1. Determine the voltage VO in the circuit of FIGURE 1 for: (a) β = 50, VBE = 0.7 V Vs (b) β = 250, VBE = 0.7 V. RB1 RC Comment on the significance of Vo your result. IB IC VB VS 12 V VBE IE RC 1 kΩ...

PFA


1. Determine the voltage VO in the circuit of FIGURE 1 for: (a) β = 50, VBE = 0.7 V Vs (b) β = 250, VBE = 0.7 V. RB1 RC Comment on the significance of Vo your result. IB IC VB VS 12 V VBE IE RC 1 kΩ RB2 VE RE RE 200 Ω RB1 15 kΩ 0 V RB2 3.3 kΩ FIG. 1 2. State the effects of negative feedback, when applied in a voltage amplifier, upon: · the overall amplification · variations in transistor gain · non-linearity · output impedance. 3. Estimate the power developed in the 8 Ω speaker of the circuit of FIGURE 2 for a 1 kHz sinusoidal input signal of 100 mV peak. All capacitors may be assumed to act as a short circuit at the frequency of operation. Compare your estimate with that derived from a PSpice simulation. [A Simetrix version of the circuit can be downloaded from the module’s Learning Materials on BlackBoard.] 12 V 18 kΩ 1 kΩ 8.2 kΩ BC109 BC109 Vin T3 T1 T2 BFY51 47 Ω 18 kΩ 27 Ω Loudspeaker 6.8 kΩ 470 Ω FIG. 2 Teesside University Open Learning © Teesside University 2013 (Engineering) 5 4. FIGURE 3 shows the circuit of a multistage amplifier. Identify the stages and describe the operation and principle features of the amplifier. You should also make an estimate of the maximum output current of the amplifier. Inverting input Non-inverting input T16 T15 T6 T11 T14 T1 T2 T13 D2 T12 T17 T5 T7 D1 T 3 T4 T 8 1 kΩ 50 kΩ 1 kΩ Offset null/ Comp Offset null Comp +VCC T9 25 Ω Output 25 Ω T10 –VCC FIG. 3 Teesside University Open Learning © Teesside University 2013 (Engineering) 6 5. FIGURE 4 shows an amplifier circuit. operation and performance of the circuit. should: Write a short report on the In completing the report you · Explain the operation of the circuit and in particular the role of resistors R1 and R2. · Build the circuit in PSpice and use it to determine: (i) the quiescent value of Vout. (ii) the voltage gain for a 100 mV, 1 kHz , input signal. · Sketch the small-signal equivalent circuit of the amplifier and use it to estimate the voltage gain. Compare your answer with that of (ii) above. · Attempt to calculate the quiescent value of Vout. Compare your answer with that given by the PSpice model. Try to explain any discrepancies. [Hint : Apply the appropriate equation (1 or 2) of Lesson 4.] 1.5k R3 J2N3819 Vout Vin 12 V1 10 Meg 100 R2 R1 FIG. 4 Teesside University Open Learning © Teesside University 2013 (Engineering) 7 N.B The parameters used by Simetrix for the transistor model can be obtained in the Schematic Command window. Use the function key F11 to reveal this window. But first import the description of the model by selecting from the menu bar; Simulator/Import Models…/Import direct copy. Note also that VP = VTO and IDSS = β × VP2 . ________________________________________________________________________________________ COMPONENT DATA ________________________________________________________________________________________ BFY51 ________________________________________________________________________________________ ELECTRICAL CHARACTERISTICS BC 109 ________________________________________________________________________________________ Teesside University Open Learning © Teesside University 2013 (Engineering) 8 2N3819 ________________________________________________________________________________________ Teesside University Open Learning © Teesside University 2013 (Engineering) 1. With reference to the block diagram of FIGURE 1, state the two conditions that must be satisfied to give an oscillatory output. si – Hso si – G so Hso H FIG. 1 2. With reference to the block diagram of FIGURE 1, determine the required value of G to give an oscillatory output if H = –10 dB. 3. FIGURE 2 shows a public address system. (a) It is found that if the microphone is brought into proximity of the loudspeaker, the systems will ‘howl’. Carefully explain, making reference to feedback theory why this is so. (b) Suggest two actions that could be adopted