Solid State devices

Module 3 Lab: Determining Beta for BJTs

Wire the circuit  shown, with Rc = 470 Ω  and Q =  2N3904

· Record Ib and Ic  in the table below for the given values of RB

RB	Ib	Ic	Vbe	Vce	Beta
1MΩ
470KΩ
200KΩ
100KΩ
47KΩ

· Return the RB value to 470KΩ,

(A) Squeeze the transistor between your fingers to raise the case temperature, record any change in collector current  _______

(B)  Put a piece of ice in a plastic bag and hold it on the transistor to reduce the case temperature, record any change in collector current __________

· Describe your perception of  the relationship between Beta and case temperature.

· Describe the behavior of Beta vs Ic

Written Assignment 4

Answer all of the following questions/problems and submit them to your mentor.  

Questions:

1. In a transistor circuit, the two extreme ends of the loadline are called:

2. When used as an amplifier, the transistor is usually biased in the region called the _______________ region.

0. The Q  point for an amplifier  is identified as _______ & _______ , which together determine the power dissipated  in the transistor.

Problems:

1. For the circuit shown,  VBB = +10v,  Vcc = +30v, RB = 470kΩ, Rc = 6kΩ  Calculate the operating point for the circuit shown for a Beta value of 90 and for a Beta value of 130.

2. For the circuit shown,  Vcc = -10v, R1 = 10kΩ,  R2 = 2.2kΩ,   Rc = 3.6kΩ, RE = 1kΩ.  Calculate the operating point for the circuit shown.  Use IB = IC / Beta and calculate IB for Beta = 90.

Module 4 Lab: AC Riding on DC R1 = 2.2KΩ    R2 = 4.7KΩ   C = 0.1uF   Vdc =  +5v  Vac = 3v peak  f = 1KHz

· Use superposition to calculate the  DC  voltage at  point X     VDC =  ______

· Use superposition to calculate the  AC  (peak)  voltage at  point X     VAC =  ______

· Sketch what you think the scope will show at X:

· Wire the circuit and connect a scope probe at X, be sure to DC couple the input and set the ground level at the bottom of the scope display, then sketch the results.

Written Assignment 5

Answer all of the following questions/problems and submit them to your mentor.  

Questions:

1. Describe what is meant by the voltage gain of an amplifier.

0. What is the primary purpose of coupling capacitors in transistor amplifiers?

Problems:

1. If  the input voltage to an amplifier is  50mV and the resulting output voltage is 3V, calculate the voltage gain.

 

0. A transistor is biased with  IE = 5mA.     Calculate r’e

0. For the circuit shown, calculate:  

a. The input resistance at the base of the transistor.

a. The input resistance of the amplifier.

a. The voltage gain.

    

0. If a circuit requires a capacitor to give  good coupling at 200Hz and the resistance the capacitor is connected to is  3Kohms,  what value capacitor is needed?

Module 5 Lab: Amplifier Gain and Comparing Constant Bias to VDB

Part I

(A) Wire the circuit shown. [ you will determine the β value from DC measurements]

R1 =  20KΩ,  R2 = 5MΩ  ( two 10MΩ in parallel ),  R3 = 20KΩ, C = 0.1uF, use 2N3904 for the transistor

· Measure  D.C.    Vc =  _____   and  Vbe = _____

· Calculate Ic = ( Vcc -  Vc )/ Rc = ______      and IB = ( Vcc -  Vbe )/ RB =  _____    then β =  _____

(a)  Verify that Rbe = Rin base = β re’

· Measure A.C. voltages   vc = ____   vbe =  _____

· Calculate ib = ( 10mV – vbe ) / 20 Kohms = _____

· Calculate ic = vc / 20kΩ = _____  calculate βac = ic / ib = ______

· Rbe = vbe / ib = _____     re’ = 25mV/ IE   =  _______   ic = βac ib = ____

· Also calculate  Rbe = βac re’ = _____  does this check with the value above?

(B)  To show the effects of changing beta on the Q point.

· Hold your finger on the case to raise the temperature and the Q point should change (Vc goes down) due to the fact that beta goes up as temperature goes up causing an increase in  Ic.

· With a piece of ice in plastic bag, hold it on the transistor to cool it; this should cause beta to decrease.  The Q point will change again ( Vc goes up ).

Part II

To show the  advantage of VDB over CCB

· Wire the VDB circuit shown. The transistor beta value was determined in part A.

· Measure [ D.C.]    VB = ____  and  VE = _____

· Calculate IE =  _____               and  re’  =  _____

· Measure [ A.C. ]    vin =  _____   and  vo = ____

· Now calculate voltage gain by two methods:

. Av =  vc / vin =  ______    and Av =  Rc / re’ =   _______.   

. Do these two values match?

· Next hold your finger on the transistor and notice that the Q point does not change.  Change the transistor and again note that the Q point stays constant.

Written Assignment 6

Answer all of the following questions/problems and submit them to your mentor.  

Questions:

1. For each circuit shown, indicate if it is CE, CB or CC amplifier.

   

Figure A

Figure B

Figure C

 

0. Show two transistors connected as a Darlington amplifier.  If one transistor has a beta of 25 and the other has a beta of 60, calculate the beta value of this combination.

  

Problems:

1. For the amplifier shown, if beta = 150

(a)  Calculate the input impedance at the base

(b)  Calculate the input impedance of the stage.

0. Shown here is a frequency response curve for an amplifier.  ESTIMATE the following:

        (a)  What is the high cutoff frequency ?

(b)  What is the low cutoff frequency?

(c)   What is the  bandwidth?

 

0. An amplifier has a gain of 500, what is the dB gain?

0. A three stage amplifier system has dB gains of 15dB, 32dB and 6db.  What is the overall gain of the system in dB?

Module 6 Lab: Class A BroadBand Amplifier

Wire the circuit shown:

R1 = 68KΩ,  R2 = 10KΩ, R3 = 10KΩ, R4 = 200Ω ( two 100Ω in series ) , R5 = 2.2KΩ,  C1 = 0.1uF, C2 = 0.1 uF  Q1 = 2N3904

· Set the sig. Gen to 50mv  RMS.  With frequency equal to 10KHz.

· Record   vo  for the frequencies listed.

vo

f ( KHz)   10KHz      25 KHz      50 KHz    100 KHz     500 KHz       1MHz    2 MHz   3MHz

Calc  Av

· Plot   Av  versus  frequency on semi log paper.