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Transistor Noise Figure for Reverb circuit
Transistor Noise Figure for Reverb circuit
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Old 21st March 2020, 09:41 PM   #11
Enzo is offline Enzo  United States
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Quote:
Quote:
Originally Posted by Enzo View Post
...ZIllions of amps like Fender and Peavey have driven reverbs with plain old 4558s for years.
I was referring to recovery circuits rather than drivers.
Yes. In those the same IC does both. I intended that to be implicit in my response.


Any other noisy models besides the 35 year old Session?
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Old 23rd March 2020, 06:18 PM   #12
dotneck335 is offline dotneck335  United States
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Originally Posted by MarcelvdG View Post
Are you comparing them at the same collector current? The higher the collector current, the less equivalent input noise voltage and the more equivalent input noise current.
Yes, both were quoted specs at Ic = 100ľA.
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Old 24th March 2020, 05:15 AM   #13
davidsrsb is offline davidsrsb  Malaysia
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A few k source impedance is where old low noise audio bjts like the BC109 do best. Too low for jfet and too high for low rbb types.
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Old 24th March 2020, 07:14 AM   #14
MarcelvdG is offline MarcelvdG  Netherlands
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It's a source impedance where BC109 / BC549 / BC550 / BC850 / BC860 work fine, but so do high transconductance JFETs and low-rbb bipolar transistors with a high hFE.
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Old 24th March 2020, 07:45 AM   #15
MarcelvdG is offline MarcelvdG  Netherlands
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Originally Posted by dotneck335 View Post
Yes, both were quoted specs at Ic = 100ľA.
I looked up the On Semiconductor datasheet, https://www.onsemi.com/pub/Collateral/2N3903-D.PDF , and it specifies a maximum noise figure of 5 dB at 1 kHz for a 1 kohm source impedance, IC = 100 ľA, VCE = 5 V at 25 degrees C ambient temperature. According to figure 10, the typical value is 3.6 dB under the same circumstances. Figure 9 shows that 1/f noise is not negligible at 1 kHz.

I think the best data you have for On Semiconductor 2N3904 transistors are those in figure 9 and figure 10. The 5 dB maximum spec is just a rejection limit set far enough above the normal noise level not to get too much yield loss. Still, 3.6 dB is a bit high.

It could very well be that the 1/f noise and base resistance noise vary between manufacturers. Apparently On Semiconductor's 2N3904 isn't as good as the ones measured by Horowitz and Hill, or whoever measured the data for figures 9 and 10 of the datasheet didn't measure very accurately.

Last edited by MarcelvdG; 24th March 2020 at 07:48 AM.
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Old 24th March 2020, 02:36 PM   #16
davidsrsb is offline davidsrsb  Malaysia
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The 2N3904 datasheet headline says it all

"General Purpose Transistors"

There was no intention to waste test time and money screening for noise
The BC549 was described as "Low Noise Transistor"
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Old 24th March 2020, 04:57 PM   #17
dotneck335 is offline dotneck335  United States
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Originally Posted by davidsrsb View Post
The BC549 was described as "Low Noise Transistor"
I wonder why Horowitz-Hill didn't include it in their survey of Low-Noise Transistors?? It does have a very low datasheet spec for Noise Figure......
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Old 24th March 2020, 08:47 PM   #18
Mark Tillotson is offline Mark Tillotson
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I believe it used to be the case that low (flicker) noise had to be screened for on a part-by-part basis, taking a lot of time (since flicker noise is intermittent), and thus putting up the cost significantly - thus a part designed for low noise back when flicker and 1/f noise were more prevalent (poorer processes, not using buried junctions?) would have a different code and cost noticably more.


For instance the NE5534A is/was a screened version of the NE5534.


It makes sense that any "general purpose" device with good typical noise performance isn't necessarily a good choice for manufacturing a product where noise performance is critical, even if its better on average than the device actually used(!). People can self-test components, but often its easier to pick a device with guaranteed adequate performance and pay for someone else to do the testing.
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Old 24th March 2020, 09:31 PM   #19
MarcelvdG is offline MarcelvdG  Netherlands
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As far as I recall, according to the Philips datasheet, the noise of the NE5534/NE5534A was sample tested.
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