6 0 2 MB
Electronics-Problems
Second Semester 2018-2019
Problem-1 Determine IC(sat) for the transistor in Figure below. What is the value of IB necessary to produce saturation? What minimum value of VIN is necessary for saturation? Assume VCE(sat) = 0 V. Solution:
πΌπΆ(π ππ‘) =
ππΆπΆ 5π = = 0.5 ππ΄ π
πΆ 10 πΞ©
πΌπΆ(π ππ‘) 0.5 ππ΄ = = 3.33 ππ΄ π½π·πΆ 150
πΌπ΅(πππ) =
πΌπ΅(πππ) =
πππ(πππ) β ππ΅πΈ π
π΅
π
π΅ πΌπ΅(πππ) = πππ(πππ) β 0.7π πππ(πππ) = π
π΅ πΌπ΅(πππ) + 0.7π = (1 πΞ©) (3.33ππ΄) + 0.7π = 4.03 π Problem-2 The transistor in Figure below has a Ξ²DC of 50. -
Determine the value of RB required to ensure saturation when VIN is 5 V.
-
What must VIN be to cut off the transistor? Assume VCE(sat) = 0 V
Solution: πΌπΆ(π ππ‘) =
ππΆπΆ
πΌπ΅(πππ) = π
π΅(πππ) =
π
πΆ
=
πΌπΆ(π ππ‘) π½π·πΆ
15 π 1.2 πΞ©
=
= 12.5 ππ΄
12.5 ππ΄ 50
πππ(πππ) β ππ΅πΈ πΌπ΅
=
= 250 ππ΄
5πβ 0.7 π 250 ππ΄
= 17.2 πΞ©
ππΌπ(ππ’π‘πππ) = 0 π
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Problem-3 The transistor in Figure below has the following maximum ratings: PD(max) = 800 mW, VCE(max) =15 V, and IC(max) =100 mA. Determine the maximum value to which VCC can be adjusted without exceeding a rating. Which rating would be exceeded first? Solution: First, find IB so that you can determine IC πΌπ΅ =
ππ΅π΅ β ππ΅πΈ π
π΅
=
5πβ0.7π 22πΞ©
= 195 ππ΄
πΌπΆ = π½π·πΆ πΌπ΅ = (100) (195ππ΄) = 19.5 ππ΄ IC is much less than IC(max) and ideally will not change with VCC. It is determined only by IB and Ξ²DC. The voltage drop across RC is VRC = IC RC = (19.5 mA) (1.0 kβ¦) = 19.5 V Now you can determine the value of VCC when VCE = VCE(max) = 15 V VRC = VCC - VCE So,
π½πͺπͺ(πππ) = π½πͺπ¬(πππ) + π½πΉπͺ = (πππ½) + (ππ. ππ½) = ππ. π π½ VCC can be increased to 34.5 V, under the existing conditions, before VCE(max) is exceeded. However, at this point it is not known whether or not PD(max) has been exceeded.
ππ· = ππΆπΈ(πππ₯) πΌπΆ = (15π) (19.5ππ΄) = 293 ππ Since PD(max) is 800 mW, it is not exceeded when VCC = 34.5 V. So, VCE(max) =15 V is the limiting rating in this case. If the base current is removed causing the transistor to turn off, VCE(max)will be exceeded first because the entire supply voltage, VCC, will be dropped across the transistor.
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Problem-4 a) For the transistor circuit in Figure below, what is VCE when VIN = 0 V? (b) What minimum value of IB is required to saturate this transistor if Ξ²DC is 200? Neglect VCE(sat). (c) Calculate the maximum value of RB when VIN = 5 V.
Solution: When vin = 0 V, the transistor is in cutoff (acts like an open switch) and
VCE = VCC = 10 V
Since VCE(sat) is neglected (assumed to be 0V), π°πͺ(πππ) = π°π©(πππ) =
π½πͺπͺ ππ π½ = = ππ ππ¨ πΉπͺ π ππ
π°πͺ(πππ) ππ ππ¨ = = ππ ππ¨ π·π«πͺ πππ
This is the value of IB necessary to drive the transistor to the point of saturation. Any further increase in IB will ensure the transistor remains in saturation but there cannot be any further increase in IC. When the transistor is on, VBE β 0.7V. the voltage across RB is
ππ
π΅ = πππ β ππ΅πΈ β
5π β 0.7π = 4.3π Calculate the maximum value of RB needed to allow a minimum IB of 50 Β΅A using ohm`s law as follows: πΉπ©(πππ) =
π½ πΉπ© π°π©(πππ)
=
3
π. ππ½ = ππ ππ ππππ¨
Problem-5 The LED in Figure below requires 30 mA to emit a sufficient level of light. Therefore, the collector current should be approximately 30 mA. For the following circuit values, determine the amplitude of the square wave input voltage necessary to make sure that the transistor saturates. Use double the minimum value of base current as a safety margin to ensure saturation. VCC = 9 V, VCE(sat) = 0.3 V, RC = 220 β¦, RB = 3.3 k β¦, Ξ²DC =50, and VLED =1.6 V.
