simplest amplifier possible with BJT's?

[millwood]

the high open loop gain (750x in my simulation) does need to be address. a few thoughts:

1) lower the collector resistor on the input stage;
2) lower the collector and bootstrap resistors on the phase splitter stage. Be careful however as you may accidentally increase the bias on theoutput stage;
3) use degenerative resistors throughout the amp.
4) use mosfets or jfets as they have lower gains.
5) get rid of the bootstrap on the phase splitter.
6) use a power resistor in serial with the load.

adding attenuation isn't bad idea as long as it isn't too much.

I think it can be made to work. Just how one wants it to be done.

[pjacobi]

...as it has whopping 5 BJTs and NFB:

http://www.linearaudio.de/scratch/new-se-1b-lp.pdf

Gives 3W @ 6Ohm from +-6.5 Supply,
Iq=1.1A, simulated THD 0.025% @1.5W

So, NFB is good for you, isn't it?

Best Regards,
Peter Jacobi

[AudioGeek]

hmm... i like this thread.

any more ideas on simple BJT amp?

How about a cascode BJT front-end driving darlington emitter followers?

take some feedback back to the BJT frontend.

looking around 5-10W of power. Run from 12v SLA batteries.

something similar to the AR-3 headphone amp..

[Easyamp]

I have designed and built this amp as a learning step for some friends that have shown interest in building there own amps.


Its design is simple and cheap and works quite well. If class A use is needed then a high bias current and larger heat sink could be used to cover the output goal by changing the current of the vas. If not the amp puts out roughly 5 watts peak in AB mode.

There are only 2 different resistor values used.

[AudioGeek]

A simple BJT gain stage... say a MPSA18 with a 1.2K load from a +24v supply. This DC coupled to a darlington 2N3055 running at 3A or about there.

Can I use the output stage load.. i.e. the 3A through a 4 ohm emitter load to tap the current for the input stage? kind of a feedback?

so instead of the input stage emitter to ground... run it through the output emitter resistor...

dunno if this would work...

maybe this is a type of feedforward?

perhaps If I use JFETS in the input stage it would work.. hmmm...

[Fuling]

Here´s something I designed and prototyped some years ago back in school. Nothing fancy at all and I guess it could be improved in all eternity, but it sounded good as it was.
Since it´s SE class A it requires alot of heatsinks and PSU.

[thoriated]

I've never formally drawn it out, but I had an idea for a very simple 2 stage all FET amplifier several years ago consisting of quasi 'common source' complementary JFET inputs with their gates both driven by the input signal directly driving common source complementary MOSFET outputs with the signal & bias stabilization feedback being taken back to the JFET sources. The lowish breakdown voltages of available jfets probably can be enhanced with a capacitor bypassed zener & small resistor to realize an amplifier up to the 1-200 watt range with only 'four' topologically required active devices (although more input stage transconductance may be gotten, if needed by paralleling input JFETs, & output devices parallelable as needed also for higher currents). Output stage biasing may be achievable with local DC loops with common AC feedback. DC signal coupling may also be feasible with this approach.

[AudioGeek]

Thats an interesting schematic...

How do I make the input stage more linear??? Do I just increase the supply voltage and increase the load resistor on the input stage?

How do I take feedback from the output back to the input transistor on this circuit?

Is it easier to use a JFET input?

What kind of power and distortion levels did you get out of that circuit?

and more importantly, how did it sound?

Would it be "better" to cascode the input stage?

Thanks!!!

[Fuling]

AudioGeek:

Whoa, that´s alot of questions!!

I guess higher supply voltage and load resistance might work, yes. The input stage employs local current feedback (no decoupling cap at the emitter resistor) so I guess it´s quite linear already.
BTW: If you change the ratio between the two specified resistors (2,7k and 120R) you also change the gain.

If you want global voltage feeback I guess you can put a resistor between the output and the emitter of the BD165. Something in the 1-2k range maybe. Haven´t tried it since I like the absence of (global) negative feedback.

No idea about JFET´s or cascoding. I remember that I used an BJT input buffer sometime to increase the input impedance.
Just a BC546C and some resistors.

How did it sound?? Well, to quote one of my classmates:
"Wow, now I can hear what the bassguitar plays!"
I got about 10W @ 8 ohms.

[AudioGeek]

how do I change the circuit for NO local feedback of the input BJT but global feedback from output for gain of 10?

doesn't the TIP42 have too much base current for the input transistor??? Or is it a darlington?

Thanks!!!