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AMPLIFIERS 01.PDF 1 © E. COATES 2007 -
Amplifiers
1.0 Introduction to Amplifiers
An amplifier is used to increase the amplitude of a signal waveform, without changing other parameters of the waveform such as frequency or wave shape. They are one of the most commonly used circuits in electronics and perform a variety of functions in a great many electronic systems.
The general symbol for an amplifier is shown in Fig 1.0.1. The symbol gives no detail of the type of amplifier described, but the direction of signal flow can be assumed (as flowing from left to right of the diagram). Amplifiers of different types are also often described in system or block diagrams by name.
Amplifiers
Module
What you’ll learn in Module 1
Section 1.0 Amplifier Basics.
- Typical functions of amplifiers in electronic systems.
- Graphical representations of amplifiers.
- Amplifier applications and types of signal. Section 1.1 Amplifier Parameters. Typical amplifier parameters.
- Gain, Frequency response, Bandwidth, Input and Output impedance, Phase shift, Feedback. Section 1.2 Class A Biasing.
- BJT Common emitter and FET common source biasing.
- Emitter, DC and temperature stabilisation.
- Class A bias.
- Common emitter input and output characteristics. Section 1.3 Gain and Decibels.
- Amplification.
- Logarithmic scales.
- Specifying voltage and power using dBs.
- Common dB values. Section 1.4 Bandwidth.
- Typical Response curves.
- Factors affecting bandwidth. Section 1.5 Amplifier Basics Quiz.
- Test your knowledge of Amplifiers (^) Fig 1.0.1 Amplifier general symbol, used in system diagrams
For example look at the block diagram of an analogue TV receiver in Fig 1.0.2 and see how many of the individual stages (shaded green) that make up the TV are amplifiers. Also notice that the names indicate the type of amplifier used. In some cases the blocks shown are true amplifiers and in others, the amplifier has extra components to modify the basic amplifier design for a special purpose. This method of using relatively simple, individual electronic circuits as "building blocks" to create large complex circuits is common to all electronic systems; even computers and microprocessors are made up of millions of logic gates, which are simply specialised types of amplifiers. Therefore to recognise and understand basic circuits such as amplifiers is an essential step in learning about electronics.
One way to describe an amplifier is by the type of signal it is designed to amplify. This usually refers to a band of frequencies that the amplifier will handle, or in some cases, the function that they perform within an electronic system.
A.F. Amplifiers
Audio frequency amplifiers are used to amplify signals in the range of human hearing, approximately 20Hz to 20kHz, although some Hi-Fi audio amplifiers extend this range up to around 100kHz, whilst other audio amplifiers may restrict the high frequency limit to 15kHz or less.
Voltage amplifiers are used to amplify the low level signals from microphones, tape and disk pickups etc. With extra circuitry they also perform functions such as tone correction equalisation of signal levels and mixing from different inputs, they generally have high voltage gain and medium to high output resistance.
Power amplifiers are used to receive the amplified input from a series of voltage amplifiers, and then provide sufficient power to drive loudspeakers.
I.F. Amplifiers
Intermediate Frequency amplifiers are tuned amplifiers used in radio, TV and radar. Their purpose is to provide the majority of the voltage amplification of a radio, TV or radar signal, before the audio or video information carried by the signal is separated (demodulated) from the radio signal.
Fig 1.0.2 Analogue TV receiver block diagram, showing amplifiers used in many stages.
Video Amplifiers
Video amplifiers are a special type of wide band amplifier that also preserve the DC level of the signal and are used specifically for signals that are to be applied to CRTs or other video equipment. The video signal carries all the picture information in TV, video and radar systems. The bandwidth of video amplifiers depends on use. In TV receivers it extends from 0Hz (DC) to 6MHz and is wider still in radar.
Buffer Amplifiers
Buffer amplifiers are a commonly encountered, specialised amplifier type that can be found within any of the above categories, they are placed between two other circuits to prevent the operation of one circuit affecting the operation of the other. (They ISOLATE the circuits from each other). Often buffer amplifiers have a gain of one, i.e. they do not actually amplify, so that their output is the same amplitude as their input, but buffer amplifiers have a very high input impedance and a low output impedance and can therefore be used as an impedance matching device. This ensures that signals are not attenuated between circuits, as happens when a circuit with a high output impedance feeds a signal directly to another circuit having a low input impedance.
