BJT Tutorial Solutions 4. For the circuit shown in Fig 4, draw a complete small-signal equivalent circuit using T model for the BJT (use α = 0.99). Your circuit should show the values of all components, including model parameters. What is the input resistance Rin and output resistance Rout? Determine the overall voltage gain (vo/vsig) Figure 1 Solution: +9V 10k RC RC C Q1 10k vo B C Q1 E Rsig B RL 10k E 0 0.5mA vsig DC Equivalent Circuit Rsig 0 vsig AC Equivalent Circuit E re C ie vo αie B T Model RC RL 10k 10k ππ = ππ 25.7ππ = = 0.0514πβ¦ = π ππ πΌπΈ 0.5ππ΄ π ππ’π‘ = π πΆ ||π πΏ = 5πβ¦ ππ = 0 − π£ππ π£ππ =− ππ ππ ππ = πΌππ = − π£π = −ππ π ππ’π‘ = 0.99π ππ’π‘ π£ππ = 96.3π£ππ ππ π΄π£ = πΊπ£ = 0.99π£ππ ππ π£π = 96.3 π£ππ ππ π΄ = 48.8 ππ + π π ππ π£ 5. In the circuit shown in Fig 5, the transistor has a current gain β of 200. What is the dc voltage at the collector? Find the input resistances Rib and Rin and the overall voltage gain (vo/vsig). Figure 2 Solution: +5V 10 mA +1.5V RB 10k E RB Rsig 10k Q1 B B 1k C E Q1 C vo vsig RC RC 00 DC equivalent circuit 00 AC equivalent circuit πΌπΆ = π½πΌπΈ = 9.95ππ΄ π½+1 ππΆ = πΌπΆ π πΆ = 0.995π ππ΅ = 1.5π + πΌπ΅ π π΅ = 2π => π΅πΆ ππ’πππ‘πππ πππ£πππ π ππππ => π΅π½π ππ πππ‘ππ£π ππππ Rsig C ic βib RC B vo 1k RB vsig rπ 10k ib 00 E Hybrid-π Model ππ = πΌπΆ ⁄ππ = ππ = π½ ⁄ππ = 9.95ππ΄ = 387ππ΄/π 25.7ππ 200 = 0.52πβ¦ = π ππ 387ππ΄/π π ππ = π ππ ||π π΅ = 0.494πβ¦ π ππ’π‘ = π πΆ = 100β¦ ππ = −π£π ⁄π ππ = −π£π ⁄ππ = −0.001923π£π ππ = π½ππ = −0.3846π£π π£π = π ππ’π‘ ππ = −38.46π£π π΄π£ = π£π ⁄π£π = −38.46 πΊπ£ = π£π ⁄π£π ππ = π ππ 0.494πβ¦ (−38.46) = −12.72 π΄π£ = π ππ + π π ππ 0.494πβ¦ + 1πβ¦
