eat the frog

1. Circuits in play

pm1000 eq
Figure 1. Yamaha PM-1000 channel EQ
hf1 mic amp
Figure 2. HF-1 [1]
ce amp
Figure 3. Generic common-emitter amplifier

2. Small-signal analysis algebra

Select one of Figure 1, Figure 2, or Figure 3 for your analysis today and to build tomorrow. Don’t worry! You will analyze, simulate, and build all 4 amplifiers. Perhaps choose the one that looks easiest now while you figure out the techniques.

How to draw a circuit’s small-signal equivalent circuit:

  1. Solve for the DC bias condition: compute collector currents and node voltages.

  2. Find the AC equivalent circuit. (Redraw the circuit.)

  3. Replace each transistor (only one!) with its small-signal model — either the hybrid-π or the T-model.
    Tourbook: §6.4 Small-signal models

  4. Decide what to do with any reactive components. This depends on your frequency range of interest.[2]

    • See day12 for discussion on how to handle the reactive components for these circuits and the simplifications for Figure 1.

    • (Redraw the circuit.)

You now have a linear circuit model with component names and values.

  1. Do linear circuit analysis to find the small-signal performance:

    1. Input impedance Zin

    2. Voltage gain Av0 (the “0” means without a load attached)

    3. Output impedance Zout

All of this analytically, on paper. Then use a circuit simulator to find these values, ensuring that they match closely.

ce amplifier lab3a
Figure 4. Common-emitter amplifier schematic

3. In-class example

Figure 4

Draw a circuit’s small-signal equivalent circuit:

  1. Solve for the DC bias condition: compute collector currents and node voltages.

    • Want \(V_{\mathit{OUT}} = 0\,\mathrm{V}\) (why?)

  2. Find the AC equivalent circuit. (Redraw the circuit)

  3. Replace each transistor (only one!) with its small-signal model. (Redraw the circuit)

  4. Decide what to do with any reactive components. This depends on your frequency range of interest. (Redraw the circuit)

We now have a linear circuit model with component names and values.

1. from Ashhar Farhan VU2ESE source
2. *You* are doing the analysis, so you are apparently _interested_ in the meaning of the results. Therefore, you know or decide the frequencies that interest you.