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Ekt204 Ekt 204 BIPOLAR JUNCTION TRANSISTOR AMPLIFIER (AC ANALYSIS) 1. Refer to Figure 1, given RE=0.6kΩ, RC=5.6kΩ, β=120, VBE(on)=0.7V, R1=250kΩ, R2=75kΩ. a) For VA=∞, determine small-signal voltage gain, Av b) Determine the input resistance looking into the base of the transistor. (Ans: Av= -8.27 and Rib=80.1kΩ) Figure 1 Draw Thevenin equivalent circuit. Refer to Amplifier dc equation slide in BJT Commonemitter lecture note. Label all the current and voltages for the circuit. We know that, I CQ I BQ I EQ (1 ) I BQ The given circuit has RE connected at emitter terminal. Apply KVL to base-emitter loop, we will get: 1 Ekt204 VTH I BQ RTH V BE ( on) I EQ R E 0 VTH I BQ RTH V BE ( on) (1 ) I BQ R E 0 I BQ RTH (1 ) R E VTH V BE ( on) I BQ VTH V BE ( on) RTH (1 ) R E If we need to find VCEQ, Apply KVL around collector-emitter loop for common-emitter circuit yield to, VCC I CQ RC VCEQ I EQ RE 0 VCEQ VCC I CQ RC I EQ R E VCC I BQ RC (1 ) I BQ R E VCC I BQ RC (1 ) R E Thevenin resistance, RTH and thevenin voltage, VTH are given as below: 2 Ekt204 3 Ekt204 2. Let β=100, VBE(on)=0.7V and VA=100V. Determine: a) Small-signal voltage gain b) Input resistance seen by the signal source, Rin c) Output resistance looking back into the output terminal, Ro (Ans: ICQ = 0.439mA, Av= -148, Rin=6.09kΩ, Ro = 9.58kΩ) Figure 2 4 Ekt204 5 Ekt204 3. Assume that β=100, VA=∞, R1=10kΩ and R2=50kΩ for the circuit in Figure 3. a. Plot the Q-point on dc load line b. Determine the small-signal voltage gain (Ans: ICQ=1.19mA, VECQ = 8.42V and Av= -1.94) Figure 3 6 Ekt204 For small-signal equivalent circuit shown above, make sure label all the current, voltages and Ri and Rib. 100(0.026) 2.18k 1.19 VO g mV RC Write KVL from input around B-E loop, V V S V g mV R E r r given g m r , V VS 1 R E 1 r Voltage gain, Av RC Av 1.94 r (1 ) RE 7 Ekt204 4. Refer to Figure 4, given β=180, VA=∞. a. Find ICQ and VCEQ b. Plot dc and ac load lines c. Calculate small-signal voltage gain. (Ans: ICQ= 15.6mA, VCEQ= 10.1V, rπ=0.30kΩ, Rib=34.2kΩ and Av= 0.806) Figure 4 RTH R1 R2 5k R2 10 (V V ) V VTH 18 9 0 10 10 R 1 R2 V V BE V 0 0.7 9 I BQ TH 0.0869mA RTH (1 ) R E 5 181(0.5) I CQ 15.6mA , I EQ (1 ) R E 15.7 mA VCEQ 18 (15.7)( 0.5) 10.1V 8 Ekt204 9