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Chapter3: Small-signal Audio-frequency Amplifiers Chapter3: Small-signal Audio-frequency Amplifiers 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Integrated circuit Principles of operation (Quiescent Operating Point) Choice of configuration Determination of gain using a load line Bias and stabilization Voltage gain of BJT amplifier Voltage gain of f.e.t. amplifier Voltage, current and power amplifiers Multi-stage amplifiers Measurements on audio-frequency amplifiers Slide - 1 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 2 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 3 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers n-P-n bipolar transistor n-P-n bipolar transistor with a buried layer Slide - 4 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Integrated Diode Slide - 5 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Integrated Resistor R l Wd l ld d Slide - 6 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Integrated Capacitor Slide - 7 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers simple circuit shown in Fig.2.10a is to be integrated. Slide - 8 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 9 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 10 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 11 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 12 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 13 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 14 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 15 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 3.1 Principles of operation ◆ Transistors and f.e.t.s may be used as amplifiers because th eir output currents can be controlled by an a.c. signal applied t o their input terminals. ◆ By suitable choice of collector current,and hence of input im pedance,a transistor may be considered as either a current-ope rated device or a voltage-operated device. ◆ If the source impedance is much larger than the input impe dance of the transistor,the transistor is current operated. If mu ch smaller, it is voltage operate. ◆ A f.e.t. has such a high input impedance that its input curre nt is negligible ;it can therefore give only a voltage gain. Slide - 16 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 17 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers In the common-emitter connection: Ic h fe IB I C h feI B I C VBE VBE gm I B IC IB h fe input impedance: Ic gm VBE h fe g m ri VBE ri I B Slide - 18 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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,o n the other hand, the power output iS the important factor. ◆ change in output current current gain Ai = change in input current ◆ ◆ change in output voltage Ai RL voltage gain A v = change in input voltage RIN change in output power Ai 2 RL power gain A p = Ai Av change in input power RIN Slide - 19 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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. Slide - 20 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 21 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 3.2 Choice of configuration ◆ The short-circuit a.c. current gain hfe of a transistor connected in the common-emitte r configuration (Fig.3.3) is much greater than the short-circuit a.c. current gain of the same transistor connected with common base, i.e. hfe=hfb/(1- hfb ). Resistance-capacita nce coupling of the cascaded stages of an amplifier is possible and nowadays transforme rs 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 Slide - 22 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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 Slide - 23 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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 amplifyin g device that can be conveniently connected between a high-impedance source and a low-impedance load. Fig. 3.4 common-collector amplifier Slide - 24 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 3.2 Choice of configuration ◆ In the normal mode of operation (Fig.3.5), the source is common to the input and out put circuits, the input signal is applied to the gate, and the output is taken from between drain and earth.This connection provides a large voltage gain and has a high in put impedance. Fig. 3.5 common-collector amplifier Slide - 25 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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 Slide - 26 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 3.3 Determination of gain using a load line 3.3.1 The relationship of output voltage and output current VCC VCE IC RL (a) giving one of the required points. Let I c I D 0 ;then VCC VCE (b) giving the second point Let VCC VCE 0 ;then VCC IC RL Fig. 3.6 common-emmitter amplifier (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. Slide - 27 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 3.3 Determination of gain using a load line 3.3.1 The relationship of output voltage and output current VDD VDS I D RL (a) giving one of the required points. Let I c I D 0 ; VDD VDS (b) giving the second point VDD I D RL Fig. 3.7 common-source amplifier Slide - 28 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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 Slide - 29 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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 Slide - 30 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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 Slide - 31 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 3.3 Determination of gain using a load line 3.3.3 A.C.Load Lines Fig. 3.10 A.C.Load Lines Slide - 32 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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. 3.11 Current Gain of a Transistor Amplifier ◆ When an input signal is applied to a transistor amplifier, the signal current iS super imposed 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. Slide - 33 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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. 3.11 Current Gain of a Transistor Amplifier peak-to-peak collector current Ai peak-to-peak base current I C ( MAX ) I C ( MIN ) I B 3 I B1 Slide - 34 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 3.3.4 Current Gain of a Transistor Amplifier Fig. 3.12 example 3.2 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. Slide - 35 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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 peak-to-peak drain voltage Av peak-to-peak gate-source voltage Fig. 3.13 Potential-divider bias amplifier Slide - 36 VDS ( MAX ) VDS ( MIN ) VGS 3 VGS 1 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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. 3.14 example 3.3 Slide - 37 BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 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. 3.4.1 Transistor Bias 1) Why the transistor amplifier should be biased? __ To amplify the input signal undistorted. 2) Fixed bias common emmitter amplifier. VCC I B R1 VBE VCC VBE R1 IB 3) This circuit does not provide any d.c. stabilization against changes in collector current due to change in ICBO or in hFE and its usefulness is limited. I C I B I CEO ICEO (1 ) I CBO Slide - 38 Fig. 3.15 Fixed bias BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers 3.4.1 Transistor Bias EXAMPLE 3.4 The circuit shown in Fig 3.16 is designed for operation with transistors having a nominal hFE of 100. Calculate the collector current. If the range of possible hFE is from 50 to 160, calculate the collector c urrent 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 would be 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. Slide - 39 Fig. 3.16 example 3.4 BTEC-Electronics