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Preamplifier Stage

The preamp is usually the voltage amplifier part of the audio amp. Since the maximum output voltage from the CD player is about 0.5V, we will require our preamp to have a maximum voltage gain of approximately 20 for all frequencies over the audio range. The Common Emitter amps from Lab 2 are probably not good choices for achieving this gain because they are capacitively or AC coupled, and thus amplify lower frequencies less than higher frequencies in the audio range. (Typically sized coupling capacitors block signals at low range audio frequencies.)

From Lab 1, we know that such a gain is attainable using an op-amp circuit for two main reasons:

(1) Op-amp circuits can be DC coupled thereby allowing us to amplify low frequency signals as well as high frequency signals.

(2) Op-amp circuits are connected with feedback that ensures a flat frequency response over the audio range. Also the use of feedback also reduces distortion.

The preamp stage is also where you would probably want to include volume and tone controls. Volume control can be achieved either by varying the gain of the preamp stage, or by following the preamp with an adjustable voltage divider which will attenuate the signal before it reaches the power stage. 741 op-amps are a reasonable choice for designing the preamp.


next up previous contents
Next: Power Amplifier Stage Up: Design Considerations Previous: Complementary Symmetry Power Amplifiers
Neil Goldsman
10/23/1998