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Small-Signal Audio-Frequency Amplifiers: Principles & Configurations

Learn about the operation principles and configuration choices for small-signal audio-frequency amplifiers using integrated circuits. Understand voltage gain, biasing, and measurement techniques.

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Small-Signal Audio-Frequency Amplifiers: Principles & Configurations

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  1. Chapter3: Small-signal Audio-frequency Amplifiers • 3.0 Integrated circuit • 3.1 Principles of operation (Quiescent Operating Point) • 3.2 Choice of configuration • 3.3 Determination of gain using a load line • 3.4 Bias and stabilization • 3.5 Voltage gain of BJT amplifier • 3.6 Voltage gain of f.e.t. amplifier • 3.7 Voltage, current and power amplifiers • 3.8 Multi-stage amplifiers • 3.9 Measurements on audio-frequency amplifiers

  2. n-P-n bipolar transistor n-P-n bipolar transistor with a buried layer

  3. Integrated Diode

  4. Integrated Resistor

  5. Integrated Capacitor

  6. simple circuit shown in Fig.2.10a is to be integrated.

  7. 3.1 Principles of operation ◆Transistors and f.e.t.s may be used as amplifiers because their output currents can be controlled by an a.c. signal applied to their input terminals. ◆ A f.e.t. has such a high input impedance that its input current is negligible ；it can therefore give only a voltage gain. ◆ By suitable choice of collector current,and hence of input impedance,a transistor may be considered as either a current-operated device or a voltage-operated device. ◆ If the source impedance is much larger than the input impedance of the transistor,the transistor is current operated. If much smaller, it is voltage operate.

  8. In the common-emitter connection: • input impedance:

  9. 3.1 Principles of operation ◆The mutual characteristics of a f.e.t or a transistor always exhibit some non-linearity. If a suitable operating point is chosen and the amplitude of the input signal is limited, the operation of the circuit may be taken as linear without the introduction of undue error. ◆The function of a small-signal amplifier is to supply a current or voltage to a load, the power output being unimportant. In a large-signal amplifier,on the other hand, the power output iS the important factor. ◆◆◆

  10. 3.2 Choice of configuration ◆The various ways in which a transistor or f.e.t.may be connected to provide a gain are shown in Fig.3.1.

  11. 3.2 Choice of configuration ◆The short-circuit a.c. current gain hfeof a transistor connected in the common-emitter configuration (Fig.3.3) is much greater than the short-circuit a.c. current gain of thesame transistor connected with common base, i.e. hfe=hfb／(1- hfb ). Resistance-capacitance coupling of the cascaded stages of an amplifier is possible and nowadays transformers are rarely used. Generally, common-emitter stages are biased, so that the transistor is current operated. Then the input impedance is in the region of 1000-2000Ω while the output impedance is some 10-30 kΩ. Fig. 3.3 common-emitter amplifier

  12. 3.2 Choice of configuration ◆A transistor connected as a common-base amplifier (Fig. 3.2) has a short circuit a.c. current gain hfb less than unity (typically about 0.992), a low input impedance of the order of 50Ω, and an output impedance of about 1 MΩ. Because the current gain is less than unity, common-base stages cannot be cascaded using resistance-capacitance coupling but transformer coupling can be used. Transformers, however, have the disadvantages of being relatively costly, bulky and heavy and having a limited frequency response, particularly the miniature types used in transistor circuits. Fig. 3.2 common-base amplifier

  13. 3.2 Choice of configuration ◆The common-collector circuit,or emitter follower as it is usually called, is shown in Fig.3.4. This connection hasa high input impedance, a low output impedance, and a voltage gain less than unity. The main use of an emitter follower is as a power amplifying device that can be conveniently connected between a high-impedance source and alow-impedance load. Fig. 3.4 common-collector amplifier

  14. 3.2 Choice of configuration ◆In the normal mode of operation (Fig.3.5), the source is common to the input and output circuits, the input signal is applied to the gate, and the output is taken frombetween drain and earth.This connection provides a large voltage gain and has a high input impedance. Fig. 3.5 common-collector amplifier

  15. 3.2 Choice of configuration ◆Fig.3.6 shows the f.e.t. equivalent of the emitter follower, this is known as the source follower circuit. The follower circuit will be treated in greater detail in Chapter 4. Fig. 3.5 common-collector amplifier

  16. 3.3 Determination of gain using a load line 3.3.1 The relationship of output voltage and output current (a) giving one of the required points. (b) giving the second point(c) If these two points are located on the characteristics and joined by a straight line,the load line for the particular load resistance and supply voltage is obtained. Fig. 3.6 common-emmitter amplifier

