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Problem 1 - (Design) DC Bias of BJT in Active Region: (0 - 18 points) For the following circuit in Figure 1, bias your transistor such that circuit gain gmRc=10 (gain of -10V/V); Rc is the resistance connected at the collector of the transistor. Fix the collector-emitter voltage such that the transistor operates in the active region. Find Ic, Rc, Rb, and Vce. a) Simulate in PSPICE DC bias and show the bias currents and voltages. Compare these to your design predictions. b) Simulate the circuit for AC frequency response analysis. Plot the frequency response of the circuit from 10Hz up to 10MHz. Use dB(Vo) for the y-axis. Find the upper and lower -3dB points on your AC plot. Figure 1: Common Emitter Amplifier Q2N3904 Problem 2: Push-Pull Power Amplifier (0 to 18 points) 2.1. Configure the circuit in Figure 2.1. Make sure to flip correctly the pnp BJT, so that both transistors connect emitter-to-emitter and base-to-base. Figure 2.1. BJT push-pull power amplifier. Let source V3 be VSIN with amplitude of 5.5V and frequency of 1 KHz. Next, try also 3.6V and 1.7V amplitude values. Observe your plot of V(out)(t) and comment on the changes in the crossover distortion phenomenon. (At least three plots and three schematics for problem 2.1) 2.2. Simulate the following circuit in Figure 2.2: Figure 2.2: Distortion free Class B push-pull power amplifier. Observe and comment on what happened to the crossover distortions. Also observe what happened to the VBE error that you observed in 2.1 (error between Vin and Vout). Try also real small input amplitude (say 0.2V) – is there still some trace of the crossover distortion? . (At least two plots and two schematics for problem 2.2) V1 12Vdc Q2 Q2N3906 vin 0 Q1 Q2N3904 V3 FREQ = 1kHz VAMPL = 5.5V VOFF = 0V R1 4k R2 5 vout 0 0 V2 12Vdc R3 5 V1 12Vdc vout1 R2 5 R1 4k 0 V3 FREQ = 1kHz VAMPL = 5.5V VOFF = 0V 0 vin Q1 Q2N3904 vout 0 V2 12Vdc U1 uA741 3 2 7 4 6 1 5 + - V+ V- OUT OS1 OS2 Q2 Q2N3906 R3 5 Problem 3: Common-Emitter Voltage Amplifier Analysis (0 to 64 points) The circuit to be simulated in Figure 3 is similar to the example studied theoretically in Unit 22 Part 2: Figure 3: Common Emitter Amplifier This PSPICE activity has multiple objectives: 1) See the practical DC performance of the BJT amplifier - exact VBE and β values. 2) Learn how to easily modify the Q-Point of the amplifier. 3) Learn how to measure the voltage gain in Transient. 4) See the effect of the Early effect on the voltage gain. 5) Learn how to easily modify the amplifier's voltage gain. 6) Observe the BJT total voltages - small-signal AC on top of a DC. 7) Compute small-signal parameters and later calculate the small-signal gain of the amplifier. Refer to the above Schematic diagram in Figure 3. The input signal is VSIN with amplitude of 5mV and a frequency of 1 KHz. The resistors Rb1, Rb2, Re are all part of the amplifier DC biasing, selected to provide an approximate emitter DC current of around 1mA. The collector resistor Rc and the load resistor Rload play a major role in determining the voltage gain of the amplifier, designed to be approximately -100. All capacitors are 100uF. These capacitors act like short circuits at the frequency of 1 KHz, but they block DC signals. Re 2k Rc 3k Rload 20k C2 100uF VE Rb1 200k VB 10Vdc V2 VC C3 100uF vin C1 100uF 0 Rs 50 0 vout Vi FREQ = 1k VAMPL = 5mV VOFF = 0V Q1 Q2N3904 Rb2 100k The following step-by-step assignments are designed to enhance your understanding of BJT amplifiers. A new PSPICE schematic is required for parts 3.3, 3.5, 3.6, 3.7, 3.8, 3.9, and 3.10. 3.1. For the BJT circuit in Figure 3, calculate the DC values for VC, VB, VE, IC, IB, and IE. 3.2. Next, compute the AC small signal parameters gm, rπ, re. Assume the transistor β=150, VBE=0.7V, and Vth=26mV. 3.3. Start your simulations by setting up the amplifier's circuit in Figure 3. Simulate the amplifier in Bias Point. Turn on annotation for the DC voltages and currents. A new PSPICE schematic is required. 3.4. Measure VBE and β and record these values from the PSPICE schematic. 3.5. How would you modify the circuit to have a collector current of exactly 1.4 mA? [Hint: The most convenient component to change, if we want to modify the DC current, is RE]. A new PSPICE schematic is required. 3.6. What should you do if we want to change IC to 0.55mA? Demonstrate by simulating how this tweaking is done. A new PSPICE schematic is required. 3.7. Simulate the amplifier in Transient. Watch the voltages V(in) and V(out), and compute the small-signal gain using ratio of peak-to-peak amplitudes. Compare the measured value to the theoretical value. How do you explain the very noticeable gain difference? [Hint: Think about the Early effect and ro. You may click-select the BJT, do Edit: PSPICE Model, just to see the model parameters. The parameter Vaf is VA and it is around 74V] A new PSPICE schematic is required. 3.8. Increase the load resistance Rload to 1MEG, simulate in transient and observe the influence on the gain. A new PSPICE schematic is required. 3.9. Keeping Rload at 1MEG, change the collector resistor R5 to 4k, simulate in transient and comment on the influence on the gain. A new PSPICE schematic is required. 3.10. Go back to the initial Rc and Rload values from 3.3 above. Change the emitter resistor Re to 3k and simulate in transient and comment on its influence on the DC emitter current, and on the voltage gain. The formulas ETe = / IVr and eLC −≅ /)||( rRRgain may help you understand what's going on. A new PSPICE schematic is required. Hint: One more source of error has to do with the computation of re. Typically “room temperature” in electronics is 200C and that yields a thermal voltage of VT≈25mV. In PSPICE however the default temperature is 3000K, and for that VT≈26mV.

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