Solution: ππΆπΆ β ππΏπΈπ· β ππΆπΈ (π ππ‘)
πΌπΆ(π ππ‘) =
π
πΆ
πΌπ΅(πππ) =
πΌπΆ(π ππ‘) π½π·πΆ
=
32.3 ππ΄ 50
=
9 πβ1.6 πβ0.3 π 220 πΞ©
= 32.3 ππ΄
= 646 ππ΄
To ensure saturation, use twice the value of IB(min), which is 1.29 mA. Use ohm's law to solve for Vin. πΌπ΅ =
ππ
π΅ π
π΅
=
πππ β ππ΅πΈ π
π΅
=
πππ β0.7 π 3.3 πΞ©
2πΌπ΅(πππ) π
π΅ = πππ β 0.7π = (1.29 ππ΄)(3.3πΞ©) πππ = (1.29 ππ΄)(3.3πΞ©) + 0.7π = 4.96 π
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Propblem-6 I.
Determine the following dc values for the amplifier in Figure below. (a) IE (b) VE (c) VB (d) IC (e) VC (f) VCE
II.
Determine the following ac values for the amplifier in Figure below. (a) Rin(base) (b) Rin (c) Av (d) Ai (e) Ap
III.
Assume that 600 Ξ©, 12ΞΌV rms a voltage source is driving the amplifier in Figure below. Determine the overall voltage gain by taking into account the attenuation in the base circuit. What is the phase relationship of the collector signal voltage to the base signal voltage?
Solution: I π·πΉπ¬ β« ππ πΉπ ππ‘β = ( π
π
2
1 + π
2
???
) β πππ = 3.66 π
π
βπ
π
π‘β = ( 1 2 ) = 9.56 πΞ© π
+π
1
2
|ππ‘β + ππ
π‘β + ππ΅πΈ + ππ
πΈ | = 0 |ππ‘β + πΌπ΅ π
π‘β + ππ΅πΈ + (π½ + 1) πΌπ΅ π
πΈ | = 0 πΌπ΅ =
ππ‘β β ππ΅πΈ π
π‘β + (π½+1)π
πΈ
= 34.6 ππ΄
πΌπΈ = (π½ + 1) πΌπ΅ = 2.63 ππ΄ ππΈ = ππ
πΈ = πΌπΈ β π
πΈ = 2.6 π ππ΅ = ππΈ + ππ΅πΈ = 3.3 π πΌπΆ = π½ β πΌπ΅ = 2.59 ππΆπΈ = ππΆπΆ β πΌπΆ π
πΆ β πΌπΈ π
πΈ = 6.8 π ππΆ = ππΆπΆ β πΌπΆ π
πΆ = 9.45 π
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Note: π
ππ(πππ π) =
πππ πΌππ
=
ππ πΌπ
πππ π ππππ πΌπ β
πΌπ β«β«β« ππ = πΌπ ππ, πΌ πΌπ β
π½ π ππ
Substituting for Vb and Ib, π
ππ(πππ π) =
ππ πΌπ
=
πΌπ π,π
πΌπ /π½ππ
Cancelling Ie, π
ππ(πππ π) = π½ππ ππ,
II π
ππ(πππ π) = π½ππ ππ, = 70 [
25ππ πΌπΈ
] = 665Ξ©
π
ππ(π‘ππ‘ππ) = π
1 β₯ π
2 β₯ π
ππ (πππ π) = 622 Ξ© π΄π£ = π΄π =
π
πΆ β₯ π
πΏ ππ, πΌπ πΌπ
= 261
Note: π΄π£ =
= π½ππ = 70
ππ ππ
ππ = πΌππ πΌπ π
πΆ β
πΌπ π
πΆ ππ = πΌπ ππ, πΌ π
π΄π£ = πΌπ π ,πΆ β«β«β« π΄π£ =
π΄π = π΄π£ β π΄π = 18281
π π
π
π = π
πΆ β₯ π
πΏ
III π
ππ(π‘ππ‘ππ)
ππ = ( π
+π
π
ππ(π‘ππ‘ππ)
622 Ξ©
) πππ = (600 Ξ©+ 622 Ξ©) 12 ππ = 6.1 ππ
Attenuation of the input network is π
ππ(π‘ππ‘ππ)
(π
+ π
π
ππ(π‘ππ‘ππ)
) = 0.51
π΄,π£ = 0.51 β 261 = 133.6 Vb / Vs
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π
πΆ ππ,
ππΆπΈ = ππΆπΆ β πΌπΆ (π
πΆ + π
πΈ ) Decrease
Increase
Constant
π = 180π
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Propblem-7 Determine the exact voltage gain for the unloaded emitter-follower in Figure below.
