Choose JFET for Audio Amp Design: J112 vs J113

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Discussion Overview

The discussion revolves around selecting between JFET models J112 and J113 for audio amplifier design. Participants explore the implications of different specifications, such as Idss and input capacitance, and consider the impact of these parameters on amplifier performance. The conversation includes technical details related to circuit design and biasing adjustments.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks guidance on choosing between J112 and J113 for an audio amplifier design, noting a previous design using BJT with a collector current of 1.2mA.
  • Another participant suggests that J111 may be better than J112 due to a higher Idss, but notes that this is conditional on the specific needs of the design.
  • There is a discussion about the need for biasing adjustments when substituting JFETs into existing designs.
  • One participant mentions their experience with JFETs being limited and expresses uncertainty about their reliability in designs.
  • Another participant shares their intention to test J111, J112, and J113 to compare performance in their amplifier setup.
  • A suggestion is made to consider dual matched, low noise pairs like the LSK489 series for potentially better performance.

Areas of Agreement / Disagreement

Participants express differing views on the suitability of J112 versus J113, with no consensus reached on which model is definitively better for the application. The discussion remains unresolved regarding the optimal choice of JFETs for audio amplification.

Contextual Notes

Participants mention the need for biasing and compensation adjustments, indicating that these factors are critical but not fully resolved in the context of the discussion. There are also references to personal experiences with JFETs that may influence their perspectives.

Who May Find This Useful

Individuals interested in audio amplifier design, particularly those exploring the use of JFETs versus BJTs, may find this discussion relevant.

yungman
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I am designing audio amplifiers, I am not familiar with JFETs, how do I choose between J112 and J113

https://www.digikey.com/products/en/discrete-semiconductor-products/transistors-jfets/288?k=jfet&k=&pkeyword=jfet&sv=1&pv1183=u35V&sf=0&FV=1140050%2Cffe00120%2C9780001&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25

My original design using BJT is running 1.2mA collector current, which one will fit better?

Also if you get the datasheet of J111, from Fig.5 in the data sheet on input capacitance is lower the more negative the Vgs, meaning choosing J111 is better than J112 as Idss is higher with J111. Is that true?

Thanks
 
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yungman said:
J111 is better than J112 as Idss is higher with J111. Is that true?

It is capable of switching a higher current. It is only "better" if you need that parameter to be larger. If you need it to switch with a lower gate voltage than it is "worse" than the others.

yungman said:
which one will fit better?

Can you share a schematic?

BoB
 
rbelli1 said:
It is capable of switching a higher current. It is only "better" if you need that parameter to be larger. If you need it to switch with a lower gate voltage than it is "worse" than the others.
Can you share a schematic?

BoB
This is the typical "Blameless" front end, I am just going to substitute Q48 and Q49 with jFET to see what is the difference in the sound.
Blameless JFET.JPG


I just change R90 and R91 to very low value like 5.1ohm to get the gain back. Q48 and Q49 are matched and wrapped together by copper tape to track temperature to reduce offset drift.
 
Last edited:
Your biasing will need to be adjusted.

@berkeman can you help here?

Thanks

BoB
 
rbelli1 said:
Your biasing will need to be adjusted.

@berkeman can you help here?

Thanks

BoB
Yes, bias, compensation all needed to be adjusted. At this point, I just want to pick the best fit. Most likely I will increase the current to about 2mA through each of the JFET.
 
rbelli1 said:
Your biasing will need to be adjusted.

@berkeman can you help here?
Sorry, I've only used JFETs for one product design, and had so-so luck with them. I haven't used them since.
 
berkeman said:
Sorry, I've only used JFETs for one product design, and had so-so luck with them. I haven't used them since.
I know, JFET is dying...if not dead already. Only place I've seen using is the first stage of audio amplifiers. I tested opamps with JFET input differential pair vs BJT, it seems to sound a little livelier. I've seen some high end audio amps using JFET as front end also, I just want to try it as I already design the pcb, it's easy to just replace two transistors and listen to it.

Anyway, I ordered all J111, 112 and 113. I just going to try it.
 
Did you ever get it working?

BoB
 
You might want to consider dual matched, very low noise pairs like the LSK489 series.
 
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