ECE-3220

QUIZ #11 PREP SHEET

All Prior Quiz Prep sheets, homework, lectures, and reading assignments. Don't forget that some problems similar to Exam #1 problems will be interspersed among the next several quizzes. 

As always, you should be able to do all problems with either NPN or PNP transistors - know the difference!

So far we have discussed Sections 7.1 through 7.5. Any of that material is fair game.

Knowing that I stress fundamentals, you can expect the following to be emphasized:

Frequency Response Characteristics of a Band Pass Amplifier:

• Constraints on the poles and zeros in the Transfer Function, A(s).
• Splitting A(s) into three parts and the constraints on each:
  • Midband gain (A_m)
  • Low frequency transfer function (F_L(s))
  • High frequency transfer function (F_H(s))
• Which capacitors typically are playing a role in each of the three regions of interest.

Analysis of the Frequency Response Characteristics of a Band Pass Amplifier:

• Determining the Midband Gain
• Determining the lower cutoff frequency using short-circuit time constants
• Determining the upper cutoff frequency using open-circuit time constants

Description and justification of the above analysis method.

• What assumptions are involved in performing the above analysis.
• Why internal capacitors are treated as opens and external capacitors are treated as shorts when finding the midband gain.
• Why short-circuit time constants are using in finding the lower cutoff frequency.
• Why open-circuit time constants are used in finding the upper cutoff frequency.
• Determining whether you add the time constants or add the frequencies is estimating the cutoff frequency of interest.

1. Given A(s) = A_M*F_L(s)*F_H(s) where F_L(s) & F_H(s) have been properly normalized, What is the overall transfer function approximately equal to in the following regions:
   1. Below the midband region.
   2. Within the midband region.
   3. Above the midband region.

    

2. Which of the capacitors in the circuit, (i.e., internal or external) or active in each of the following regions:
   1. Below the midband region.
   2. Within the midband region.
   3. Above the midband region.

    

3. Which of the following is NOT an assumption that is generally made when using the analysis techniques presented in Chapter Seven?
   1. A dominant pole exists in both the upper- and lower-frequency transfer functions.
   2. The external capacitors play a role only below the lower cutoff frequency.
   3. All capacitors appear as open circuits within the midband.
   4. The internal capacitors play a role only above the upper cutoff frequency.

    

4. When computing the lower and upper cutoff frequencies, you know that in one case the cutoff frequency is the sum of the reciprocals of the time constant associated with each capacitor and in the other case the reciprocal of the cutoff frequency is the sum of those time constants. Which is which? How do you KNOW that you are correct?

    

5. When determining the resistance seen by a given capacitor for the purpose of calculating the associated time-constant, sometimes you use "open-circuit" resistance and other times you use "short-circuit" time constants. What is the difference between the two? When do you use the first and when do you use the second? How do you KNOW you are using the right one?

    

6. Draw the complete high frequency hybrid-pi model of the BJT transistor. (See Sec 4.15 p323).

    

7. Explain Miller's Theorem. Given two nodes with a bridging element where the voltage on one node can be written in terms of a constant gain and the voltage on the other node, show what the values of the two elements that can be used to replace the bridging element are in terms of the bridging element and the voltage gain between the two nodes. Notice that you are asked to "show" what the values are - this means to use the given information and fundamental analysis techniques to come up with the results as opposed to blindly applying Miller's Theorem from memory.

    

8. Why does the common-emitter configuration generally have a poor upper cutoff frequency?

    

9. Why does the common-base configuration generally have a good upper cutoff frequency?

    

10. How does the cascode configuration overcome the bandwidth deficiency typically exhibited by the common-emitter configuration?

     

11. Write an exam question for this material that you feel is both appropriate for an exam (that should require approximately ten minutes to solve) and that you feel adequately tests the knowledge about a particular topic within this chapter. List what knowledge is being tested.

     

12. Recall how, in the common-emitter configuration, an external capacitor can be used to permit an emitter resistor to establish the DC bias conditions while effectively removing that resistor from the circuit within the midband of the amplifier. Describe how this works. Keeping in mind that the small signal gain of this configuration is roughly the ratio of the total resistance in the collector circuit to the total resistance in the emitter circuit, how might an external capacitor be used to establish the upper cutoff frequency for this configuration?