BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide 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 Chapter3: Small-signal Audio-frequency Amplifiers

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BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 4 n-P-n bipolar transistor n-P-n bipolar transistor with a buried layer

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 5 Integrated Diode

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 6 Integrated Resistor

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 7 Integrated Capacitor

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 8 simple circuit shown in Fig.2.10a is to be integrated.

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BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 16 ◆ Transistors and f.e.t.s may be used as amplifiers because the ir 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 curren t is negligible ； it can therefore give only a voltage gain. ◆ 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 imped ance of the transistor,the transistor is current operated. If muc h smaller, it is voltage operate. 3.1 Principles of operation

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 17

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 18 In the common-emitter connection: input impedance:

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 19 ◆ 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 t o a load, the power output being unimportant. In a large-signal amplifier,o n the other hand, the power output iS the important factor. ◆ ◆ ◆ 3.1 Principles of operation

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 20 ◆ The various ways in which a transistor or f.e.t.may be connected to provide a gain are shown in Fig Choice of configuration

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 21

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 22 ◆ The short-circuit a.c. current gain h fe 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. h fe =h fb ／ (1- h fb ). 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 Ω while the output impedance is some kΩ. 3.2 Choice of configuration Fig. 3.3 common-emitter amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 23 ◆ A transistor connected as a common-base amplifier (Fig. 3.2) has a short circuit a.c. current gain h fb 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. 3.2 Choice of configuration Fig. 3.2 common-base amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 24 ◆ The common-collector circuit,or emitter follower as it is usually called, is shown in F ig.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. 3.2 Choice of configuration Fig. 3.4 common- collector amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 25 ◆ 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. 3.2 Choice of configuration Fig. 3.5 common- collector amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 26 ◆ 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 Choice of configuration Fig. 3.5 common- collector amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 27 (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. 3.3 Determination of gain using a load line The relationship of output voltage and output current Fig. 3.6 common- emmitter amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 28 (a) giving one of the required points. (b) giving the second point 3.3 Determination of gain using a load line The relationship of output voltage and output current Fig. 3.7 common- source amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide Determination of gain using a load line The relationship of output voltage and output current Tab. 3.1 data of the common emmitter amplifier 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.

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide Determination of gain using a load line Choice of Operating Point Fig. 3.8 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.

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide Determination of gain using a load line A.C.Load Lines Fig. 3.9 Potential-divider bias amplifier ◆ 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.

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide Determination of gain using a load line A.C.Load Lines Fig A.C.Load Lines

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide Determination of gain using a load line Current Gain of a Transistor Amplifier Fig. 3.9 Potential-divider bias 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 I B2 and that an input signal current swings the base current between the values I B1 and I B3. ◆ 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 I B1 and I B3. Fig Current Gain of a Transistor Amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide Determination of gain using a load line Current Gain of a Transistor Amplifier Fig. 3.9 Potential-divider bias amplifier Fig Current Gain of a Transistor Amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 35 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 Current Gain of a Transistor Amplifier Fig example 3.2

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide Determination of gain using a load line Voltage Gain of a FET Amplifier Fig Potential-divider bias amplifier ◆ The voltage gain of a fet can also be found with the aid of a load line. For example, Fig 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

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 37 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 example Voltage Gain of a FET Amplifier

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 38 ◆ 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 I CBO or in h FE and its usefulness is limited. 3.4 Bias and stabilization Transistor Bias Fig Fixed bias

BTEC-Electronics Chapter3: Small-signal Audio-frequency Amplifiers Slide - 39 EXAMPLE 3.4 The circuit shown in Fig 3.16 is designed for operation with transistors having a nominal h FE of 100. Calculate the collector current. If the range of possible h FE is from 50 to 160, calculate the collector current flowing if a transistor having the maximum h FE is used. Assume I CBO =10nA and V BE =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 Transistor Bias Fig example 3.4