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UW-Madison PHYSICS 623 - Single Transistor Amplifier

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Single Transistor Amplifier Lab Worksheet Note: Work out the exercises on this sheet before the lab. It must be turned in when you arrive for the lab and will be graded. If you have any problems with these exercises, please [see, call, email] the instructor well in advance of the lab. A. Read sections 2.01 and 2.02 in the text. We'll need the first two transistor rules, which can be summarized as follows: B. Figure out the operating ("Q") point of the circuit in the lab writeup. Get the data sheet for the 2N1480 transistor as well as the lab writeup from the web site. Outline of procedure for doing this: 1. Assume that the "If's" in the rules are satisfied. You should check this at the end. 2. For the first cut, assume that  is very large, and therefore from 2nd rule, IB = IC/ 0. 3. With no current into the base, the bias circuit of R1 and R2 is a simple unloaded voltage divider, so you can find VB (single subscripts on V's mean with respect to ground). 4. Use VB and rule 1) to find VE. 5. VE is the voltage across RE, so you can find the current through it, which is IE. 6. Kierkoff's laws (just conservation of charge), require that the current into the collector plus the current into the base equals the current out of the emitter. Since IB 0, IC = IE. 7. Use IC to find the voltage drop across RC. Subtract this from VCC (the power supply voltage at the top of RC) to get VC (with respect to ground). 8. The difference VC – VE  VCE. Calculate this and check the "If" in rule 2. Golden Rules for Transistors 1) If the transistor is not "cut off" (i.e., if IC > 0), then VBE ~ 0.6 V. 2) If the transistor is not "saturated" (i.e., if VCE > 0 — really > ~0.2 V), then IC = IB. (usually  >> 1)C. Now calculate the 1st-order correction for finite : 1. Use your first-cut value for IC to calculate IB for a  of say, 30. (IB = IC/, from rule 2.) 2. Make a Thevenin equivalent circuit of the bias divider: VTH = _____________ RTH = _____________ 3. Find VB. (VB = VTH – IB RTH, from ohm's law.) 4. Repeat 4 and 5 from part B to get VE and IE. 5. IC = IE – IB from conservation of charge. 6. Again find VC as in B7. 7. You could use the new IC to find a new IB and iterate, or (horrors!) write down the equations and solve simultaneously to get an exact VC. In practice, neither of these is normally necessary! D. Read Appendix F in the text on load lines. E. Figure out what the "Voltage Gain" (AV) of the circuit is: AV VC/VIn. In this circuit, VIn = VB. Remember that ohm's law is linear, and with linear equations, you can solve for just the changes from your operating point. IB RTH


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UW-Madison PHYSICS 623 - Single Transistor Amplifier

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