Discrete Opamp Open Design

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bcarso

diyAudio Member RIP
Joined 2005
scott wurcer said:
How about CPC3730 from C P Clare? With that and some 140V transistors this could be the front end of a 100W amp.
Click to expand...

Ahaa, didn't realize Clare sold individual parts, a logical move though, based on the optos stuff. That's reassuring that we have at least two manufacturers of depletion mode DMOS.

EDIT: Also the geometric mean of the threshold voltages listed is 2.5V, a bit higher than the other parts.
 
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scott wurcer

scott wurcer

R.I.P
Joined 2004
bcarso said:
Ahaa, didn't realize Clare sold individual parts, a logical move though, based on the optos stuff. That's reassuring that we have at least two manufacturers of depletion mode DMOS.

EDIT: Also the geometric mean of the threshold voltages listed is 2.5V, a bit higher than the other parts.
Click to expand...

Good point, BTW Infineon has some too, SOT23 and very low capacitances.
 
K

kgrlee

Member
Joined 2011
scott wurcer said:
Rather than answer questions piecemeal how about I put a device by device description in the articles section, this is a holiday weekend I might have some time.
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Scott, when you do this for #1397, could you also explain the reasons for various choices? I would much like to know the reasons for
  • the fancy output stage compared to simple Class B as in 990
  • why FET i/p instead of simple BJT in your rehash of 990
 
fas42

fas42

Banned
Joined 2012
Just tried something else in my version of SW-OPA, looking at PSRR. At 20kHz, only a bit over 15dB on the -ve supply, 30dB on the +ve when working at a gain of 20. So an extremely well regulated supply will be necessary for the circuit as is ...

Frank
 
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F

fitzfish

Member
Joined 2010
Frank,

dadod's version sims at about ~-25db at 20KHz on the negative side with your 20K/2K feedback values. Switching to K170/J74 models for the front end @ 7mA, everything else otherwise the same, brought it up around ~-31db at 20KHz. Low frequency is about -89db for the LSK389/J109 with the knee at about 65Hz. The K170/J74 was in the -96db range with the knee around 70Hz. High side PSRR is better.

All based on the LTSpice sim of course… Anybody else get similar numbers from it? It would be nice to know it works the same in multiple locations, even if just in the sim at this point.

Dave
 
scott wurcer

scott wurcer

R.I.P
Joined 2004
fas42 said:
And of further interest, a member PM'ed me about a breadboard he's already knocked up, and his PSRR figures, measured at lower frequencies, correlate beautifully with what the sim predicts ...

Frank
Click to expand...

PSRR is something to look at and there could be some tweaks. I would expect non-cascoded inputs to be fairly sensitive to source impedance mis-match. Welcome to the design process.

EDIT- OK we need to track down the discrepancy, the very first circuit (all N flavor FET's but input cascoded) shows me flat -100dB RTI on both supplies out to 20K.
 
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S

stinius

Disabled Account
Joined 2008
PSRR

This is what I'm getting when simulating Scott's BF862 circuit with the buffer.
I have also attached the schematic I'm using, showing how I do the PSRR simulation. Maybe I'm doing it wrong?

Stein
 

Attachments

  • NEG PSRR.pdf
    35.8 KB · Views: 109
  • POS PSRR.pdf
    37.8 KB · Views: 92
  • SW BF862 with buffer.pdf
    34.6 KB · Views: 187
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