Solution: ππ΅ = ( π
π
2
4.7 πΞ©
1 + π
2
πΌπΈ = ππ, β
π΄π£ =
ππ΅ β ππ΅πΈ π
πΈ 25ππ 1.06 ππ΄ π
πΈ π
πΈ + ππ,
) ππΆπΆ = (14.7 πΞ©) 5.5 π = 1.76π =
1.76πβ0.7π 1.0 πΞ©
= 1.06 ππ΄
= 23.6 Ξ© =
1.0 πΞ© 1.0 πΞ©+23.6 Ξ©
= 0.977
Note: π΄π£ =
πππ’π‘ πππ
πππ’π‘ = πΌπ π
π πππ = πΌπ (ππ, + π
π ) πΌ π
π΄π£ = πΌ (ππ, +ππ
) β«β«β« π΄π£ = π
π
π
π
π = π
πΈ β₯ π
πΏ π
π β« ππ, π΄π£ β
1
8
π
π , ππ + π
π
Propblem-8 Find the overall current gain Ai in below.
Solution: π
ππ(πππ π) = π½ππ1 π½ππ2 π
πΈ = (150)(100)(1.5 πΞ©) = 22.5 πΞ©
R in = R1 β₯ R 2 β₯ R in(base) = 33kΞ© β₯ 22kΞ© β₯ 22.5 MΞ© = 13.2 kΞ© Note:
πΌππ =
πππ π
ππ
=
πΌππ(πππ π1) =
1π 13.2 πΞ©
= 75.8 ππ΄
πππ π
ππ(πππ π1)
=
1π 22.5 πΞ©
= 44.4 ππ΄
πΌπ β
π½ππ1 π½ππ2 πΌππ(πππ π1) = (150)(100)(44.4ππ΄) = 667ππ΄ π΄,π =
πΌπ πΌππ
=
667 ππ΄ 75.8 ππ΄
= 8.8
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Propblem-9 Find power gain Ap for the unloaded amplifier in Figure below
The same circuit
Solution: ππΈ = ( π
π
2
1 + π
2
πΌπΈ =
6.8 π 620Ξ©
10πΞ©
) ππΆπΆ β ππ΅πΈ = (32πΞ©) 24π β 0.7π = 6.8π
= 10.97ππ΄
π
ππ(πππ π) = ππ, β
π¨π =
πΉπͺ π,π
=
25 ππ 25ππ = = 2.28 Ξ© πΌπΈ 10.97 ππ΄
π.π ππ π.ππ π
Note: πΌπ π
πΈ β« ππ, , then π
π΄π£ β
ππΆ,
= πππ
π
π
π = π
πΆ β₯ π
πΏ
π΄π β
1 π¨π = π¨π π¨π β
πππ
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Propblem-10 For the two-stage, capacitively coupled amplifier in Figure below, find the following values: (a) voltage gain of each stage (b) overall voltage gain (c) Express the gains found in (a) and (b) in dB.
