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Old 11th October 2013, 11:19 PM   #1
danny92 is offline danny92  Portugal
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Default Faster VAS/driver transistors

Hi everyone

I'm using MJE340/350 pair for the VAS and MJE15032/MJE15033 for the EF driver. But they are slow and with low beta. There's something I could use to replace them?
I also need the for LTSpice models.

The 2SA1381/2S3503 pair are good enought, but they are discontinuated devices.

Best regards,
Daniel

Last edited by danny92; 11th October 2013 at 11:22 PM.
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Old 12th October 2013, 02:26 AM   #2
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The 2SA1381/2S3503...l
Should still be available as KSC3503 / KSA1381.

Best wishes
David
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Old 12th October 2013, 02:48 AM   #3
danny92 is offline danny92  Portugal
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Yes, you're right, thank you very much

Best regards,
Daniel
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Old 12th October 2013, 03:53 PM   #4
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I've begun to use the KSA1220A / KSC2690A devices, the Cob is higher than the most highly prized devices but they do in fact sound good. They are relatively cheap, in current production, and readily available in matching hFE ranks. They are even sturdy enough for pre-driver / driver duty if you so choose.
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Old 12th October 2013, 08:22 PM   #5
Bensen is offline Bensen  Belgium
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I'm using the KSC2682/KSA1142, these have great specs, are not expensive and in current production.
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Old 12th October 2013, 08:30 PM   #6
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I found I didn't need faster transistors than mje340 as I was having to slow down the VAS with a base collector capacitor.
If I didn't do this then I got oscillation on the output transistors.
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Old 12th October 2013, 08:55 PM   #7
danny92 is offline danny92  Portugal
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Hi everyone,

The only base to collector capacitor that I know is the CMiller compensation capacitor, used to roll off gain, before phase shift arround the closed loop reach phase invertion (180), and that's right it can lower the slew rate indeed, but using advanced compensation schemes like TMC or TPC you can have good phase/gain margin and also a fast amplifier with an high slew rate.
Faster VAS transistors don't suffer from large gain peakings (at dozens of MHz) in open loop gain, because of their lower junction capacitances [Cpi (Cbe) and Cmiu (Cce)], which improves linearity in open loop and phase/gain margins.

I only have models for KSC3503 and KSC1381
I need at least a maximum Vce of 120 V.

Best regards,
Daniel

Last edited by danny92; 12th October 2013 at 09:05 PM.
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Old 12th October 2013, 09:08 PM   #8
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Originally Posted by Bensen View Post
I'm using the KSC2682/KSA1142, these have great specs, are not expensive and in current production.
The KSC2682 is in fact obsolete and not in production. Too bad because the specs were really good.
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Old 12th October 2013, 09:15 PM   #9
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I just found out today the KSC2682 is indeed gone. I don't understand it. Nothing better offered either.
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Old 12th October 2013, 09:18 PM   #10
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Quote:
Originally Posted by danny92 View Post
Hi everyone,

The only base to collector capacitor that I know is the CMiller compensation capacitor, used to roll off gain, before phase shift arround the closed loop reach phase invertion (180), and that's right it can lower the slew rate indeed, but using advanced compensation schemes like TMC or TPC you can have good phase/gain margin and also a fast amplifier with an high slew rate.
Faster VAS transistors don't suffer from large gain peakings (at dozens of MHz) in open loop gain, because of their lower junction capacitances [Cpi (Cbe) and Cmiu (Cce)], which improves linearity in open loop and phase/gain margins.

I only have models for KSC3503 and KSC1381
I need at least a maximum Vce of 120 V.

Best regards,
Daniel
Here are some models that came from Fairchild's download area for the KSA1220A, KSC2690A, KSA992, and KSC1845.
Attached Files
File Type: txt Fairchild Models.txt (1.4 KB, 24 views)
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