Uno en clase A con dos etapas:
[This discrete operational amplifier consists of a differential input amplifier (build with a 2SK389 dual JFET - Q1) with a current source (Q2) and a single ended class A output stage with a pnp bipolar transistor (in my design a ZTX550 - Q5) also with an active current source (Q4). The power supply voltage is, for normal operational amplifiers unusual, ±25V – unlike to the design of moamps who operates with the normal ±15V. The npn transistor Q3 make sure that both current source transistors sees a constant voltage at the basis (constant base-emitter voltage ≈0.65V).]
http://www.rstaudio.de/FreqWeiche/DiskreterOP.pdfReplacement for Dual-JFET 2SK389 and 2SJ109
[The Toshiba Dual-JFETs 2SK389 (n-channel) and 2SJ109 (p-channel) are no longer in production. Nevertheless specially these types are in use in nearly every relatively new design from Nelson Pass. Fortunately the corresponding single JFET's are still in production and therefore it is easy to replace the Dual-JFET's. To do that the single transistors must be matched and thermally coupled. In a new design it is also possible to use the SMD replacement for the Dual-JFET.]
Original Replacement Comment
n-Channel 2SK389 - 2× 2SK170 identical single JFET
2× 2SK370 replacement for 2SK170
2SK2145 Dual-JFET, SMD, both sources internally connected
p-Kanal 2SJ109 - 2× 2SJ74 identical single JFET
2× 2SJ108 replacement for 2SJ74
Saludos, Raúl