Solution: ππΈ = ( π
π
2
8.2πΞ©
1 + π
2
πΌπΈ =
ππΈ π
πΈ
=
) ππΆπΆ β ππ΅πΈ = (33πΞ©+8.2 kΞ©) 15π β 0.7π = 2.29π
2.29 π 1.0πΞ©
ππ, β
= 2.29ππ΄
25 ππ πΌπΈ
=
25ππ 2.29 ππ΄
= 10.9 Ξ©
R in(2) = R 5 β₯ R 4 β₯ π½ππ ππ, = 8.2kΞ© β₯ 33kΞ© β₯ 175(10.9) = 1.48 kΞ© π΄π£1 =
π΄π£2 =
π
πΆ β₯ π
ππ(2) ππ, π
πΆ ππ,
=
=
3.3 πΞ© 10.9 Ξ©
3.3πΞ© β₯1.48πΞ© 10.9 Ξ©
= 93.6
= 302
A,v = Av1 Av2 = (93.6)(302) = 28267 Av1(dB) = 20log(93.6) = 39.4 dB Av2(dB) = 20log(302) = 49.6 dB A,v(dB) = 20log(28267) = 89 dB
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Propblem-11 Figure below shows a CE power amplifier in which the collector resistor serves also as the load resistor. Assume Ξ²DC = Ξ²ac = 100, Determinea. The dc Q-point (ICQ and VCEQ). b. The voltage gain and the power gain. c. The power dissipated in the transistor with no load d. the total power from the power supply with no load e. the signal power in the load with a 500mV input
Solution: (a) ππΈ = ( π
π
2
330Ξ©
1 + π
2
) ππΆπΆ = (1.0πΞ©+330 Ξ©) 15π = 3.72π
ππΈ = ππ΅ β ππ΅πΈ = 3.72π β 0.7π = 3.02π πΌπΆπ β
πΌπΈ = ππ, β
25 ππ πΌπΈ
ππΈ π
πΈ1 + π
πΈ2
=
=
25ππ 68.3 ππ΄
3.02π 8.2Ξ©+36Ξ©
= 68.3mA
= 0.37 Ξ©
ππΆπΈπ = ππΆπΆ β (πΌπΆ )(π
πΈ1 + π
πΈ2 + π
πΏ ) = 15π β (68.3ππ΄)(8.2Ξ© + 36Ξ© + 100Ξ©) = 5.15π
(b) π΄π£ =
π
πΏ π
πΈ1 + ππ,
=
100 Ξ© 8.2Ξ©+0.37 Ξ©
= 11.7
R in = R1 β₯ R 2 β₯ π½ππ (π
πΈ1 + ππ, ) = 330Ξ© β₯ 1.0kΞ© β₯ 100(8.2Ξ© + 0.37Ξ©) = 192Ξ© π
192 Ξ©
π΄π = π΄2π£ ( ππ ) = 11.72 ( ) = 263 π
100Ξ© πΏ
The computed voltage and power gains are slightly higher if ππ, ππ πππππππ. (c) If RL is removed, there is no collector current; hence, the power dissipated in the transistor is zero.
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(d) Power is dissipated only in the bias resistor plus a small amount in RE1 and RE2. Since the load resistor has been removed, the base voltage is altered. The base voltage can be found from the Theveninβs equivalent draw for the bias circuit in figure below πΉππ =
πΉπ β πΉπ πΉπ + πΉπ
= πππ π½ Rth = 248 V
0.7 V RE1 + RE2 = 44.2 Ξ©
Applying the voltage βdivider rule and including the base-emitter diode drop of 0.7 V result in a base voltage of 1.2 V. The power supply current is then computed as π°πͺπͺ =
π½πͺπͺ β π. π π½ πππ½ β π. ππ½ = = ππ. πππ¨ πΉπ π. πππ΄
Power from the supply is then computed as π·π» = π°πͺπͺ π½πͺπͺ = (ππ. π ππ¨)(πππ½) = πππ ππΎ (e) π΄π£ = 11.7 π½ππ = πππ ππ½πβπ =
π½πβπ πβ βπ
=
πππ π½ πβ βπ
= πππ ππ½πππ
πππ’π‘ = π΄π£ πππ = (11.7)(177ππ) = 2.07π πππ’π‘ =
2 πππ’π‘
π
πΏ
=
(2.07)2 100Ξ©
= 42.8 ππ
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Problem-12 For the BJT amplifier of Fig. below, if vin (t) = 0.05 sin (Οt) determine: a) Zi b) Zo c) Av d) Draw the total collector voltage waveform and the total output voltage waveform. Solution: ππ΅ =
ππΆπΆ β π
2 π
1 + π
2
=
15 πβ (5β 103 πΊ) 10β103 πΊ +5β 103 πΊ
=5π
ππΈ = ππ΅ β ππ΅πΈ = 5 π β 0.7 π = 4.3 π πΌπΈ = ππ =
ππΈ π
πΈ
=
26 ππ πΌπΈ
4.3 π
= 4.3 ππ΄
1000 πΊ
=
26 ππ 4.3 ππ΄
= 6.05 πΊ
π
1 β π
2
π
π΅ = π
1 β₯ π
2 =
π
1 + π
2
= 3.33 ππΊ
π½ππ = 1815 πΊ πππ = π
π΅ β₯ π½ππ = 1.12 ππΊ πππ’π‘ = π
πΆ = 1 ππΊ π΄π£ = β
π
πΆ ππ
= β
1000 πΊ 6.05 πΊ
= β165
πππ’π‘ = π΄π£ β πππ = β (165) β (0.05) π ππ (ππ‘) = β8.25 π ππ (ππ‘)
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