Download BJTAMP-fre1q-lab

ENGR 311
BJT Amplifiers - Complete Model
The common emitter amplifier is one of the most widely used amplifier configurations due to its
high gain. Other configurations are the common collector and common base amplifiers which
respectively have the collector and base of the transistor grounded, or common to the input and
output AC signals. This exercise will look only at the common emitter amplifier.
Amplifier performance is measured by a number of parameters: gain, bandwidth, distortion, input
impedance, output impedance, noise figure/signal to noise ratio, power dissipation/efficiency, and
phase shift. Many of these parameters are difficult to measure without specialized equipment, so we
will concentrate on only a few of these in this exercise.
Experiment
1. Study the data sheet for the 2N3904.
2. Calculate the DC operating point of the amplifier shown in Figure 1 (use MathCAD).
3. Calculate the AC small signal gain (vo/vs instead of transducer gain) of the amplifier (setup
equations and use MathCAD). Make the simplifying assumption that all capacitors are short circuits
at the frequency of interest. Keep in mind that, although these are the usual assumptions made for
hand calculations, the value calculated is commonly referred to as the mid-band gain.
4. Use PSpice to obtain a plot of the gain magnitude of the circuit as a function of frequency.
Repeat the simulation with the emitter capacitor set at 1μF and again with the emitter capacitor
removed.
5. Set vin at 0.5v and take gain magnitude measurements at three frequencies: a) somewhere in the
middle of the band, b) at the lower 3 dB point (-3dB with respect to your first measurement), and c)
at the upper 3dB point. You will have to use a function generator, a frequency counter and an
oscilloscope. Be sure to use scope probes and verify the compensation of the probes before taking
measurements. The cutoff frequencies should occur at approximately 1KHz and 1MHz.
6. Remove the emitter capacitor and qualitatively observe the effects on midband gain and
bandwidth. Can you explain the observed changes?
7. Replace the emitter capacitor with a 1μF capacitor. Measure the new lower cutoff frequency.
Can you explain this result?
8. Use a frequency somewhere in the midband. Measure the AC voltage at the base of the transistor
and the voltage drop across the 10kΩ resistor connected to the input (the AC current). From this
data, calculate the small signal input resistance of the amplifier for inclusion in your report.
9. Remove the 1kΩ load resistor and measure the output voltage. Now attach a load resistor of
about 300Ω (any low value of resistance is fine as long as its exact value is known). Measure the
new output voltage. From the change in voltage and the known value of RL, calculate the small
signal output resistance.
10. Report all your finding and observations. Report due in two weeks.