Bipolar transistor

1. Introducing a resistor in the emitter of a common emitter amplifier stabilizes the dc operating point against variations in(a) only the temperature(b) only the β of the transistor (c) both temperature and β(d) none of the above

2. The current gain of a bipolar transistor drops at high frequencies because of(a) transistor capacitances of (b) high current effects in the base(c) parasitic inductive elements(d) the Early effect

3. In the circuit of the figure, assume that the transistor is in the active region. It has a large β and its base-emitter voltage is 0.7 V. The value of Ic is

(a) Indeterminate since Rc is not given(b) 1 mA(c) 5 mA(d) 10 mA

4. The current gain of a BJT is

(a) gmro (c) gmr0

(b) gmrπ (d) gmr π

5. An npn BJT has gm = 38 mA/V, Cµ = 10-14F, Cπ = 10-13F, and DC current gain β0 = 90. For this transistor fT and fβ are(a) fT = 1.64 x 108 Hz and fβ = 1.47 x 1010 Hz(b) fT = 1.47 x 1010 Hz and fβ = 1.64 x 108 Hz(c) fT = 1.33 x 1012 Hz and fβ = 1.47 x 1010 Hz(d) fT = 1.47 x 1010 Hz and fβ = 1.33 x 1012 Hz

6. Three identical RC-coupled transistor amplifiers are cascaded. If each of the amplifiers has a frequency response as shown in the figures, the overall frequency response is as given in

7. Choose the correct match for input resistance of various amplifier configurations shown belowConfigurationCB: Common BaseCC: Common collectorCE: Common EmitterInput resistanceLO: LowMO: ModerateHI: High(a) CB-LO, CC-MO, CE-HI(b) CB-LO, CC-HI, CE-MO(c) CB-MO, CC-HI, CE-LO(d) CB-HI, CC-LO, CE-MO

8. Generally, the gain of a transistor amplifier falls at high frequencies due to the(a) internal capacitances of the device(b) coupling capacitor at the input(c) skin effect (d) coupling capacitor at the output

9. In the amplifier circuit shown in the figure, the value of R1 and R2 are such that the transistor is operating at VCE = 3V and IC = 1.5 mA when its β is 150. For a transistor with β of 200, the operating point (VCE, IC) is

(a) (2 V, 2mA) (c) (3 V, 2 mA)(b) (4 V, 2mA) (d) (4 V, 1 mA)

10.Assuming VCEsat = 0.2 V and β = 50, the minimum base current (IB) required to drive the transistor in the figure to saturation is

(a) 56 µA (c) 140 µA(b) 60 µA (d) 3 µA

11.A bipolar transistor is operating in the active region with a collector current of 1 mA. Assuming that the β of the transistor is 100 and the thermal vltage (VT) is 25 mV, the transconductance (gm) and the input resistance (rπ) of the transistor in the common emitter configuration, are(a) gm = 25 mA/V and rπ = 15.625kΩ(b) gm = 40 mA/V and rπ = 4.0 kΩ(c) gm = 25 mA/V and rπ = 2.5 kΩ(d) gm = 40 mA/V and rπ = 2.5 kΩ

12. Assuming that the β of the transistor is extremely large and VBE = 0.7 V, IC and VCE in the circuit shown in the figure are

(a) IC = 1 mA, VCE = 4.7 V(b) IC = 0.5 mA, VCE = 3.75 V(c) IC = 1 mA, VCE = 2.5 V(d) IC = 0.5 mA, VCE = 3.9 V

13.The cascade amplifier is a multistage configuration of(a) CC-CB (c) CE-CB(b) CB-CC (d) CE-CC

14. For an npn transistor connected as shown in the figure, VBE = 0.7 volts. Given that reverse saturation current of the junction at room temperature 3000K is 10-13 A, the emitter current is

(a) 30 mA (c) 39 mA(b) 49 mA (d) 20 mA

15.In an ideal differential amplifier shown in the figure, a large value of (RE)

(a) increases both the differential and common-mode gains(b) increases the common-mode gain only(c) decreases the differential-mode gain only(d) decreases the common-mode gain only

16.The circuit using a BJT with β = 50 and VBE = 0.7 V is shown in the figure. The base current IB and collector voltage VC are respectively

(a) 43 µA and 11.4 Volts (c) 45 µA and 11 Volts(b) 40 µA and 16 Volts (d) 50 µA and 10 VoltsCommon Data for Questions 17,18,19.In the transistor amplifier circuit shown in the figure below, the transistor has the following parameters:βDC = 60, VBE = 0.7V, hie→ ∞, → hfe → ∞The capacitance CC can be assumed to be infinite.

In the figure above, the ground has been shown by the symbol 17. Under the DC conditions, the collector-to-emitter voltage drop is

(a) 4.8 Volts (c) 5.3 Volts(b) 6.0 Volts (d) 6.6 Volts

18.If βDC is increased by 10%, the collector-to-emitter voltage drop

(a) increases by less than or equal to 10%(b) decreases by less than or equal to 10%(c) increases by more than 10%(d) decreases by more than 10%

19. The small-signal gain of the amplifier Vc/Vs is(a) -10 (c) -5.3(b) 5.3 (d) 10

20. For the BJT circuit shown, assume that the β of the transistor is very large and VBE = 0.7 V. The mode of operation of the BJT is

(a) cut-off (c) saturation(b) normal active (d) reverse activeStatement for linked Answer Questions 21 and 22:In the following transistor circuit, VBE = 0.7 V, re = 25 mV/IE, and β and all the capacitances are very large.

21. The value of DC current IE is(a) 1 mA (c) 2 mA(b) 5 mA (d) 10 mA

22. The mid-band voltage gain of the amplifier is approximately(a) -180 (c) -120(b) -90 (d) -60

23. A small signal source vi(t) = A cos 20t + B sin 106t is applied to a transistor amplifier as shown below. The transistor has β = 150 and hie = 3 kΩ. Which expression best approximates v0(t)?

(a) V0(t) = -1500(A cos 20t + B sin 106t)(b) V0(t) = -150(A cos 20t + B sin 106t)(c) V0(t) = -1500 B sin 106t(d) V0(t) = -150 B sin 106t

24. In the circuit shown below, capacitors C1 and C2 are very large and shorts at the input frequency. v1 is a small input. The gain magnitude |V 0 /V i| at 10 M rad/s is

(a) Maximum (c) minimum(b) Unity (d) zero

25. For the BJT Q1 in the circuit shown below, β = ∞, VBEon = 0.7 V, VCEsat = 0.7 V. The switch is initially closed. At time t = 0, the switch is opened. The time t at which Q1 leaves the active region is

(a) 10 ms (c) 25 ms(b) 50 ms (d) 100 ms