AMPLIFIER CIRCUITS circuit board. Student Manual FACET by Lab-Volt 29

Transcription

1 When you have completed this exercise, you will be able to determine the dc operating conditions of a common base (CB) transistor circuit by using a typical CB circuit. You will verify your results with a multimeter. AMPLIFIER CIRCUITS circuit board. FACET by Lab-Volt 29

2 Common Base Circuit The base terminal is common to the input and output signals and is connected to ground for ac signals by capacitor C2. necessary to forward bias the base-emitter junction. This circuit is the common a. base circuit. b. emitter circuit. c. collector circuit. 30 FACET by Lab-Volt

3 You can calculate base voltage (V B ) from the voltage divider equation: V B = VA R2 R1 + R2 where V A is the dc supply voltage and R1 and R2 are the base voltage divider resistors. The base-emitter difference voltage (V BE ) in a silicon NPN transistor is normally about 0.6 Vdc. Consequently, the emitter voltage (V E ) is 0.6 Vdc less than the base voltage. V E = V B 0.6 To determine the emitter voltage (V E ), a. add 0.6 Vdc to the base voltage. b. subtract 0.6 Vdc from the collector voltage. c. subtract 0.6 Vdc from the base voltage. FACET by Lab-Volt 31

4 Common Base Circuit To calculate emitter current (I E ), use the value of the emitter resistor (R5) and Ohm s law. VE IE = R5 The collector current (I C ) equals the difference between the emitter current (I E ) and the base current (I B ). I C = I E I B The collector current (I C ) nearly equals the emitter current (I E ) due to the base current (I B ) being much smaller than the emitter current. 32 FACET by Lab-Volt

5 Collector current (I C ) can be calculated from Ohm s law, where R4 is the collector resistor. Collector current essentially equals emitter current (I E ). I C VA VC = = I R4 E Collector current essentially equals the a. base current. b. base current plus the emitter current. c. emitter current. A dc load line is drawn on the collector current characteristic curves of the common base circuit. FACET by Lab-Volt 33

6 Common Base Circuit The dc Q-point (quiescent point) of the transistor is on the dc load line at the intersection of the operating collector current (I C ) on the Y-axis and the base-collector voltage (V BC ) on the X-axis. The cutoff point, on the X-axis, is where the collector and emitter currents are essentially zero (the baseemitter junction comes out of forward bias). The collector voltage is nearly equal to V A. The collector-base voltage [V CB(cutoff) ] equals the difference between the dc supply voltage and the base voltage (V A V B ). The saturation point, on the Y-axis, is where the base-collector junction comes out of reverse bias. Collector current (I C ) is maximum. 34 FACET by Lab-Volt

7 The active region of the transistor lies between the cutoff and saturation points. The point on the dc load line at which the base-collector voltage (V BC ) is zero is the a. cutoff point. b. Q-point. c. saturation point. base circuit as shown. FACET by Lab-Volt 35

8 Common Base Circuit Measure the value of the supply voltage (V A ). V A = Vdc (Recall Value 1) Calculate the base voltage (V B ) of Q1. Use the values of R1 and R2 and the voltage divider formula. V B R2 = VA R1 + R2 = Vdc (Recall Value 2) Measure the base voltage (V B ) of Q1. V B = Vdc (Recall Value 3) 36 FACET by Lab-Volt

9 Measure the base-emitter voltage (V BE ). V BE = Vdc (Recall Value 4) Based on the values you have just measured, is the base-emitter junction forward biased? a. yes b. no V B = Vdc (Step 4, Recall Value 3) V BE = Vdc (Step 5, Recall Value 4) Measure the base-collector voltage (V BC ). V BC = Vdc (Recall Value 5) FACET by Lab-Volt 37

10 Common Base Circuit Based on the value you have just measured, is the base-collector junction reverse biased? a. yes b. no V BC = Vdc (Step 7, Recall Value 5) Is NPN transistor Q1 biased to operate in the active region? a. yes b. no Measure the emitter voltage (V E ). V E = Vdc (Recall Value 6) 38 FACET by Lab-Volt

