Operational Amplifier - Power Amplifier : Class A,B,AB - Distortion

Introduction to Amplifiers In "Electronics", signal amplifiers are widely used devices as they have the ability to amplify a relatively small input voltage signal, for example from a sensor or microphone, into a much larger output signal to drive a relay, lamp or loudspeaker for example. There are many forms of amplifiers, from Operational Amplifier and Small Signal Amplifiers up to Large Signal and Power-Amplifiers. Amplifiers can be thought of as a simple box or block containing the amplifying device, such as a Transistor, Field Effect Transistor or Opamp, and which has two input terminals and two output terminals with the output signal being greater than that of the input signal, being "Amplified".

Introduction to Amplifiers An amplifier has three main properties, - Input Resistance (Rin) - Output Resistance(Rout) - Gain(A). No matter how complicated an amplifier circuit is, a general amplifier model can be used to show the relationship of these three properties. Ideal Amplifier Model

Amplifier Gain(A) The gain of an amplifier can be said to be the relationship that exists between the signal measured at the output with the signal measured at the input.

Amplifier Gain(A) The power Gain of the amplifier can also be expressed in Decibels, (db). To calculate the gain of the amplifier in Decibels or db, we can use the following expressions. Voltage Gain in db: a v = 20 log Av Current Gain in db: a i = 20 log Ai Power Gain in db: a p = 10 log Ap

Operational Amplifier(OP-AMP) Two Basic Operational Amplifier Circuits

Operational Amplifier(OP-AMP) The Operational Amplifier, or Op-amp as it is most commonly called, is an ideal amplifier with infinite Gain and Bandwidth when used in the Open-loop mode with typical d.c. gains of 100,000, or 100dB. The basic construction is of a 3-terminal device, 2-inputs and 1-output. An Operational Amplifier operates from a dual positive (+V) and an corresponding negative (-V) supply but they can also operate from a single DC supply voltage. It has Infinite Input impedance, (Z ) resulting in "No current flowing into either of its two inputs" and zero input offset voltage "V1 = V2". It also has Zero Output impedance, (Z=0).

Operational Amplifier(OP-AMP) Op-amps sense the difference between the voltage signals applied to the two input terminals and then multiply it by some pre-determined Gain, (A). This Gain, (A) is often referred to as the amplifiers "Open-loop Gain". Op-amps can be connected into two basic circuits, Inverting and Noninverting.

Operational Amplifier(OP-AMP) Gain Bandwidth Product

Operational Amplifier(OP-AMP) For Negative feedback, where the fed-back voltage is in "Anti-phase" to the input the overall gain of the amplifier is reduced. For Positive feedback, where the fed-back voltage is in "Phase" with the input the overall gain of the amplifier is increased. By connecting the output directly back to the negative input terminal, 100% feedback is achieved resulting in a Voltage Follower (buffer) circuit with a constant gain of 1 (Unity). Changing the fixed feedback resistor (Rf) for a Potentiometer, the circuit will have Adjustable Gain. The Differential Amplifier produces an output that is proportional to the difference between the 2 input voltages.

Operational Amplifier(OP-AMP) Differential and Summing Operational Amplifier Circuits

Operational Amplifier(OP-AMP) Adding more input resistor to either the inverting or non-inverting inputs Voltage Adders or Summers can be made. Voltage follower op-amps can be added to the inputs of Differential amplifiers to produce high impedance Instrumentation amplifiers. The Integrator Amplifier produces an output that is the mathematical operation of integration. The Differentiator Amplifier produces an output that is the mathematical operation of differentiation. Both the Integrator and Differentiator Amplifiers have a Resistor and Capacitor connected across the op-amp and are affected by its RC time constant.

Operational Amplifier(OP-AMP) Differentiator and Integrator Operational Amplifier Circuits

Power Amplifiers Small signal amplifiers are generally referred to as "Voltage" amplifiers as they convert a small input voltage into a much larger output voltage. Sometimes an amplifier is required to drive a motor or feed a loudspeaker and for these types of applications where high switching currents are needed Power Amplifiers are required. The main function of Power amplifiers (also known as large signal amplifiers) is to deliver power. The power amplifier works on the basic principle of converting the DC power drawn from the power supply into an AC voltage signal delivered to the load.

Amplifier Classes By changing the amplifiers Base bias voltage different ranges or modes of operation can be obtained and these are categorized according to their Class. Audio Power Amplifiers are classified in order according to their circuit configurations and mode of operation being designated different classes of operation in alphabetical order such as A, B, C, AB, etc. These different classes of operation range from a near linear output but with low efficiency to a non-linear output but with a high efficiency. There are typical maximum efficiencies for the various types or class of amplifier, with the most commonly used being: Class A - a maximum theoretical efficiency of less than 40% Class B - with a maximum theoretical efficiency of about 70% Class AB - which an efficiency rating between that of Class A and Class B

Class A Class A Amplifier operation is were the entire input signal waveform is faithfully reproduced at the amplifiers output as the transistor is perfectly biased within its active region, thereby never reaching either of its Cut-off or Saturation regions.

Class A Single Ended Amplifier Circuit

Class B Unlike the Class A amplifier above that uses a single transistor for its output stage, the Class B Amplifier uses two complimentary transistors (an NPN and a PNP) for each half of the output waveform. One transistors for the positive half of the waveform and another for the negative half of the waveform. This means that each transistor spends half of its time in the Active region and half its time in the Cutoff region.

Class B

Class AB The Class AB Amplifier is a compromise between the Class A and the Class B configurations. While Class AB operation still uses two complementary transistors in its output stage a very small biasing voltage is applied to the Base of the transistor to bias it close to the Cut-off region when no input signal is present. An input signal will cause the transistor to operate as normal in its Active region thereby eliminating any crossover distortion. A small Collector current will flow when there is no input signal but it is much less than that for the Class A amplifier configuration. This means then that the transistor will be "ON" for more than half a cycle of the waveform. This type of amplifier configuration improves both the efficiency and linearity of the amplifier circuit compared to Class A.

Class AB

Amplifier Distortion Distortion of the signal waveform may take place because: 1. Amplification may not be taking place over the whole signal cycle due to incorrect biasing. 2. The input may be too large, causing the amplifier to limit. 3. The amplification may not be linear over the entire frequency range of inputs. This means then that during the amplification process of the signal waveform, some form of Amplifier Distortion has occurred.

Amplitude Distortion Amplitude distortion occurs when the peak values of the frequency waveform are attenuated causing distortion due to a shift in the Q-point and amplification may not take place over the whole signal cycle. This non-linearity of the output waveform is shown below. Amplitude Distortion due to Incorrect Biasing

Frequency Distortion Frequency Distortion occurs in a transistor amplifier when the level of amplification varies with frequency. Many of the input signals that a practical amplifier will amplify consist of the required signal waveform called the "Fundamental Frequency" plus a number of different frequencies called "Harmonics" superimposed onto it.

Frequency Distortion Normally, the amplitude of these harmonics are a fraction of the fundamental amplitude and therefore have very little or no effect on the output waveform. However, the output waveform can become distorted if these harmonic frequencies increase in amplitude with regards to the fundamental frequency.

Phase Distortion Phase Distortion or Delay Distortion occurs in a non-linear transistor amplifier when there is a time delay between the input signal and its appearance at the output.

Phase Distortion If we call the phase change between the input and the output zero at the fundamental frequency, the resultant phase angle delay will be the difference between the harmonic and the fundamental. This time delay will depend on the construction of the amplifier and will increase progressively with frequency within the bandwidth of the amplifier.