Exam Write down one phrase/sentence that describes the purpose of the diodes and constant current source in the amplifier below.

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Brittney Hicks
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1 Exam 3 Name: Score /94 Question 1 Short Takes 1 point each unless noted otherwise. 1. Write down one phrase/sentence that describes the purpose of the diodes and constant current source in the amplifier below. Answer: Reduction of cross-over distortion 2. What is the maximum theoretical efficiency for a class-b amplifier? Answer: 78% 3. Many BJT datasheets do not list explicitly, but list an equivalent h-parameter instead. What is this parameter? Answer: 4. Consider a frequency. How many octaves higher is the frequency? Answer: Each octave means a doubling in frequency. Thus, we have to find. Substituting values gives ( ) ( ) octaves. in 5. Consider a frequency. How many decades higher is the frequency? Answer: Each decade means a frequency higher. Thus, we have to find in. Substituting values gives ( ) decades. 1

2 6. The small-signal output resistance of a BJT biased at = 1 ma is100k. What is when the transistor is biased at? Answer: is inversely-proportional to ( ) so that will be smaller at 5 ma, or. 7. A MOSFET has rated power of 25 W at an ambient temperature and a maximum specified junction temperature of 105 o C. What is the thermal resistance between the case and the junction? Answer: 8. What does SOA in the context of power transistors stand for? Answer: Safe Operating Area 9. True or false: if class-a amplifiers are not carefully biased, they will suffer from crossover distortion. Answer: False 10. Identify the false statement (a) FETs do not suffer from thermal runaway, but BJTs do (b) Everything else being equal, BJTs have an order of magnitude more gain than FETs (c) BJT technology has superior performance in power application when compared to the modern MOS technology, which explains why BJTs are still widely incorporated in power designs. Answer: (c) is false 11. An engineer designs a MOSFET-based class-ab amplifier to deliver 6.25 W (sinusoidal) signal power to a resistive load. What is the required peak-to-peak voltage swing across the load? (2 points) Answer:, so that, so that 2

3 12. In the circuit below, what is the maximum current that can flow through? Make reasonable assumptions. (2 points) Answer. Assume that for, ( ). Thus, will turn on and starve from additional base current when the current through (which is also the current through ) is 13. Consider the current mirror below, and neglect base currents. What is? (2 points) Answer: 14. In the current mirrors below, neglect the base currents. What is? (2 points) Answer: 3

4 15. In the circuit below. Estimate the midband gain. (2 points) (a) (b) (c) (d) Need additional information Answer: (b) 16. A BJT has rated power of 115 W at and maximum allowable junction temperature. It is mounted on a heat sink with. It is dissipating 5 W at an ambient temperature. What is the temperature of the heat sink? (2 points) (a) Need additional information (b) (c) (d) Answer: 5 W through the heat sink will elevate the sink s temperature by = above the ambient, so the answer is (c). 17. Estimate assuming and. (2 points) (a) (b) (c) (d) (e) Need additional information Answer: Using BJT scaling the resistance looking into the transistors base is about and assuming and this about 7.3K. This is in parallel with the 82K and 22K resistors, so (d) is the correct answer. 4

5 18. Consider a first-order RC low-pass filter with 3-dB frequency. What is the phase shift in degrees at 50 Hz? (2 points) Answer: The phase shift at 60 Hz is and increases at / decade. 50 Hz is ( ) decades higher than 60 Hz. (The negative sign implies 50 Hz is 0.08 decades before 60 Hz.) Thus, the phase shift is. An alternate and more accurate calculation is ( ). 19. The following circuit has a time-constant of. What is the attenuation (in db) at (3 points) Answer. This is a 1 st order low-pass network with a corner frequency of ( ). The attenuation is 20 db/decade above and 30 khz is 2 decades higher than 300 Hz. Thus, the network will attenuate at 40 db at 30 khz. An alternate calculation is ( ( ) ). 20. In the circuit below and all the capacitors are large enough to be considered shorts. Estimate the midband gain (3 points) (a) (b) (c) (d) ( ) ( ) Answer: (d) 5