Operational Amplifiers
Operational amplifiers (Op-amps) have developed from circuits designed for the early analogue computers where they were used for mathematical operations such as adding and subtracting. Today they are widely used in integrated circuit form where they are available in single or multiple amplifier packages and often incorporated into complex integrated circuits for specific applications.
The design is based on a differential amplifier, which has two inputs instead of one, and produces an output that is proportional to the difference between the two inputs. Without negative feedback, op amps have an extremely high gain, typically in the hundreds of thousands. Applying negative feedback increases the op amp’s bandwidth so they can operate as wideband amplifiers with a bandwidth in the MHz range, but reduces their gain. A simple resistor network can apply such feedback externally and other external networks can vary the function of op-amps.
LM324N Low power Quad Operational Amplifier IC by ST Microelectronics.
The Output Properties of Amplifiers
Amplifiers are used to increase the amplitude of a voltage or current, or to increase the amount of power available usually from an AC signal. Whatever the task, there are three categories of amplifier that relate to the properties of their output;
- Voltage amplifiers.
- Current amplifiers.
- Power amplifiers.
The purpose of a voltage amplifier is to make the amplitude of the output voltage waveform greater than that of the input voltage waveform (although the amplitude of the output current may be greater or smaller than that of the input current, this change is less important for the amplifier’s designed purpose).
The purpose of a current amplifier is to make the amplitude of the output current waveform greater than that of the input current waveform (although the amplitude of the output voltage may be greater or smaller than that of the input voltage, this change is less important for the amplifier’s designed purpose).
In a power amplifier, the product of voltage and current (i.e. power = voltage x current) at the output is greater than the product of voltage x current at the input. Note that either voltage or current may be less at the output than at the input. It is the product of the two that is significantly increased.
To show how the gain of an amplifier varies with frequency, a graph, showing the frequency response of the amplifier is used. Fig. 1.1.1a shows the typical frequency response curve of an audio amplifier, and Fig. 1.1.1b, that of a RF amplifier. In such graphs, it is common that very large values may be encountered for both gain and frequency. For this reason it is usual for both the frequency and gain axes of the graph to use logarithmic scales. It can be seen from Fig. 1.1.1a that scales on the (horizontal) x-axis do not increase in a linear manner; each equal division represents a tenfold increase in the frequency plotted. This ensures that a very wide range of frequency can be plotted on a single graph. The (vertical) y-axis uses linear divisions but logarithmic units (deciBels dB). The curve of the graph shows how gain, measured in deciBels, varies with frequency.
Comparing Figs. 1.1.1a and b drawn in this manner, shows how each type of amplifier (audio, RF etc) has its own characteristic shape of frequency response curve. An amplifier which has a very narrow, sharply peaked response curve is said to be very "selective". This is typical of an RF amplifier and is precisely what is needed in an amplifier designed for the tuning stages of a radio where only one radio carrier wave among many hundred others, crowded along the medium wave band for example, must be selected.
Fig. 1.1.1a Response curve for an audio amplifier
Fig. 1.1.1b Response curve for a RF amplifier tuned to 774kHz
Bandwidth
An important piece of information that can be obtained from a frequency response curve is the Bandwidth of the amplifier. This refers to the ‘band’ of frequencies for which the amplifier has a useful gain. Outside this useful band the gain of the amplifier is considered to be insufficient compared with the gain at the centre of the bandwidth. Bandwidth specified for voltage amplifiers is the range of frequencies for which the amplifier’s gain is greater than 0.707 of the maximum gain (see Fig. 1.1.1.b). Alternatively, decibels are used to indicate the gain, the ratio of output to input voltage, (see Fig. 1.1.1.a). The useful bandwidth in Fig. 1.1.1a would be described as extending to those frequencies at which the voltage gain is −3dB down compared to the gain at the mid band frequency. Several ways of describing the bandwidth can be used, firstly it could be said (of Fig 1.1.1a), that "The bandwidth is from 10Hz to 20kHz." Alternatively it could be said (of Fig. 1.1.1b) "The bandwidth is 9kHz, centred on 774kHz." or even that it is "774kHz plus or minus 4.5kHz."
