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Master the debugging method of amplifier static working point , Learn to analyze the effect of static operating point on amplifier performance .
Master the amplifier voltage amplification factor 、 Input resistance 、 Test method for output resistance and maximum undistorted output voltage .
Familiar with the use of common electronic instruments and analog circuit experimental equipment .

Two 、 Experimental apparatus

Dual trace oscilloscope
A multimeter
AC millivoltmeter
Signal generator
DIGILENT The experiment platform
Self made experimental board 1

3、 ... and 、 Experimental principle

1、 Amplifier static Testing of indicators

chart 1 Experimental circuit diagram for resistive voltage divider type single transistor amplifier with stable operating point . Its bias circuit uses RB2 and RB1 A voltage dividing circuit , And a resistor is connected in the emitter RE, To stabilize the static operating point of the amplifier . When the input signal is added to the input end of the amplifier Ui after , At the output end of the amplifier, you can get a and Ui Opposite phase , An output signal whose amplitude is amplified Uo, Thus, the voltage amplification is realized .

In the figure 1 In circuit , When flowing through the bias resistor RB1（20kΩ） and RB2（10kΩ and 100kΩ Potentiometer RW） The current of is much larger than that of the transistor T The base current IB when （ commonly 5~10 times ）, Then its static working point can be estimated by the following formula ,VCC Provide power supply , This is a +12V.

UB=RB1/(RB1+RB2) *VCC

IE=IC=(UB-UBE)/RE

UCE=VCC-IC(RC+RE)

Voltage amplification Av=-B*(RC//RE)/rbe

Input resistance ：Ri=RB1//RB2//rbe

Output resistance ：Ro=RC

2、 Measurement and debugging of the static working point of the amplifier

(1) Measurement of static working point

Measure the static operating point of the amplifier , The input signal shall be Ui=0 In the case of , Short circuit the input end of the amplifier to the ground , Then select a digital multimeter with appropriate range , Measure the collector current of the transistor respectively IC And the potential of each electrode to the ground UB、UC and UE. In general experiments , To avoid disconnecting the collector , Therefore, measure the voltage , Then work out IC Methods , for example , Just measure UE, That is, available IC=IE=UE/RE Work out IC（ Can also be based on IC=(VCC-UC)/RC, from UC determine IC）, Can also calculate .

(2) Debugging of static working point

The adjustment of the static working point of the amplifier refers to the collection current of the triode IC（ or UCE） Adjustment and testing .

Whether the static working point is appropriate , It has a great influence on the performance and output waveform of the amplifier . For example, the working point is too high , The amplifier is easy to produce saturation distortion after adding AC signal , here Uo The negative half cycle of will be cut to the bottom , Pictured 2(a) Shown , If the operating point is low, it is easy to produce cut-off distortion , namely Uo The first half of the circle is capped （ Generally, the cut-off distortion is not as obvious as the saturation distortion ）, Pictured 2(b) Shown . These conditions do not meet the requirements of undistorted amplification . Therefore, dynamic debugging must be carried out after the working point is selected , That is, add a certain amount of... At the input of the amplifier Ui, Check the output voltage Uo Whether the size and waveform meet the requirements . If not satisfied , The position of the static working point shall be adjusted .

Change circuit parameters VCC,RC,RB（RB1,RB2） Will cause changes in the static operating point , Pictured 3 Shown , But it is usually used to adjust the bias resistance RB2 To change the static working point , For example, reduce RB2, The static working point can be improved .

Last but not least , The working points mentioned above “ High ” or “ Low ” Not absolutely , It should be relative to the amplitude of the signal , For example, the signal amplitude is very small , Distortion does not necessarily occur even at higher or lower operating points . So to be exact , Waveform distortion is caused by improper coordination between signal amplitude and static operating point setting . If it is necessary to meet the requirements of large signals , The static operating point should be as close to the midpoint of the AC load line as possible .

3、 Amplifier dynamic index test

Amplifier dynamic index test includes voltage amplification factor 、 Input resistance 、 Output resistance 、 Maximum undistorted output voltage （ Dynamic range ） And passband .

(1) Voltage amplification AV The measurement of

Adjust the amplifier to the proper static operating point , Then add the input voltage Ui, At output voltage uo Without distortion , Measure with an AC millivoltmeter Ui and Uo Effective value Ui and Uo, be Av=Uo/Ui.

(2) Input resistance Ri The measurement of

To measure the input resistance of the amplifier , According to the figure 4 A known resistance is connected in series between the input of the amplifier under test and the signal source R（10kΩ）, When the amplifier is working normally , Measure with an AC millivoltmeter US and Ui, Then according to the definition of input resistance

Ri = Ui/Ii = Ui/(UR/R) = Ui/(Us-Ui)*R.

Attention shall be paid to during measurement

measurement R Voltage at both ends UR Must be measured separately US and Ui, Then press UR=US-Ui Find out UR value .
resistance R The value of should not be too large or too small , To avoid large measurement error , Usually take R And Ri It is better to be of the same order of magnitude , This experiment can be R=1～2KΩ.

(3) Output resistance Ro The measurement of

According to the figure 4 circuit , Under normal operating conditions of the amplifier , Measure that the output terminal is not connected to the load RL The output voltage of Uo' And output voltage after load connection UL', according to UL=RL/(Ro+RL)*Uo', You can find out. Ro=(Uo/UL-1)*RL.

During the test, attention shall be paid to , Must keep RL The size of the input signal remains unchanged before and after access .

