CHAPTER - 4
PERFORMANCE OF AMPLIFIERS
The following particulars are important to assess the performance of an Amplifier: i) ii) iii) iv) v) vi) vii) viii) ix) Input Impedance. Input voltage to produce full rated output (volume control at maximum position) Output impedance. Rated output in watts. Percentage distortion at full output power expressed in relation to harmonics. Percentage of harmonics present at a quarter, half and three quarters of full rated output. Frequency response. Noise level at full output. Power Consumption.
The gain may be expressed in dB, but the input impedance should be specified to obtain the actual output in watts. The harmonic distortion is due to the transistor characteristics. Since harmonic content varies with load impedance, an increased load impedance results in decreased harmonic distortion, but this would result in decreased output wattage. Conversely, decreased load impedance will cause more harmonic distortion. Transformers caused harmonic distortion due to non-linearity of the magnetisation curve of the iron. The output rating in watts is not a complete indication of the capabilities of that amplifier under operating conditions. Amplifiers are rated at some specified output in watts with a declared harmonic content, of say, 5%. In class A amplifiers, harmonic distortion increases with output. In cass B, the harmonic distortion is constant. The response curve is usually specified for a range of frequencies, the output of which is linear for plus or minus say 3 dB. Another method is to specify the range of departure (in dBs) from the level at a mean frequency say 400 Hz or 1000 Hz. The response level is taken with amplifier delivering 50 percent of its rated output. The amplifier delivers a small output even for no signal input condition. This is due to circuit noises-thermal agitation noise, valve transistor noise, etc. If the noise level is high, the amount of amplification also is limited, so that its level should be as low as possible. The recommended level is -50 dB. The power consumption of an amplifier is an important factor is determining the rating of power supplies especially if they are to be operated from rotary converters or engine generators. The efficiency is 20%, and in the event of class AB amplifier, the current consumption decreases slightly with decrease in signal output. The common measurements made to judge the performance of amplifier are: i) ii) iii) iv) v) vi) vii) Frequency Response Amplification Internal Noise Level Output Power Harmonic Content Output Regulation Power Consumption.
Fig. 43 Equipment for frequency response measurement The set up of equipment for determining the frequency response is shown in the Fig 43. Meters A and B register the input and output voltages. Suppose the amplifier has a rated output of 15 watts for an input of 75 volts. If the input impedance is 500,000 Ohms and output impedance is 10 Ohms, voltmeters B should indicate 12.3 volts for .75 volts reading for meter A. The volume control should be maximum. It is now turned to half the volume, so that meter B reads 8.7 volts corresponding to 7.5 watts output. Let these readings be obtained, at a frequency, say 1000Hz. Keeping this input voltage constant the output level is measured for frequencies ranging from 40 Hz to 10000 Hz. The results are tabulated as under:Frequency in Hz. Output Volts. Voltage ratio relative to 1000 Hz output of dB level relative to 1000 Hz. 8.7 volts.
The overall amplification can be calculated from the given data as follows: i) ii) iii) iv) Input impedance: Output " Input Voltage: Output " 500,000 Ohms. 10 " 75 volts 12.3 volts
Item iii & iv are determined experimentally. Input Watts = .752 500000
= .112 X 10 -5 watt Output Watts Amplification D.B. gain = 12.32 10 = = 15.1 Watts.
15.1 = 1.35 X 107 -5 .112 X 10 = 10 log 1.35 X 107 = 10 (.1303 + 7)
2
= 71.303. For output power measurements, the arrangement of the equipments is similar to that given under para above for fidelity measurements. The output volts is measured for three spot frequencies say 50, 1000 and 8000 Hz. If the amplifier is having class A output, a meter is provided to register any change in the anode current. During reproduction at full output, the anode current should not vary by more than 5 percent. Harmonic distortion may be measured by means of an analyser incorporating filters so that each harmonic can be isolated and measured. The output regulation is a very important criterion in the performance of an amplifier, since the loudspeakers are switched in and out frequently in actual practice. The method employed consists in driving the amplifier from an oscillator at 1000 Hz, and connecting the output to a variable resistance with a voltmeter across it. With input volts and frequency constant, the load resistance is adjusted to various values and the voltage measured in volts and watts calculated from these values. The data is plotted as shown in Fig.44. In practice a voltage increase of 40% can be permitted due to change in load, since the resulting change of 3 dB, will not be apparent in the loudspeaker output.
Fig. 44 Output Regulation Curve The power consumption is a straightforward measurement with watt-meter in AC operated equipment, and the current is measured and wattage calculated in the case of DC equipment. ***