Input Impedance
The word impedance means opposition to AC current flow. At 0 Hz, (that is, DC) impedance (symbol Z) is the same as resistance (R), but at frequencies other than 0Hz impedance and resistance are not the same. The input impedance of an amplifier is the effective impedance between the input terminals. "Effective" means that the impedance is not necessarily just that of the amplifier components (resistors, capacitors etc.) actually connected across the input terminals, but is the impedance experienced as the amount of current able to flow into the input terminals for a given signal voltage applied at a particular frequency. Input Impedance is influenced by a number of factors including the frequency of the applied signal, the gain of the amplifier, any signal feedback used and even what is connected to the output of the amplifier.
Output Impedance
The output impedance of an amplifier is not solely dependent on the actual components connected within the output of an amplifier. It is an ‘apparent’ impedance and can best be demonstrated as being responsible for a fall in signal voltage at the output terminals of an amplifier, when a current is drawn from the output terminals. The more current drawn from the output terminals, the greater the reduction in output signal voltage. The effect is that of an impedance or resistance in series with the output terminals.
Fig. 1.1.2 Amplifier Input and Output Impedances
Phase Shift
Phase shift in an amplifier is the amount (if any) by which the output signal is delayed or advanced in phase with respect to the input signal expressed in degrees. If a phase shift of 90 degrees occurs then the peak of the output wave occurs one quarter of a cycle after the peak of input wave. Such a shift can be caused by the effect of components such as resistors inductors and capacitors in the amplifier circuit. The action of the transistor in a single stage amplifier can cause 180 degrees of phase shift, and therefore the input and output will be in "anti-phase." Whether a phase shift in an amplifier is important depends on the purpose of the amplifier.
The design of multi stage amplifiers must take phase shift into consideration, as the amount of phase shift will vary with frequency it is possible that at some frequencies the total phase shift may add up to 360 degrees. If the output signal of such a system is allowed to re-enter the input then positive feedback occurs and the amplifier will become unstable and is likely to oscillate.
Feedback
Feedback is the process of taking a proportion of an amplifier’s output signal and feeding it back into the input. Feedback can be arranged to either increase or decrease the input signal. When feedback is used to increase the input signal it is called POSITIVE FEEDBACK, and when the effect of the feedback reduces the input signal it is called NEGATIVE FEEDBACK.
POSITIVE FEEDBACK occurs when the feedback signal is in phase with the input signal, this increases the amplitude of the input and hence the output signal, effectively increasing the gain of the amplifier.
NEGATIVE FEEDBACK occurs when the feedback signal is in anti-phase with the input signal, effectively reducing the amplitude of the input and hence also the output signal. This causes a reduction in gain. See Fig. 1.1.5.
Fig. 1.1.4 Phase Shift
Fig. 1.1.5 Negative feedback reduces gain, distortion and noise, it also increases bandwidth.
In high quality amplifiers negative feedback is often used to reduce the gain of the amplifier. A particular benefit of this, is that any distortion of the signal or background noise produced by the amplifier is also reduced. A further beneficial effect is that applying negative feedback increases the bandwidth of the amplifier. The reason for this can be seen in Fig. 1.1.6 where reducing the height
of the gain curve produces wider spacing of the 0.707 points, therefore widening the bandwidth.
Fig. 1.1.6 The effect of negative feedback on amplifier bandwidth.
Common Emitter Fixed Biasing
Amplifiers are needed in most pieces of electronic equipment, not only for sound and picture reproduction but also in control systems and communications. The design of amplifiers is aimed at producing a circuit that has a predicted gain over a particular band of frequencies with minimum distortion. The amplifier must also be stable and not prone to oscillation. Bipolar PNP or NPN transistors or FETs may be used in a wide variety of designs depending on their intended purpose.
Consider the simple bipolar NPN common emitter amplifier shown in Fig. 1.2.1 consisting of a transistor and two resistors. To function correctly the amplifier should produce at its output, an amplified version of the signal at its input without distortion. In order to do this, its quiescent or no signal (DC) conditions, must first be correct. Its output can only be undistorted if its input is undistorted.
Using the Input Characteristics.
Fig. 1.2.2 shows a typical input characteristic curve for a small signal amplifier transistor where changes in base voltage Vb are plotted against the resulting changes in base current I (^) b.
If changes in the AC signal voltage (changes in Vb ) applied to the base, are to produce proportional changes in AC base current I (^) b then some DC value of VB must be used so that positive and negative excursions of the signal voltage occur only on the linear part of the input curve (waveform b in Fig. 1.2.2). This DC voltage (0.7V in Fig. 1.2.2) applied to the base is called the base bias voltage. It can be seen from Fig. 1.2.2 that if the bias voltage is insufficient, then only the positive tips of the input voltage waveform would produce base current, and consequently severe distortion will occur in the base current waveform a.
