homework solution 04

7/24/2019 Homework Solution 04

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55:041 Electronic Circuits. The University of Iowa. Fall 2010.

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Homework Assignment 04 Solutions

Question 1

Unless stated otherwise, each question below is 1 point.

1. Consider the current mirror below, and neglect base currents. What is ?

Answer:

2.

In the current mirrors below, neglect base currents and

, What is?

Answer:

3. In the current mirrors below, neglect base currents and

, What is ?

Answer:

4. Below are two schematics of current sources implemented with BJTs. Which

current source has the best compliance voltage?


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(a) (b)

Answer: (b)

5. Provide a range for the CE saturation voltage of a BJT.

Answer: 6. Tue of false: For both the amplifiers below, the voltage gain increases when the

drain/collector bias current is increased.

Answer: True

7. The small-signal output resistance ( r o) of a BJT biased at I C = 1 mA is 100 k .What is r o when the transistor is biased at I C = 10 mA? (2 points)

Answer:

First, determine Early voltage . Next determine output resistance at :


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8. Explain briefly (1 2 sentences) what is meant by the term matched transistors .

Answer: These are transistor made using the same geometry, same doping, same processand on the same substrate. They are in the same thermal environment.

9. True or false: the Early voltage for a small-signal BJT is in the order of 100 V.

Answer: True

10. True or false: one can use an ordinary glass Si diode as a photodetector.

Answer: True

11. True or false: when transforming a circuits diagram to a diagram of its small-signalmodel, we replace constant current sources with open circuits.

Answer: True

12. Tue or false: when transforming a circuits diagram to a diagram of its small-signalmodel, we replace voltage sources with shorts to ground.

Answer: True

13. True or false: the of a transistor is a function of temperature, but essentiallyindependent of collector current.

Answer: False

14. True or false: the voltage gain of the following amplifier is about -5.

Answer: False


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15. The abbreviation/term ESR is often encountered in data sheets of capacitors.What does ESR stand for?

Answer: Equivalent Series Resistance

16. When researching part numbers for three-terminal regulators, an engineerencounters the term LDO. What does LDO stand for?

Answer: Low Drop Out

17. Define load regulation of a voltage regulator.

Answer:

NL means No Load, and FL means Full Load

18. The output voltage of a three-terminal voltage regulator is 5 V @ 5 mA load, and4.96 V @ 1.5 A load. What is the regulators load regulation? (2 points )

19. True or false: the turn-on voltages of Schottky diodes are less than that of Si diodes.

However, their reverse leakage/saturation currents are also higher.

Answer: True

20. True or false: The turn-on voltage of red LEDs is larger than the turn-on voltage of blue LEDs. ( 1 point )

Answer: False

21. Which of the following depicts the correct current direction? Circle one. (1 point)

Answer: (a)


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22. Write down dc load line equation for the BJT in the circuit below, incorporating .That is, do not assume that . (3 points)

Answer:

23.


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Question 2 For the circuit below, determine the dc load line equation for the BJT, incorporating. That is, do not assume that . (5 points)

Solution

The load line equation is an equation that expresses as a function of . If were absent,

one would start with, for example:

and note that , substitute that into the equation above an express as afunction of . With present, one has to take a different approach. The simplest is toreplace , and with a Thevenin equivalent circuit as shown below.

Now

Combining these equations, substituting values, and rearranging, gives


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Question 3 Using the fact that , derive an expression for transconductance ofthe BJT (see below). (5 points)

Solution

By definition, the slope of the transfer curve at the Q-point is the transconductance

The first component of this expression is equal to . Thus

At the operating point of the value of the transconductance is


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Question 4 (N 5.48) For the BJT amplifier below, determine . For the Si transistor,, and assume the transistor is operating in the forward active mode. (10 points)

SolutionReplace the base bias network with its Thevenin equivalent network as shown below. For a Sitransistor, .

Now

However, so that

Solving yields . From this follows


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Question 5 The transistor in the following circuit has and Draw the small-signal model (common-emitter current gain version) for the circuit. Be sure to add importantdetails such as the polarity , expression for , etc. (10 points)

Solution

In the model, because , . Further

A analysis with will reveal the value of


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Question 6 Determine the output resistance for the amplifier below. Start by drawing asmall-signal model for the following circuit. Be sure to supply all necessary details , polaritiesof , labeling the gate, source, and drain, an expression for and so on. is a

coupling capacitor and for the FET. (10 points)

Solution

To determine set the independent source to 0, and add a test voltage and determinethe test current that flows. This is shown in (a) below. Note that , so that the voltage-

controller current source is off, and the equivalent circuit is shown in (b).

(a) (b)


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Question 7 . For the small-signal model below, determine an expression for (10 points)

Answer

While not required, it may help to redraw the small-signal model:

Next, set independent sources to zero, add a test voltage The resulting small-signal model isshown below.


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The KCL equation for the emitter ( E ) node, using the convention that currents flowing into thenode are positive and currents flowing out of the node are negative is:

Note that

Thus

Solving for yields

Thus

homework solution 04

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