Bjt Tr Circuits

8/8/2019 Bjt Tr Circuits

http://slidepdf.com/reader/full/bjt-tr-circuits 1/9

[ENGN2211 Home]

Aim

To learn to use PS PICE to simulate BJT transistor circuits and to enhance your

understanding of the operation of BJT circuits.

ANU Engineering - ENGN2211

[ENGN2211 Home]

BJT Characteristics

Load the PSPICE file bjt-dc2.sch, Figure 31.

Figure 31: Circuit for measuring BJT

characteristics.

Exercise:

1.



Simulate the circuit for the given setup. Is the transistor Q 1 on or off? Why?2.

We now want to obtain the family of characteristics, IC vs V CE for variousvalues of IB, (like Figure 32, which shows IC as a function of V CE; note that in

this simulation V CE=V CC). We use the nested DC sweep feature:

V CC is set as the main sweep variable, ranging from 0 V to 5 V in increments

of 0.1 V. IB is the nested sweep variable, ranging from 0 to 20 A in

increments of 5 A.

Enable DC sweep and simulate again. Determine the correct value of IB for each of the curves.

Lecture Notes : BJT Transistor

Circuits : BJT Characteristics

3.

Find the saturation and active regions for a fixed value of IB.4.

Obtain a graphs of IC versus IB.5.

What is the current gain ?Use the graphs to determine the ratio IC /IB in the active region for several curves.

Figure 32: BJT characteristics ( IC versus V CE for several values of IB).



[ENGN2211 Home] ANU Engineering - ENGN2211

[ENGN2211 Home]

Switching

Load the PSPICE file bjt-dc1.sch, Figure 33.

Figure 33: BJT switching circuit.

Exercise:

1.Simulate the circuit for the given values of V BB and V CC and note the DCcurrents and voltages. Is the transistor Q 1 on or off? Why?

Lecture Notes : BJT Transistor Circuits : Introductory BJT Circuits :

Transistor as a Switch



On or off refers to extreme operating points corresponding to saturation or cut-off respectively. When on a substantial current flows (order mA), while

when off only a small current flows (order few A or less).

Note also the values of V BE and IB (add trace).

2.Enable DC sweep and simulate again ( V BB is varied, and the resulting V CE isplotted). You should obtain the graph shown in Figure 34.

Also graph IC versus V BE.

3.At what range of values of V BB and V BE does switching occur?

4.

Describe the switching behaviour.

Figure 34: BJT switching curve (V C versus V BB).


[ENGN2211 Home]



DC Bias

Load the PSPICE file bjt-dc3.sch, Figure 35. This circuit will have the

same DC bias as the amplifier circuit of section 7.5.

Lecture Notes : BJT Transistor Circuits :

DC Bias

Figure 35: Circuit for BJT DC operating point.

Exercise:

1.Simulate the circuit for the given setup. Is the transistor Q 1 in the activemode? Why?It may be helpful to refer to the transistor characteritics obtained earlier (itis the same transistor).

2.

We now wish to investigate the effect of varying the emitter resistor R E onthe DC operating point, and in particular on IC and V CE. We do this using DC

sweep, with R E set up as a parameter , varying between 100 and 5 k .

Enable DC sweep and simulate again. Obtain plots of IC and V CE vs R E,(something like Figure 36). (Recall that V CE=V C-V E!)

3.



Explain what you see.Note carefully the effect of R E on IC and V CE.

4.Draw load lines on the characteristic graphs (IC vs V CE) for three values of R E: 0.5, 1.0 and 3.0 k .

The horizontal intercept is V CC=5 V, and the vertical intercepts are given by

Plot the operating point Q corresponding to item 1 (on the R E=3 k loadline).

Figure 36: DC operating point varies with R E.


[ENGN2211 Home]



Small Signal Amplifier

Load the PSPICE file bjt-amp1.sch, Figure 37. Your task is to design

values of R B1, R B2 and R E for good gain and input level.

Figure 37: Small signal amplifier.

Exercise:

1.Using the relations

select (standard) values of R B1 and R B2 so that V, k(as in the previous bias section). Set these values in PS PICE.

2.

Now choose R E so mA (for maximum possible AC swing). Set inPSPICE.

You may use your curves from the DC bias exercise above, section 7.4, tofind R E.



3.Simulate and check that your DC bias point is correct. Record your values.

Draw the load line on the characteristic graph ( IC vs V CE) for your value of R E. Plot the DC operating point Q on this load line.

Check for consistency with the DC bias exercise a bove, section 7.4.

4.Enable transient analysis and simulate. Observe the input and outputwaveforms. What is the voltage gain from the sou rce voltage vs to theoutput load voltage vout (As,out)? Compare with theory.

Lecture Notes : BJT Transistor

Circuits : S mall Signal A mplifier :

AC A nalysis

To measure the gain, measure the peak -to-peak values of vs and vout and calculate the ratio vout /vs.

5.Repeat the transient simulation for increasing magnitudes of the sourcevoltage vs, and determine the maximum magnitude of the input signalbefore the output waveform is clipped. Note any distortion.

6.Set the magnitude of vs back to its original value, and now vary the sourceR S (increase it) and load R L (decrease it) resistances (individually). Note the

influence on gain. Discuss.


[ENGN2211 Home]

Amplifier Distortion - Optional

Study the distortion of the output waveform in the small signal amplifier. Since theinput is a pure sine wave, an ideal linear amplifier would produce a pure sine waveoutput. However, because the amplifier has some non -ideal nonlinearities, theoutput is distorted from being a pure sine wave. This effect is more pronounced



for larger signals - why? In the frequency domain, distortion appears as

harmonics, see Figure 38.

Figure 38: Amplifier distortion (frequency domain).

ANU Engineering - ENGN2211

Bjt Tr Circuits

Documents

Transcript of Bjt Tr Circuits