It can also be seen that for this transistor, a DC base bias voltage (R (^) B ) of 0.7V produces a quiescent (DC) base current of 40μA. These values are set by the correct choice of resistance value for RB (Fig. 1.2.1).
Setting the Quiescent Output Conditions
The quiescent output conditions must also be considered, as the quiescent base current I (^) b will produce a quiescent collector current Ic that will depend on the value of I (^) b and the current gain h (^) fe of the transistor. Also, because I (^) c flows through the load resistor (RL) it will produce a potential difference across RL that when subtracted from the supply voltage (Vcc) gives the value of the transistor collector/emitter voltage (Vce).
Fig 1.2.1 Simple bias for a common emitter amplifier.
Fig. 1.2.2 Input Characteristics.
Fig. 1.2.3 shows the two extreme conditions for the values of Ic and Vce. It can be seen in the first case (Fig. 1.2.3a), that if the collector current IC is zero, owing to the base voltage being low enough to cut off base current, the voltage developed across RL will be zero and the whole of Vcc will be developed across the transistor so Vce will rise to the supply voltage Vcc.
If a signal is applied under these conditions (Fig. 1.2.3a), positive going half cycles of the output signal (which is in anti phase to the voltage waveform at the base) cannot make Vce rise any further than Vcc and so the positive going half cycles of collector voltage will not be reproduced, causing severe distortion.
Alternatively if I (^) c is very high (Fig 1.2.3b) due to excessive base bias, the transistor will be in a saturated condition and Vce will fall to almost zero. As the collector voltage cannot fall below 0V the negative going half cycles of the output signal will be lost. It follows therefore, that to reproduce the full waveform at the collector, the ideal quiescent value for Vce will be around midway between Vcc and zero volts. This will allow the maximum amplitudes of both positive and negative going half cycles of the output wave to be reproduced without distortion.
Using the Output Characteristics
In the output characteristics shown in Fig. 1.2.4 changes in I (^) c are plotted against changes in Vce for various constant base currents I (^) b.
A ´load line´ is drawn on Fig. 1.2. between the two extreme points described in Fig. 1.2.3.
Point P is where VCE = Vcc (which in this case equals 10V) and I (^) c= zero, and because no current collector is flowing, the transistor is said to be "Cut off".
Point R is the maximum value of I (^) c (where I (^) c = Vcc ÷ RL) and Vce is zero (because practically the whole of Vcc is developed across RL). This is called "Saturation" as no further increase in collector current will occur.
With the load line drawn from P to R, it can be seen that a value of Vce can be chosen mid-way along the load line at point Q, which in this case coincides with the curve for IB.
Fig. 1.2.3 Incorrect Bias Conditions
Fig. 1.2.4 Output Characteristics and Load Line
The ultimate result of this process called "Thermal Runaway", is that the transistor will get hotter and hotter until it is destroyed. Although Thermal Runaway is much less of a problem in modern power transistors, in small signal types it is still a possible hazard that should be avoided by building some form of bias stabilisation into the amplifier design.
DC Stabilisation
Fig. 1.2.5 shows a simple method of improving the temperature stabilisation of a common emitter amplifier. Instead of feeding the bias current from V (^) cc it is fed from the collector end of RL.
With this arrangement, any increase in collector current will cause an increase in the potential difference across RL and, as the top of RL is held steady by V (^) cc, the collector voltage Vce at the bottom of RL must fall. This in turn will cause V (^) be to fall, and so reduce collector current. The bias conditions are to a large extent self adjusting and said to be stabilised by a form of DC feedback.
Emitter Stabilised Bias
An alternative, and much more common bias arrangement used in most commercial circuits, uses a potential divider comprising two resistors (R 1 and R 2 in Fig. 1.2.6) to provide a steady value of Vbe and an emitter resistor Re to provide stabilisation by DC feedback.
If collector current increases in this circuit, so does the emitter current, which causes a rise in the emitter voltage Ve. This rise compared with the steady base voltage, causes a reduction in the base-emitter voltage Vbe and subsequent drop in collector current. DC feedback using an emitter stabilising resistor keeps circuit conditions stable when other conditions (e.g. temperature or transistor h (^) fe) may change.