  17. 3.3 Determination of gain using a load line 3.3.1 The relationship of output voltage and output current (a) giving one of the required points.(b) giving the second point Fig. 3.7 common-source amplifier

  18. 3.3 Determination of gain using a load line 3.3.1 The relationship of output voltage and output current example 3.1 A transistor connected in the common-emitter configuration has the data given in Table 3.1. Plot the output characteristics of the transistor and draw the load lines for collector load resistances of (a) 1000Ω and (b) 1800Ω.Use the load lines to determine the steady (quiescent) collector current and voltage if the base bias current is 80μA and the collector supply Ic=0 Vce=Vcc=9V and is marked A in Fig.3.3. Tab. 3.1 data of the common emmitter amplifier

  19. 3.3 Determination of gain using a load line 3.3.2 Choice of Operating Point ◆In practice, some non-linearity always exists, and to minimize signal distortion care must be taken to restrict operation to the most nearly linear part of the characteristic. ◆For this a suitable operating point must be selected and the signal amplitude must be restricted. Fig. 3.8 Choice of Operating Point

  20. 3.3 Determination of gain using a load line 3.3.3 A.C.Load Lines ◆Very often the load into which the transistor or fet works is not the same for both ac and dc conditions. ◆When this is the case two load lines must be drawn on the out characteristics：a dc load line to determine the operating point, and an ac load line to determine the current or voltage gain of the circuit. ◆The ac load line must pass through the operating point. Fig. 3.9 Potential-divider bias amplifier

  21. 3.3 Determination of gain using a load line 3.3.3 A.C.Load Lines Fig. 3.10 A.C.Load Lines

  22. 3.3 Determination of gain using a load line 3.3.4 Current Gain of a Transistor Amplifier Fig. 3.9 Potential-divider bias amplifier Fig. 3.11Current Gain of a Transistor Amplifier ◆When an input signal is applied to a transistor amplifier, the signal current iS superimposed upon the bias current. ◆ suppose that the base bias current is IB2 and that an input signal current swings the base current between the values IB1 and IB3. ◆ The resulting values of collector current are found by projecting onto the collector-current axis from the in tersection of the a.c.load line and the curves for IB1 and IB3.

  23. 3.3 Determination of gain using a load line 3.3.4 Current Gain of a Transistor Amplifier Fig. 3.9 Potential-divider bias amplifier Fig. 3.11Current Gain of a Transistor Amplifier

  24. 3.3.4 Current Gain of a Transistor Amplifier example 3.2 The transistor used in the circuit has the data given in Table. Plot the output characteristics of the transistor. Draw the dc load line and select a suitable operating point. Draw the ac load line and use it to find the alternating current that flows in the 2500Ω load when an input signal producing a base current swing of±15μA about the bias current is applied to the circuit. Assume all the capacitors have zero reactance at signal frequencies. Fig. 3.12example 3.2

  25. 3.3 Determination of gain using a load line 3.3.5 Voltage Gain of a FET Amplifier ◆The voltage gain of a fet can also be found with the aid of a load line. For example, Fig. 3.13 shows an ac load line drawn on the drain characteristics of a fet and the dotted projections from the load line show how the drain voltage swing, resulting from the application of an input signal voltage, can be found.The voltage gain Av of the fet amplifier stage is Fig. 3.13 Potential-divider bias amplifier

  26. 3.3.5 Voltage Gain of a FET Amplifier example 3.3 Draw the d.c.load line and select a suitable operatin point. Draw the a.c.load line and use it to find the voltage gain when a sinusoidal input signal of 0.3 V peak is applied. Fig. 3.14example 3.3

  27. 3.4 Bias and stabilization ◆To establish the chosen operating point it is necessary to apply a bias voltage or current to a FET or transistor. 1) Why the transistor amplifier should be biased? __ To amplify the input signal undistorted. 2) Fixed bias common emmitter amplifier.3) This circuit does not provide any d.c. stabilization against changes in collector current due to change in ICBO or in hFEand its usefulness is limited. 3.4.1 Transistor Bias Fig. 3.15Fixed bias

  28. 3.4.1 Transistor Bias EXAMPLE 3.4The circuit shown in Fig 3.16 is designed for operation withtransistors having a nominal hFE of 100. Calculate the collector current. If the range of possible hFE is from 50 to 160, calculate the collector current flowing if a transistor having the maximum hFE is used. Assume ICBO=10nA and VBE=0.62V.In the above example the effect of the increased collector current wouldbe to move the operating point along the d.c.load line,and this would lead to signal distortion unless the input signal level were reduced. Fig. 3.16example 3.4

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