Module 3 Lab: Determining Beta for BJTs
Wire the circuit shown, with Rc = 470 ? and Q = 2N3904
• Record Ib and Ic in the table below for the given values of RB
RB Ib Ic Vbe Vce Beta
1M?
470K?
200K?
100K?
47K?
• Return the RB value to 470K?,
(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.
3. 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 = 470k?, Rc = 6k? 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 = 10k?, R2 = 2.2k?, Rc = 3.6k?, RE = 1k?. 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 = 2.2K? R2 = 4.7K? 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.
2. 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.
2. A transistor is biased with IE = 5mA. Calculate r'e
3. For the circuit shown, calculate:
a. The input resistance at the base of the transistor.
b. The input resistance of the amplifier.
c. The voltage gain.
4. 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 = 20K?, R2 = 5M? ( two 10M? in parallel ), R3 = 20K?, 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 / 20k? = _____ 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:
o Av = vc / vin = ______ and Av = Rc / re' = _______.
o 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
2. 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.
2. 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?
3. An amplifier has a gain of 500, what is the dB gain?
4. 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 = 68K?, R2 = 10K?, R3 = 10K?, R4 = 200? ( two 100? in series ) , R5 = 2.2K?, 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.