However the emitter resistor will also cause unwanted AC feedback because under signal conditions the AC waveform appearing on the emitter will be in phase with the base waveform, and the two waveforms changing together will tend to reduce the variations in base-emitter voltage, causing a substantial reduction in gain. To avoid this problem it is usual for the emitter stabilising resistor Re to be bypassed by a (usually) large value capacitor connected across RE that will form a very low impedance path to any AC signal present, preventing any AC appearing on the emitter, but without changing any of the DC conditions.
Fig. 1.2.5 Collector Derived Bias.
Fig. 1.2.6 Emitter Stabilisation
FET Biasing
The biasing of FETs is simpler than in bipolar designs as no gate (input) current is flowing. Fig. 1.2.7 shows a typical JFET bias arrangement. (MOSFETs also use a similar bias circuit).
When used in depletion mode, the gate of the FET must be more negative than the source. This is achieved by holding the gate at zero volts, whilst the drain/source current through R 3 makes the source terminal positive. As no gate current flows in FETs there can be no voltage developed across R 1 and the gate remains at zero volts. The use of a very high value for R 1 maintains the very high input impedance, which is a useful property of FET amplifiers.
An AC signal applied to the gate will cause small variations of gate voltage above and below zero, which will cause AC changes in drain-source current and, as in a bipolar amplifier, these are converted to voltage changes by R 2. The source resistor R 3 performs DC stabilisation in the same way as the emitter resistor in a bipolar amplifier, and is also normally bypassed to prevent AC negative feedback.
Fig. 1.2.7 FET Bias
To describe a change in output power over the whole frequency range of the amplifier, a response curve, plotted in decibels is used to show variations in output. The powers at various frequencies throughout the range are compared to a particular reference frequency, (the mid band frequency). The difference in power at the mid band frequency and the power at any other frequency being measured, is given as so many decibels greater (+dB) or less (-dB) than the mid band frequency, which is given a value of 0dB. Notice that, on the logarithmic frequency scale in Fig 1.3.1 the middle of the 10Hz to 100kHz band is 1kHz and frequencies around this figure (where the output is usually at its maximum) are normally chosen as the reference frequency.
Converting a power gain ratio to dBs is calculated by multiplying the log of the ratio by 10:
Where P 1 is the power at mid band and P 2 is the power being measured.
Although it is common to describe the voltage gain of an amplifier as so many decibels, this is not really an accurate use for the unit. It is OK to use decibels to compare the output of an amplifier at different frequencies, since all the measurements of output power or voltage are taken across the same impedance (the amplifier load), but when describing the voltage gain (between input and output) of an amplifier, the input and output voltages are being developed across quite different impedances. However it is quite widely accepted to also describe voltage gain in decibels.
Fig 1.3.2 Audio Power Response Curve
Note:
When using this formula in a calculator the use of brackets is important, so that 10 x the log of (P 1 /P 2 ) is used, rather than 10 x the log of P 1 , divided by P 2.
e.g. if P 1 = 6 and P 2 =
10 x log(6/3) =3dB (right answer), but 10 x log 6/3 = 2.6dB (wrong answer).
Fig 1.3.3 Audio Voltage Response Curve
When voltage gain(Av ) or current gain (Ai ) is plotted against frequency the −3dB points are where the gain falls to 0.707 of the maximum (mid band) gain.
Notice that converting voltage ratios to dBs uses 20 log(Vout/Vin )
Describing the voltage gain of an amplifier that produces an output voltage of 3.5V for an input of 35mV as being 40dB, is equivalent to saying that the output voltage is 100 times greater than the input voltage.
To reverse the process, and convert dBs to a voltage ratios for example, use:
Note that the brackets are important and antilog may be shown on calculator keypads as 10x^ or 10^ and is also usually Shift +log. Use the same formula for dBs to Current gain ratio, and to convert dBs to a power ratio, simply replace the 20 in the formula with 10.
An advantage of using dBs to indicate the gain of amplifiers is that in multi stage amplifiers, the total gain of a series of amplifiers expressed in simple ratios, would be the product of the individual gains, Av1 x Av2 x Av3 x Av4 ...etc.
This can produce some very large numbers, but the total of individual gains expressed in dBs would
be the sum of the individual gains:
Av1 + Av2 + Av3 + Av4 ...etc.
Likewise losses due to circuits such as filters, attenuators etc. are subtracted to give the total loss.