6 21. In the circuit below and all the capacitors are large enough to be considered shorts. Estimate the midband gain (3 points) (a) (b) (c) (d) ( ) Answer: ( ), so (b) is the answer. 6

7 Question 2 For the BJT amplifier below, determine and. For the Si transistor,, and assume the transistor is operating in the forward active mode. (6 points) Replace the base bias network with its Thevenin equivalent network as shown below. For a Si transistor,. Now However, ( ) so that ( ) ( ) ( )( )( ) Solving yields. From this follows ( ) 7

8 Question 3 Consider the amplifier shown. Neglect the BJTs internal capacitances. Estimate the 3-dB frequency and the midband gain and then make a plot of the frequency response. You can assume and that. Further,. (15 points) Hint: using BJT scaling concepts will greatly simplify your work. Note that ( ) ( ). Turn off the input voltage and use BJT scaling to determine the resistance looking back into the emitter: ( ) sees ( ) and the circuit time constant is ( )( ) ( )( ) = 10 ms, so that ( ). Using BJT scaling, the resistance looking into the base at midband frequencies is ( )( ) ( )( ) The parallel combination of and is much larger than so the amplifier does not load the input. Since this is a CC amplifier,. The Bode plot is shown below. 8

9 Question 4 Consider a BJT with a rated power of 115 W at, and a maximum allowable junction temperature. The transistor is mounted on a heat sink with parameters, and. Determine how much power the BJT can safely dissipate at an ambient temperature of. (12 points) The thermal resistance from the device/junction to the case is not given explicitly, so we need to determine it before proceeding. The BJT is rated at 115 W at and a thermal model and the calculation of is then ( ) ( ) (3 points) Now we can determine the maximum allowable power dissipation when the BJT is mounted on a heat skink with the given parameters. A thermal model for the problem is shown below. ( ) ( ) (9 points) 9

10 Question 5 Consider the circuit below. The duty cycle and frequency of the 555 astable is 60% and 10 khz respectively. (a) Specify a value for to ensure that the average current through the IR diode does not exceed 30 ma (4 points) (b) Explain (2 sentences maximum) the purpose of the decoupling capacitor (1 point) (c) Give a reasonable value for the decoupling capacitor (1 point) Part (a) The peak current must be ( ). This value will give an average of 30 ma with a 60% on time. Assuming the ( ) for the BJT, then. Choose the closest standard value of. Part (b) When the FET switches, large current spikes may appear on the supply rail, which can propagate into the IC and disturb its operation. The decoupling capacitor provides a local reservoir of energy, and ensures a clean power supply rail. Part (c) A good first try would be. 10

11 Question 6 In the following circuit, the three transistors are matched and in the same thermal environment. Determine the values for and to produce an output current of 0.4 ma. You may ignore base currents and make reasonable assumptions about V BE. (6 points) The voltage across the diode-strapped transistors is and we ignore base currents, so the voltage drop across the base-emitter of the output transistor is, and the voltage drop across is. Assume V BE = 0.7 V, so that and ( ) 11

12 Question 7 For the amplifier below,. Determine, and estimate. (8 points) Hint, use BJT scaling. Since is large,. Then and. Using BJT scaling: and This is an Emitter Follower, so ( ) ( )( ) 12

13 Question 8 Consider the amplifier shown. The maximum power the transistor may dissipate is, and. (a) Determine a load resistance so maximum power is delivered it. (b) For, determine the signal power dissipated in the load For the power calculations, neglect the base currents (8 points) Part (a) The transistor will dissipate the maximum power (25 W) when. From this follows that and. Part (b) The gain of the amplifier is ( )( )( ), so that the amplitude of the signal output voltage is. The signal power dissipated in the resistor is ( ) ( ) 13

Homework Assignment 10

Homework Assignment 10 Question 1 (Short Takes) Two points each unless otherwise indicated. 1. What is the 3-dB bandwidth of the amplifier shown below if r π = 2.5K, r o = 100K, g m = 40 ms, and C L =