to remedy the ‘howling’. (c) Measurements show that for a particular arrangement of the equipment and at a particular amplifier setting, the system will howl if 1% of the output power is fed back to the microphone. Estimate the power gain of the P.A. amplifier in decibels. Teesside University Open Learning © Teesside University 2013 (Engineering) 4 FIG. 2 4. FIGURE 3(a) shows the circuit of an Armstrong oscillator (named after its inventor, the American engineer Edwin Armstrong in 1912). Here a transformer is used to couple the output to the input to give feedback. The transformer has a turns ratio of n:1, where n represents the primary winding. In this particular circuit the transistor’s emitter resistor is bypassed by a large capacitor at a.c. frequencies and its base is biased via the transformer windings. FIGURE 3(b) represents the a.c. equivalent circuit of the oscillator and (c) its h-parameter equivalent circuit. (a) Explain the significance of the transformer’s dot notation in relation to the operation of the oscillator. Teesside University Open Learning © Teesside University 2013 (Engineering) 5 (b) It can be shown that the loop-gain of the oscillator at resonance is given by: G 1 hfe R′  n Vloop gain h L ie where RL′ is the effective resistive load on the transistor, i.e.: RL′  RL //hoe // n 2  R1 //R2  Estimate the required value of turns-ratio if: R1 = 4.7 kΩ, R2 = 24 kΩ, RL = 2.7 kΩ, hfe = 250, hoe = 10–5 S, hie = 4 kΩ +VCC R2 CL RL L 1:n Vo 1:n Vi Vo Vi C1 R1 RE CE (b) (a) FIGS. 3(a) and 3(b) Teesside University Open Learning © Teesside University 2013 (Engineering) 6 RL CL ib n:1 R1//R2 Cin hie Vin hoe Co Vo L hfe i b (c) FIG. 3(c) 5. FIGURE 4 shows another variation the Armstrong oscillator. A transformer with two secondary windings has been use, one to give feedback and one to give the oscillator’s output. Write a short report [two to three pages] on an investigation into the operation and performance of this circuit1. The report should embrace, as far as you are able, the following themes: (a) Why the output is taken via the transformer rather than directly off the collector of the transistor. (b) The agreement between the measured and calculated quiescent voltages on the three terminals of the transistor. ________________________________________________________________________________________ 1The circuit model is available in the module’s Learning Materials on Blackboard. In this simulation two extra components, C5/R5, have been added to the left of C2. The added capacitor carries a small initial voltage to act as the necessary noise required to ‘kick start’ the oscillator. The default run time is from 100 to 102 milliseconds. The transformer has been formed from three mutually coupled inductors, rather than using the transformer model. This has been done because the parameter ‘inductance’ can be swept for an inductor in an a.c. analysis. This facility is not available in the transformer model. Teesside University Open Learning © Teesside University 2013 (Engineering) 7 (c) The agreement between the measured and calculated frequency of oscillation. (d) The shape of the output waveform in the first 10 milliseconds of start-up. (e) The given L1:L2 ratio is not necessarily the optimum value to give a good sinusoidal output. [A Fourier probe on the output will give a spectral response]. Try to devise an experiment to find the optimum ratio*. Express the ratio as a turns ratio. The report should include copies of any graphical responses produced in the investigation. *This can be done by performing an AC sweep on the inductance parameter L2. However a sweep cannot be performed without a voltage source in the circuit. For the purposes of this analysis a small voltage source can be inserted into the feedback loop as shown in the second version of the circuit on Blackboard. Teesside University Open Learning © Teesside University 2013 (Engineering) 8 2.7k 18k R1 10u Probe2-NODE R3 Q2 C3 10m C2 BC 109 L3 C4 L1 5 1u 1 10m V1 1m 8.2k 1k C1 L2 R4 R2 10u FIG. 4 Teesside University Open Learning © Teesside University 2013 (Engineering)