Model electricity experiment -- Experiment 2 JFET common source amplifier circuit

Catalog

One 、 The experiment purpose

Two 、 Experimental apparatus

3、 ... and 、 Experimental principle

1、 Experimental simulation circuit

2、 Characteristics and parameters of Junction FET

3、 Performance analysis of FET amplifier

4、 Measurement method of input resistance

Four 、 Experimental content

1、 According to the figure 1 Expand the connection , And make the potentiometer Rg1 The initial value is adjusted to 72K

2、 Measurement and adjustment of static working point

3、 Voltage amplification Av、 Input resistance Ri And output resistance Ro The measurement of

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One 、 The experiment purpose

1. Understand the performance and characteristics of Junction FET .
2. Be more familiar with the test method of amplifier dynamic parameters .

Two 、 Experimental apparatus

1. Dual trace oscilloscope
2. A multimeter
3. AC millivoltmeter
4. Signal generator

3、 ... and 、 Experimental principle

1、 Experimental simulation circuit

chart 1   Junction FET common source amplifier

2、 Characteristics and parameters of Junction FET

The ideal model is selected in the simulation experiment N Ditch JFET

 

 

  This experiment , Focus on the first three parameters of the tube , first VTO=-2V, namely JFET Pinch off voltage Vp or VGS（off）, the second BETA=0.1m A/V2, namely JFET Conductance constant of Kn, Third LAMBDA=0, That is, the channel length modulation effect is not considered .

3、 Performance analysis of FET amplifier

chart 1 It is a common source amplifier circuit composed of Junction FET . The static working point of the theoretical calculation value is as follows ：（Rg2=72k）

UGS=UG-Us=(Rg1/(Rg1+Rg2))*UCC-ID*RS1  

ID=IDSS*(1- UGS/UP)^2 ,IDSS=Kn*VP^2

The above formula is simultaneous , Obtainable UGSQ=100mv,IDQ=441u A ,UDSQ= UCC-IDQ(RS1+RD)=9.56V

UGSQ> Vp, And UDSQ>UGSQ-VP, Therefore, the pipe works in the saturation zone . The static working point is not adjusted later , On this basis, AC parameter values are calculated .

gm=2kn*(UGSQ-Vp=2*0.1*（100mv -(-2）)=0.42ms

If voltage amplification factor Av=-gm*RL'=-gm*(RD║RL)=-0.42*（5.1k//10k）=-1.42  

Input resistance          Ri=RG+(Rg1║Rg2)=RG=1MΩ（ a little bigger than ）                            

Output resistance          Ro≈RD=5.1kΩ     

4、 Measurement method of input resistance

Static operating point of FET amplifier 、 Measurement method of voltage magnification and output resistance , The measurement method is the same as that of the transistor amplifier in Experiment 1 . Measurement of its input resistance , In principle , The method described in Experiment 1 can also be used , But due to the Ri The larger , Such as direct measurement of input voltage Us and Ui, Because the input resistance of the measuring instrument is limited , It will inevitably bring about large errors （ There is generally no such concern in the simulation ）. So in order to reduce the error , Always use the isolation function of the amplifier under test , By measuring the output voltage Uo To calculate the input resistance . The measuring circuit is shown in the figure 2 Shown .             

                                        

Put a resistor in series at the input end of the amplifier R, Switch K Throw to position 1（ Even if R=0）, Measure the input voltage of the amplifier Uo1=Av*Us; keep Us unchanged , And then K Throw direction 2（ Namely access R）, Measure the output voltage of the amplifier Uo2. Since two measurements Av and Us remain unchanged , so Uo2=Av*Ui=(Ri/R+Ri)*Us*Av. From this we can find Ri=Uo2/(Uo1-Uo2) *R, In style R and Ri Don't make a big difference , This simulation experiment can be R=100KΩ.

Four 、 Experimental content

1、 According to the figure 1 Expand the connection , And make the potentiometer Rg1 The initial value is adjusted to 72K

2、 Measurement and adjustment of static working point

According to the figure 1 Operation measurement data of , Measure the value of the static working point , And fill in the following table .

3、 Voltage amplification Av、 Input resistance Ri And output resistance Ro The measurement of

（1）Av and Ro The measurement of

According to the plan 1 Circuit experiment , hold RD The value is fixed at 5.1K Access circuit , Add a frequency of... At the input of the amplifier 1KHz、 The peak to peak is 100mV The sine signal of Ui, Not yet Rs, And monitor the output with an oscilloscope Uo Waveform of . At output Uo Without distortion , Measure separately RL=∞ and RL=10KΩ The output voltage of Uo( Be careful : keep Ui unchanged ), Record in table 4-2.

（2）Ri The measurement of

According to the figure 2 Connect the experimental circuit , hold RD The value is fixed at 5.1K Access circuit , The selection frequency is 1KHz、 The peak to peak is 100mV Sinusoidal signal input voltage Us, Turn the switch on K Throw direction “1”, measure Rs=0 Output voltage at Uo1, Then throw the switch to “2”( Access Rs), keep Us unchanged , Then measure Uo2, According to the formula (Uo2/(Uo1-UOo2))*Rs, Find out Ri, Record in table 4-3.（RL=∞, Each voltage is measured and calculated using peak to peak values ）

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