Choose JFET for Audio Amp Design: J112 vs J113

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In summary: They are very expensive, but very low noise and matched.In summary, the conversation is about choosing between J112 and J113 JFET transistors for an audio amplifier design. The individual is not familiar with JFETs and is seeking advice on which one will fit better. They also mention the possibility of using J111, which has a higher Idss and lower input capacitance when Vgs is more negative. The conversation also touches on biasing and compensation adjustments that may be needed, as well as the use of JFETs in audio amplifiers and the suggestion to consider dual matched, low noise pairs like the LSK489 series.
  • #1
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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  • #2
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
 
  • #3
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:
  • #4
Your biasing will need to be adjusted.

@berkeman can you help here?

Thanks

BoB
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
Did you ever get it working?

BoB
 
  • #9
You might want to consider dual matched, very low noise pairs like the LSK489 series.
 
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FAQ: Choose JFET for Audio Amp Design: J112 vs J113

1. What is the difference between J112 and J113 JFETs?

The main difference between J112 and J113 JFETs is their channel resistance. The J112 has a lower channel resistance, making it more suitable for high-gain applications, while the J113 has a higher channel resistance, making it better for low-gain applications.

2. Which JFET should I choose for an audio amplifier?

It ultimately depends on your specific design requirements. If you need a higher gain, the J112 may be a better choice. However, if you need a lower gain, the J113 may be more suitable. It's important to consider other factors such as input impedance and noise levels when making your decision.

3. Are there any other differences between J112 and J113 JFETs?

Aside from their channel resistance, J112 and J113 JFETs have similar characteristics such as voltage range and maximum current. However, they may have slightly different capacitance values and transconductance values, which can affect their performance in certain applications.

4. Can I use either JFET for both high-gain and low-gain applications?

While the J112 and J113 JFETs have different channel resistances, they can both be used for a variety of applications. It's important to carefully consider your design requirements and test the performance of each JFET in your specific circuit before making a decision.

5. How do I know which JFET is right for my specific audio amplifier design?

The best way to determine which JFET is right for your design is to consult the datasheets for both the J112 and J113 and compare their specifications to your design requirements. You may also want to consider simulating the performance of each JFET in your circuit using a software tool before making a final decision.

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