Pass DIY Op-Amps

[prh]

For something different, I thought I'd try out some ideas presented in Nelson Pass's DIY Op-Amp article from the Pass DIY site. http://www.passdiy.com/pdf/diyopamp.pdf

A couple of questions from a novice:

1) Devices for the diffential pair. A 2SK389 dual JFET is used in one circuit variation. I have a couple of LS843 low noise dual JFETs, but these have way less transconductance than the 398 (about 1.5mS). Can I use the LS843?

2) Further gain is provided by a P-channel MOSFET in common source configuration. The MOSFET is biased at about 3 volts below the +ve rail. If the MOSFET turns off when the gate to source voltage drops below 3 volts, how is all of the input signal passed? (Does this question make sense? I thought you needed some "headroom" above Vgs to handle input + and - voltage swing.)

Thanks for any enlightenment!

Paul.

 

[uli]

For something different, I thought I'd try out some ideas presented in Nelson Pass's DIY Op-Amp article from the Pass DIY site. http://www.passdiy.com/pdf/diyopamp.pdf

A couple of questions from a novice:

1) Devices for the diffential pair. A 2SK389 dual JFET is used in one circuit variation. I have a couple of LS843 low noise dual JFETs, but these have way less transconductance than the 398 (about 1.5mS). Can I use the LS843?

Hi Paul,
Of course you can, but the load impedance of the LS843 should
be 22/1.5 = ~15 times higher than for the K389 to achieve
about the same open loop gain.

Uli

 

[Nelson Pass]

1) Devices for the diffential pair. A 2SK389 dual JFET is used in one circuit variation. I have a couple of LS843 low noise dual JFETs, but these have way less transconductance than the 398 (about 1.5mS). Can I use the LS843?

2) Further gain is provided by a P-channel MOSFET in common source configuration. The MOSFET is biased at about 3 volts below the +ve rail. If the MOSFET turns off when the gate to source voltage drops below 3 volts, how is all of the input signal passed? (Does this question make sense? I thought you needed some "headroom" above Vgs to handle input + and - voltage swing.)

1) If the LS843 can run the same Drain to Source current,
you can plug it right in, otherwise you have to adjust the
current fed to the diff pair to put it into the desired range.

2) The 3 to 5 volt bias Gate to Source on the Mosfet puts it
into the "middle" of its operating curve. In operation, this
gate voltage will vary from 0 to maybe twice the quiescent
value. All the headroom you will ever need will be found in
a small range about the quiescent value.