ENEL 463 Assignment #4

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```					ENEL 463 Assignment #4
Due Thursday, November 30, 2006
Each question is worth 25% of the total marks for this assignment.

1.    The BJT shown in Figure 1 has β = 100. Both capacitors have infinite capacitance.
a. Find the DC collector current and the DC voltage at the collector.
b. Replacing the transistor by its T model, draw the small-signal equivalent
circuit of the amplifier. Analyze the resulting circuit to determine the
voltage gain vo/vi

Figure 1

2.    Refer to the voltage-gain expression (in terms of the transistor β) given in Equation
1 for the CE amplifier with a resistance Re in the emitter. Let the BJT be biased at
an emitter current of 1mA. The source resistance Rs is 10k Ω. The BJT β is
specified to lie in the range of 50 to 150 with a nominal value of 100.
a. What is the ratio of maximum to minimum voltage gain obtained without
Re?
b. What value of Re should be used to limit the ratio of maximum to
minimum gain to 1.2?
c. If the Re found in (b) is used, by what factor is the gain reduced (compared
to the case without Re) for a BJT of nominal β?

Equation 1
vo              Rc
Av       
vs    Rs  (   1)( re  Re )
3.   Consider the CB amplifier of Figure 2 with a collector voltage signal coupled to a
1kΩ load resistance through a large capacitor. Let the power supplies be ±5V. The
source has a resistance of 50Ω. Design the circuit so that the amplifier input
resistance is matched to that of the source, and the output signal swing is as large as
possible with relatively low distortion (vbe limited to 10mV). Find Ie and Rc and
calculate the overall voltage gain obtained and the output signal swing. Assume
capacitors of infinite capacitance.

Figure 2

4.   In the emitter follower of Figure 3, the signal source has Rs = 10kΩ and a DC
component of zero. The transistor has β = 100 and VA = 125V. The bias current I =
2.5mA, and VCC = 3V. What is the output resistance of the follower? Find the gain
vo/vs with no load and with a load of 1kΩ. With the 1kΩ load connected, find the
largest possible negative output signal. What is the largest possible positive output
signal if operation is to the point that the base-collector junction is forward biased
by 0.2V?

Figure 3

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