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# Category: Bipolar Junction Transistors (BJTs)

# Section 6.3: BJT Circuits at DC 6.51 The transistor in the circuit of Fig. P6.51 has a very high β. Find V E and V C for V B (a) +2.0 V, (b) +1.7 V, and (c) 0 V.

# 6.40 Consider a transistor forwhich the base–emitter voltage drop is 0.7 V at 10 mA. What current flows for v BE = 0.5 V? Evaluate the ratio of the slopes of the i C –v BE curve at v BE = 700 mV and at v BE = 500 mV. The large ratio confirms the point that the BJT has an “apparent threshold” at v BE 0.5 V.

# **6.69 All the transistors in the circuits of Fig. P6.69 are specified to have a minimum β of 50. Find approximate values for the collector voltages and calculate forced β for each of the transistors. (Hint: Initially, assume all transistors are operating in saturation, and verify the assumption.)

# *6.68 For the circuit in Fig. P6.68, find V B v I = 0 V, +2 V, –2.5 V, and –5 V. The BJTs have β=50.

# *6.67 Using β=∞, design the circuit shown in Fig. P6.67 so that the emitter currents of Q 1 , Q 2 , and Q 3 are 0.5 mA, 0.5 mA, and 1 mA, respectively, and V 3 V 5 =−2 V, and V 7 =1 V. For each resistor, select the nearest standard value utilizing the table of standard values for 5% resistors in Appendix J. Now, for β=100, find the values of V 3 , V 4 , V 5 , V 6 , and V 7

# *6.66 For the circuit shown in Fig. P6.66, find the labeled node voltages for:

# ***6.65 Consider the circuit shown in Fig. P6.65. It resembles that in Fig. 6.30 but includes other features. First, note diodes D 1 and D 2 are included to make design (and analysis) easier and to provide temperature compensation for the emitter–base voltages of Q 1 and Q 2 . Second, note resistor R, whose purpose is to provide negative feedback (more on this later in the book!). Using V BE and V D = 0.7 V independent of current, and β=∞, find the voltages V B1 , V E1 , V C1 , V B2 , V E2 , and V C2 , initially with R open-circuited and then with R connected. Repeat for β=100, with R open-circuited initially, then connected.

# 6.64 The pnp transistor in the circuit of Fig. P6.64 has β=50. Find the value for R C to obtain V C = +2 V. What happens if the transistor is replaced with another having β=100? Give the value of V C in the latter case.

# **6.63 It is required to design the circuit in Fig. P6.63 so that a current of 1 mA is established in the emitter and a voltage of −1 V appears at the collector. The transistor type used has a nominal β of 100. However, the β value can be as low as 50 and as high as 150. Your design should ensure that the specified emitter current is obtained when β=100 and that at the extreme values of β the emitter current does not change by more than 10% of its nominal value. Also, design for as large a value for R B as possible. Give the values of R B , R E , and R C to the nearest kilohm. What is the expected range of collector current and collector voltage corresponding to the full range of β values?

# 6.25 A pnp power transistor operates with an emitter-to-collector voltage of 5 V, an emitter current of 5 A, and V EB = 0.8 V. For β=20, what base current is required? What is I S for this transistor? Compare the emitter–base junction area of this transistor with that of a small-signal transistor that conducts i C = 1 mA with v EB = 0.70 V. How much larger is it?

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