The objective is to drive 1 watt into the speaker. R1 represents the output impedance of the previous common collector stage.
Here's what I think I know:
- 1 watt into 4 ohms means 2V RMS or 2.8V peak.
- If the beta of Q1 is something like 70 then the input impedance of the emitter follower is something like 700–pretty low.
- In the quiescent state C1 is charged and the current through the speaker is zero.
- When the voltage rises to +2.8V, 700mA (2.8V/4 ohms) goes to SPKR1. (Confused about whether it all goes to SPKR1, seems like some must go through R2.)
- When the voltage falls to -2.8V, 700mA gets pulled back through SPKR1 and must be sunk through R2.
The problem in a sentence is that R2 is too high to sink the current coming back from SPKR1 when the voltage is low. If the emitter voltage is about 6V then when it swings -2.8V to 3.2V, R2 will sink only 320mV. So the returning current causes the emitter voltage to rise, turning off Q1 and creating a flat spot at the bottom of the waveform.
If I decrease R2, then the input impedance drops and it's hard to maintain the voltage swing. Also, the quiescent current increases and I start needing a big heat sink for Q1.
My question is: is there a process for determining a value for R2 that can meet the objective of driving 1 watt into the speaker, or is this just a waste of time?