4、 Maximum undistorted output voltage Uopp The measurement of （ Maximum dynamic range ）

As mentioned above , For maximum dynamic range , The static operating point shall be set at the midpoint of the AC load line . For this purpose, when the amplifier works normally , Gradually increase the amplitude of the input signal , And adjust at the same time RW（ Change the static operating point ）, Observe with an oscilloscope Uo, When the output waveform has the phenomena of bottom cutting and top shrinking at the same time （ Pictured 5） when , It indicates that the static operating point has been set at the midpoint of the AC load line . Then adjust the input signal repeatedly , Maximize the waveform output amplitude , Without obvious distortion , Measure with an AC millivoltmeter Uo（ Valid values ）, Then the dynamic range is equal to 2Uo（ Peak to peak ）. Or read directly with an oscilloscope Uopp Come on .

5、 Measurement of amplifier frequency characteristics

The frequency characteristic of an amplifier refers to the voltage amplification factor of the amplifier AV And input signal frequency f The relationship between . The amplitude frequency characteristic curve of the single tube resistance capacitance coupling amplifier circuit is shown in the figure 6 Shown ：

Avm Is the intermediate frequency voltage amplification factor , It is generally specified that the voltage amplification factor decreases to the intermediate frequency amplification factor with the change of frequency $1/\sqrt{2}$ times , namely 0.707Avm The corresponding frequencies are called lower limit frequencies fL And upper limit frequency fH, Then the passband fBW=fH-fL.

The amplitude frequency characteristic of an amplifier is the voltage amplification factor when measuring signals of different frequencies AV. For this purpose, the above measurement can be adopted AV Methods , Every time you change a signal frequency , Measure the corresponding voltage magnification , When measuring, pay attention to the proper point , More points shall be measured in the low frequency band and high frequency band , You can measure a few points less in the intermediate frequency . Besides , When changing the frequency , Keep the amplitude of the input signal constant , And the output waveform shall not be distorted .

Four 、 Experimental content

1、 attachment

According to the figure 1 Connection circuit shown , Finally, connect the power supply +12V.

2、 Measuring static working point

Static operating point measurement conditions ： Input ground even if Ui=0.

Ui=0, Turn on the DC switch , Adjust the RW, send IC=1.1mA about （ namely UE=2.4V about ）, Measure with a multimeter UB、UE、UC、RB2 value . Record in table 1.

The above results are compared with the theoretical values and based on Multisim The simulation results are compared and verified .

3、 Measure the voltage magnification

Adjust a frequency to 1KHz、 The peak value is 25mV The sine wave of is used as the input signal Ui . At the same time, observe the input voltage of the amplifier with a dual trace oscilloscope Ui And output voltage Uo Waveform of , stay Uo Use a millivoltmeter to measure the waveform without distortion , And observe with double trace oscilloscope Uo and Ui The phase relationship of , Record in table 2.

The above results are compared with the theoretical values and based on Multisim The simulation results are compared and verified .

Be careful ： Due to the dispersion of transistor element parameters , For quantitative analysis Ui by 20mV Not necessarily , According to the actual situation, appropriate Ui value , It will not be explained in the future . because Uo The measured value is a valid value , So peak value Ui It needs to be converted into effective value or measured with millivoltmeter Ui To calculate Av value . Remember the multimeter 、 Millivoltmeter measurements are valid values , And what oscillograph observes is peak to peak .

4、 Observe the influence of the static operating point on the voltage magnification

Steps in 3 Of RC=5.1KΩ,RL= ∞ Under connection conditions , Adjust a frequency to 1KHz、 The peak value is 25mV The sine wave of is used as the input signal Ui Connected to the amplifying circuit . Adjust the RW, Monitor the output voltage waveform with an oscilloscope , stay Uo Without distortion , Measurement array IC and Uo Value , Record in table 3. measurement IC when , To make Ui=0.

5、 Observe the effect of static operating point on output waveform distortion

Steps in 3 Of RC=5.1KΩ RL=∞ Under connection conditions , send Ui=0, Adjust the RW send IC=1.1mA about , measure UCE value . Adjust a frequency to 1KHz、 The peak value is 25mV The sine wave of is used as the input signal Ui Connected to the amplifying circuit , Then gradually increase the input signal , Make the output voltage Uo Large enough without distortion . Then keep the input signal unchanged , Increase and decrease respectively RW, Distort the waveform , draw Uo Waveform of , And measure the distortion IC and UCE value , Record in table 4 in . Every time I test IC and UCE Value so that the input signal is zero （ Even if Ui=0）.

**6、* Measure the maximum undistorted output voltage**

Steps in 3 Of RC=5.1KΩ RL=10KΩ Under connection conditions , Adjust the amplitude of the input signal and the potentiometer at the same time RW, Measure with oscilloscope and millivoltmeter Uopp.

**7、* Measure input resistance and output resistance**

According to the figure 4 Shown , take R=10KΩ, Set up RC=5.1KΩ,RL=10KΩ,IC=1.1mA. Input f=1KHz、 The peak value is 25mV The sine signal of , At output voltage Uo Without distortion , Measure with a millivoltmeter US,Ui and UL, use Ri The formula works out Ri.

keep US unchanged , To break off RL, Measure the output voltage Uo', See Ro The formula works out Ro.

It can be found that Ri=7.6KΩ

Obtainable Ro=4.7KΩ..

**8、* Measure the amplitude frequency characteristic curve**

take RC=5.1KΩ,RL=10KΩ,IC=1.1mA. Keep the previous input signal Ui unchanged , Change the signal source frequency f, Measure the amplitude frequency characteristic of quxianbin with a baud tester and observe .

Schematic diagram is as follows ：