High midband gain amp stabilizes DC operating point through feedback

202

01:26:54

Sometimes it's more fun to design with discrete components. This amplifier uses series shunt  feedback for DC operating point stabilization, but  is open loop for  the midband. More gain can be achieved via additional  amplifier stages in the forward loop. This method minimizes the instability issues associated with high gain feedback amplifiers. The 100uf cap essentially acts as a short to ground at midband freqs, thereby, breaking the closed loop.  Addional midband gain has been achieved through the use of the 10 uf / 1k combo at the emitter of thr first xstr.  The output LED driver stage was added as part of an optical audio link and is not part of the basic feedback amplifier. The circuit was designed, built, and tested years ago and works. The orginal had a jfet buffer, but the simulator does not have a jfet. To get the Bode plot, transient waveform, and operating point info, set up the AC source appropriately. Refer to one of my earlier submissions as a guide. This configuration has been setup to initially display op data on the schematic.

published 8 years ago

aquif1990

8 years ago

Sorry kind of noob question, why you not used coupling capacitor beteeen the transistor stages.

leed124

8 years ago

1) use of coupling caps requires that each stage will need its own individual dc biasing config. 2) Additional DC biasing parts will be required for each stage. 3) The lower the freq of operation, the larger the cap vals required. Special attention must be given to the time constants created by the interaction with the source and load impedances seen by the caps. 4) The advantage of utilizing readonable preditable xstr parameters as a reference for biasing other stages is lost. See the last stage. The Vbe (approx 0.7v at rm temp) is used to generate a preditable current through the 680 ohm resister. 5) Feed back advantages at DC are lost. 6) If you get the opportunity, design with dc coupling in mind.

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