An amplifier supplied from high voltage rails can be have an input stage using low voltage rails and still be simple and stable. This seems a contradiction as low voltage input stages seem to be difficult to interface with a high voltage VAS. There are various ways which implement this interfacing, but none is simple and easy to successfully tame and simulate without stability issues. Yet, there is a straightforward and simple solution, which using only one component to do all the required interfacing, the resultant amplifier is still stable! Yes, stable, just like a three stage amplifier using high voltage transistors for its input stage. As if this is not enough, the interfacing transistor need not support double the high rail voltage but only one high rail voltage. The interfacing transistor connection mode is common-base and is connected with its emitter to the output point of the non-inverting input transistor, the base to the low voltage rails at which the input stage's current mirror is implemented and the collector connected to the base of the VAS.
Issues:
One issue that I found was that the amplifier has a too wide bandwidth which extends to several megahertz. I did not succeed to correct this without causing the distortion to increase drastically.
I am attaching the schematic. Mind you, my post is intended only as a suggestion for those who are after a simple solution for their low voltage input stage.
Warning:
Be aware this amplifier was NOT built and tested in reality, but only simulated.
Issues:
One issue that I found was that the amplifier has a too wide bandwidth which extends to several megahertz. I did not succeed to correct this without causing the distortion to increase drastically.
I am attaching the schematic. Mind you, my post is intended only as a suggestion for those who are after a simple solution for their low voltage input stage.
Warning:
Be aware this amplifier was NOT built and tested in reality, but only simulated.
Attachments
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You can reduce the bw inserting some resistance in the base lead and wiring a little cap between collector and base. Or a rc filter in the collector to ground.
The input stage complete with the current source is supplied +15V and -15V DC. The 0V is at the same potential of the rest of the amplifier. This means, inverting the input would require the same solution or another more complex solution. A resistor alone cannot work, as it will have a too high DC component, which has to be blocked, and this adds complexity and instability.
During my journey designing my amplifier I needed such simplicity, but I could not find it. The "solutions" were too complex and resulted into terrible instability. The irony is, a BJT in common-base mode, solves the issue simply and effectively. What I was replied to was that a low voltage input stage necessarily forces the use of more complex interfacing stages.
Nevertheless, I heartily share this for anyone who wants to explore it and use it. After all, this is the spirit of these fora.
Post Scriptum:
Thanks for not using this solution and using a high voltage input stage, I can run my amplifier at very low rail voltages saving a lot of energy consumption. A low voltage has the advantage of less stress on power components and a lesser risk of fire.
During my journey designing my amplifier I needed such simplicity, but I could not find it. The "solutions" were too complex and resulted into terrible instability. The irony is, a BJT in common-base mode, solves the issue simply and effectively. What I was replied to was that a low voltage input stage necessarily forces the use of more complex interfacing stages.
Nevertheless, I heartily share this for anyone who wants to explore it and use it. After all, this is the spirit of these fora.
Post Scriptum:
Thanks for not using this solution and using a high voltage input stage, I can run my amplifier at very low rail voltages saving a lot of energy consumption. A low voltage has the advantage of less stress on power components and a lesser risk of fire.
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I think it was a Advance Acoustics amp I repaired that used Zeners to drop the voltages so lower voltage transistors could be used in the input stage.
From memory it was basically a standard LTP with CCS, and the CCS had a Zener to get a high voltage drop over it, and the 'legs' of the LTP had Zeners too if I remember correctly.
Not really 'low voltage rails' as you describe it, but low voltage transistors used with high rail voltages.
I guess the input stage by AKSA could be used on lower voltage rails too, feeding a higher voltage final stage (but I guess it could/should be defined as input stage + VAS combined)?
Alpha Nirvana 39w 8ohm Class A Amp
Sorry if I posted dumb suggestions..
From memory it was basically a standard LTP with CCS, and the CCS had a Zener to get a high voltage drop over it, and the 'legs' of the LTP had Zeners too if I remember correctly.
Not really 'low voltage rails' as you describe it, but low voltage transistors used with high rail voltages.
I guess the input stage by AKSA could be used on lower voltage rails too, feeding a higher voltage final stage (but I guess it could/should be defined as input stage + VAS combined)?
Alpha Nirvana 39w 8ohm Class A Amp
Sorry if I posted dumb suggestions..