11 Calculate the dc emitter current (I E ) by using Ohm s law. V E = Vdc (Step 10, Recall Value 6) R5 = 1.0 k I E VE = = ma (Recall Value 7) R5 Measure the collector voltage (V C ). V C = Vdc (Recall Value 8) The difference between the dc supply voltage (V A ) and the collector voltage (V C ) is the voltage drop across R4. V A = Vdc (Step 2, Recall Value 1) V C = Vdc (Step 12, Recall Value 8) Calculate the voltage drop across R4. V R4 = V A V C = Vdc (Recall Value 9) FACET by Lab-Volt 39

12 Common Base Circuit Calculate the dc collector current (I C ) by using Ohm s law. V R4 = Vdc (Step 13, Recall Value 9) I C VR4 = = ma (Recall Value 10) R4 Are the emitter and collector currents essentially the same? a. yes b. no I E = ma (Step 11, Recall Value 7) I C = ma (Step 14, Recall Value 10) Place CM switch 9 in the ON position to change the Q1 bias as shown (base resistor R2 was changed from 10 k to 3.3 k ). 40 FACET by Lab-Volt

13 Measure the collector voltage (V C ), and enter the value below. V C = Vdc (Recall Value 11) Measure the base voltage (V B ), and enter the value below. V B = mvdc (Recall Value 12) Measure the emitter voltage (V E ), and enter the value below. V E = mvdc (Recall Value 13) FACET by Lab-Volt 41

14 Common Base Circuit With Q1 biased as shown, does transistor Q1 still operate in the active region? a. yes b. no V C = Vdc (Step 16, Recall Value 11) V B = mvdc (Step 17, Recall Value 12) V E = mvdc (Step 18, Recall Value 13) Based on your measured results, the transistor is operating at the a. saturation point. b. Q-point. c. cutoff point. The transistor operating at the cutoff point can be determined by which relationship between V B and V E? a. V BE is less than 0.6 Vdc. b. V B has changed in value. c. V E has changed in value. Make sure all CMs are cleared (turned off) before proceeding to the next section. A voltage divider circuit provides a constant dc base voltage to properly bias the transistor. When the transistor is operating in the active region, the base-emitter junction is forward biased and the base-collector junction is reverse biased. The emitter and collector currents are essentially equal. The dc load line describes the relationship of the collector current to the base-collector voltage difference. 42 FACET by Lab-Volt

15 The Q-point on the dc load line is the intersection of the operating collector current (I C ) and the basecollector voltage (V BC ). The intersection of the dc load line and the Y-axis is the saturation point; the intersection of the dc load line and the X-axis is the cutoff point (zero current). 1. Place the CM switch 7 in the ON position to change the value of R1 from 120 k to 10 k. Measure the collector voltage (V C ). V C = Vdc FACET by Lab-Volt 43

16 Common Base Circuit Measure the base voltage (V B ). V B = Vdc Measure the emitter voltage (V E ). V E = Vdc 44 FACET by Lab-Volt

17 The transistor is operating a. at the saturation point. b. in the active region. c. at the cutoff point. d. at an optimum Q-point. 2. Place the CM switch 8 in the ON position to change the value of R4 from 15 k to 2.7 k. Measure the collector voltage (V C ). V C = Vdc FACET by Lab-Volt 45

18 Common Base Circuit Measure the base voltage (V B ). V B = Vdc Measure the emitter voltage (V E ). V E = Vdc 46 FACET by Lab-Volt

19 The transistor is operating a. at the saturation point. b. in the active region. c. at the cutoff point. d. at an optimum collector voltage (V C ). 3. The voltage at the base of an NPN transistor operating in the active region is 0.6 V a. more positive than the collector voltage. b. less positive than the emitter voltage. c. more positive than the emitter voltage. d. None of the above. 4. The collector-base voltage [V CB(cutoff) ] equals the difference between the dc supply voltage and the base voltage (V A V B ). At the cutoff point, the base-collector voltage (V BC ) is about equal to a. V A V B. b. 0.0 Vdc. c. V BE. d. 0.6 Vdc. 5. In a CB transistor circuit, the maximum collector current occurs at a. the Q-point. b. the saturation point. c. the cutoff point. d. a point midway between the saturation and cutoff points. Make sure all CMs are cleared (turned off) before proceeding to the next section. FACET by Lab-Volt 47