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.