If I put my notes here, I might be able to find them again later!
RJM Audio discrete op amp (circuit schematic)
This is my first op amp design which doesn't completely suck.
Now, it's a terrible op amp... don't misunderstand... (Nat Semi will not be making me any offers)... but it does perform the job I want it to do reasonably well: remain stable while providing 6-20 dB of line-level voltage amplification with low distortion, decent PSRR, and sufficient bandwidth.
The main limitation is the distortion at high frequencies rises to -70 dB. The circuit needs less open loop distortion, or more open loop gain above 10 kHz, or both.
You will note the circuit has no current sources. This is intentional. I wanted to see how far it was possible to get without them. Obviously headroom takes a big hit, but distortion and PSRR ended up better than I imagined.
This is a simulation. No guarantee it will work, and there are no safeties (current limiters, input voltage clamps, etc) shown.
PS. Frequency response in image is open loop, while circuit is shown configured closed loop, resulting in the FFT shown.
PPS. Need higher voltage rated transistors than BC327/337 shown, especially for Q5,10. Originally I had it running at 15 V, but increased it to 24 V to improve headroom ... omitting to change the transistors since transistors don't blow in simulation.
Now, it's a terrible op amp... don't misunderstand... (Nat Semi will not be making me any offers)... but it does perform the job I want it to do reasonably well: remain stable while providing 6-20 dB of line-level voltage amplification with low distortion, decent PSRR, and sufficient bandwidth.
The main limitation is the distortion at high frequencies rises to -70 dB. The circuit needs less open loop distortion, or more open loop gain above 10 kHz, or both.
You will note the circuit has no current sources. This is intentional. I wanted to see how far it was possible to get without them. Obviously headroom takes a big hit, but distortion and PSRR ended up better than I imagined.
This is a simulation. No guarantee it will work, and there are no safeties (current limiters, input voltage clamps, etc) shown.
PS. Frequency response in image is open loop, while circuit is shown configured closed loop, resulting in the FFT shown.
PPS. Need higher voltage rated transistors than BC327/337 shown, especially for Q5,10. Originally I had it running at 15 V, but increased it to 24 V to improve headroom ... omitting to change the transistors since transistors don't blow in simulation.
Total Comments 2
Comments
-
It's a very simple design.
Please take a view at this design
(AK) classA HA/Preamplifier. | Rock Grotto
AKSCMODSHAtest-v-1_zps2008161d.jpg Photo by drymdrum | Photobucket
or here
(AK) classA HA/Preamplifier. | Rock Grotto
Posted 8th August 2016 at 05:15 AM by FritzS
Updated 8th August 2016 at 05:31 AM by FritzS -
Thanks. Nice diagram, I appreciate having all the voltages written on there. Yes, I expect if I put the current sources back in to my circuit I will end up with something very similar. The main difference in the topology is just that my version has a follower/buffer before the VAS.Posted 8th August 2016 at 05:35 AM by rjm
