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Old 5th May 2004, 03:43 PM   #11
jcx is offline jcx  United States
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complicated question there Jan,

I think your concept of fewer stages is rooted in the idea that each stage adds phase shift and the fewer the better – but a question is whether the added stages can be much faster than the output device, if so extra stages don’t add significant phase shift and it should be possible to make good use of the extra gain (at low frequencies)

Common audio power amp practice however frequently uses VAS/Driver Qs only a little faster than the best output Qs since unity gain outputs require full amp voltage swing and high voltage transistors are naturally slower, Ed Cherry recommends CE outputs with large V gain and then the rest of the amp can operate at low voltages with very fast low voltage small signal Qs (or even op amps, today’s better op amps are faster than many common small signal Qs)

There are many tradeoffs in multistage amplifiers, especially with respect to stability and recovery transients in the face of clipping/slew/current limiting those limits, and typical transistor speeds probably leave us with the present loose rules that you can observe in current amplifier practice: 3 stages of amplification is the default, 1 or 2 for high frequency response(at reduced accuracy), 4 for low frequency precision

(all depending on how you count such tight local feedback stages such as a cascodes or triple CF or Darlington outputs)

squadra,

the gain margin is how much the open loop amplifier gain can increase (assuming the phase shift doesn’t change) before the amplifier will become an oscillator

the gain may be larger than the designed gain from component tolerance or for example h_fe rise with temp, or gm/ft changing with operating current

keeping feedback relations straight is tough, but lower closed loop gain means less feedback network attenuation which gives higher loop gain (loop transmission) and the closed loop gain intercept with the open loop gain curve moves higher in frequency, closer to the point where open loop phase shift exceeds 180 degrees and reducing gain margin (if could explain this clearly in a paragraph I should be a EE prof, not just an engineer – Gerald Graeme’s op amp books have an interesting graphical feedback analysis approach based on the Bode plots, perhaps you can find some of his old articles online for a slightly different view than the typical textbook)

If you aren’t pushing the envelope in this project then adjusting the dominant pole (usually Miller Cap across the VAS) should let you use lower closed loop gain with little difficulty
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