Brandeis University | Physics 29a |

Spring 2019 | Kevan Hashemi |

**Part 1:** Fetch a 2N3904 transistor. Its data sheet is on the 29A page. Build the circuit shown below, with R1 = 1.0 MΩ. Measure *I _{C}* with an ammeter. Measure

Plot *I _{C}* versus

**Part 2:** Build the *grounded-emitter* amplifier circuit shown below. Use R1 = 1 MΩ and C1 = 100 nF. Choose R2 so that *Y* is around 5 V.

Produce a 10 mVpp (ten millivolts peak to peak) sinusoid of 10 kHz with your function generator. Apply the 10 mVpp to *X*. What is the amplitude of *Y*? What is the amplifier's gain? Is the gain inverting or non-inverting? Use the transistor equations to calculate the theoretical gain of the circuit. How well do your calculations and measurements agree?

**Part 3:** Estimate the maximum and minimum voltage the grounded-emitter amplifier can generate on *Y*. Increase the amplitude of *X* until the amplitude of *Y* no longer increases. Sketch *Y*. The maximum and minimum values of *Y* are the *saturation* voltages of the amplifier. Do the saturation voltages agree with your estimates?

**Part 4:** The base resistor of a grounded-emitter amplifier must be chosen to match the transistor current gain, β, but β can vary by a factor of two from one lot of 2N3904s to the next. The *common-emitter* amplifier below will work for any β > 50. Build the amplifier with R1 = 100 kΩ, R2 = 10 kΩ, C1 = 100 nF. Choose R4 so that the emitter current is around 1 mA. Choose R3 so that *Y* sits at around 5 V.

Apply a 10-kHz sinusoid of amplitude 100 mVpp to *X*. What is the gain of the amplifier from *X* to *Y*? What is the gain of the amplifier from *X* to *Z*? Calculate theoretical values for these two gains.

**Part 5:** With 10 kHz, 100 mVpp applied to *X* in the common-emitter amplifier, connect 10 μF in series with 330-Ω from *Y* to 0 V. In this way you are applying a *load* of 330 Ω to the 10-kHz signal at *Y*, but you are applying no load to the quiescent voltage at *Y*. Measure the amplitude of the 10-kHz signal at *Y* with and without the load. Apply Thevenin's Equivalence Theorem so as to model the behavior of the 10-kHz signal on *Y* with one 10-kHz voltage source and one series resistor. What is this equivalent series resistance? Move the load to *Z* and perform the same measurements. What is the equivalent series resistance of the 10-kHz signal on *Z*? Why is the source resistance at *Z* so much lower than at *Y*? Remove the load from your amplifier before you move on to the next part of the Lab.

**Part 6:** Place a 10-μF capacitor in parallel with R4 in your common-emitter amplifier. What is the small-signal gain from *X* to *Y* with this addition? Why do you think the capacitor causes such a dramatic increase in gain?