Extra Problems for BJT Faculty of Information Engineering & Technology Semiconductors (ELCT503) IET Department Dr. Hany Bastawrous Winter 2011 Eng. Ahmed Ragheb ____________________________________________________________________________________ Problem Set 9 1. (a) The transistor shown on the right in the circuit has a very high β. Find VE and VC for VB (i) +2 V, (ii) +1 V, and (iii) 0 V. Assume VBE = 0.7 V. (b) Find the highest value of VB for which the transistor still operates in the active mode. Also, find the value of VB for which the transistor operates in saturation with a forced β of 1. 2. For the transistor shown on the left, assume α =1. What are the values of VE and VC for VB = 0 V? For what value of VB does the transistor cut off? Saturate? In each case, what values of VE and VC result? 3. In the circuit shown on the right the transistor has β = 30. Find the values of VB, VE, and VC. If RB is raised to 270 kΩ, what voltages result? With RB = 270 kΩ, what value of β would return the voltages to the values first calculated? All the problems in the problem set are taken from Microelectronic Circuits 5th edition by Sedra/Smith. 4. For the circuits shown below, find values for the labelled node voltages and branch currents. Assume β to be very high and VBE= 0.7 V for npn and VEB= 0.7 V for pnp transistors. (b) Repeat the analysis of the circuits for β = 100. Find all the labelled node voltages and branch currents. 5. In the circuit shown on the right, VBE is adjusted so that VC = 2V. If VCC = 5V, RC = 3 kΩ, and a signal vbe = 0.005sinωt volts is applied, find expressions for the total instantaneous quantities iC(t), vC(t), and iB(t). The transistor has β = 100. What is the voltage gain? All the problems in the problem set are taken from Microelectronic Circuits 5th edition by Sedra/Smith. 6. For the circcuit shown below, b α = 0.99. Usingg the brid-π modeel, draw an AC A equivaleent of the cirrcuit hyb sho own. What is i the input resistance r Rin e the i ? Calculate overall voltagee gain (vo/vs). he circuit shhown on thee left, the traansistor 7. In th has a β of 200. W What is the dc voltage at the collecto or? Find thee input resisstances Ri1 and a Ri2 and thee overall vooltage gain (vo/vs). For output signal of o ± 0.4 V, what w value of o vs is requirred? mitter-followeer circuit shhown 8. For the em t BJT used d is specifieed to on the right, the 00 (a havve β values in the rangee of 40 to 20 disstressing situation fo or the ciircuit dessigner). For the two extrreme values of β (β = 40 and β = 200), find:: (a)) IE, VE, and VB. (b)) the voltagee gain vo/vs. All the t problems inn the problem set are taken frrom Microelecctronic Circuitss 5th edition byy Sedra/Smith.