Incorporating LPF in the FET input stage

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ilya
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Incorporating LPF in the FET input stage

Post by ilya » Mon Dec 11, 2017 1:00 pm

I'm building a simple FET DI input and it has too wide bandwidth as it is. What is the best way to incorporate an LPF in this design? I've tried different things, but the most obvious is a cap across the resistor as shown (470p gives a good cut-off frequency).

Are there any drawbacks to this method? Maybe there're some other solutions?

Thanks!

Edit by mediatechnology 2/13/19: I updated the title of this thread to correct "HPF" to "LPF."
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JR.
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Re: Incorporating LPF in the FET input stage

Post by JR. » Mon Dec 11, 2017 2:48 pm

Your added cap is simply a shunt across the input so LPF results will vary with the source impedance of the instrument plugged in.

To make it better behaved you can add a fixed resistance in series with C50, but there is no free lunch that added R will contribute noise of it's own. That added series R wants to be high impedance wrt source, but not so high that it adds audible noise...

You can probably experiment with a few different Rs to see what works for you.
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Another option is to use the series output R (100 ohm) as the R in your RC LPF but that will require a larger value C. That r could be increased to 200 ohms or more with little consequence.

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Re: Incorporating LPF in the FET input stage

Post by ilya » Mon Dec 11, 2017 3:28 pm

Series Rin will be too noisy. I estimated a suitable R as 500K, which gave me a -97dBv Johnson noise.

Cap on the output worked though. Where is it better to connect it? Before R56 or after? I'm getting different values of the cap depending on its position (roughly 100 times difference in capacitance).

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Re: Incorporating LPF in the FET input stage

Post by JR. » Mon Dec 11, 2017 5:30 pm

It must be after R56 to work... that is the R of the RC.

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Re: Incorporating LPF in the FET input stage

Post by mediatechnology » Tue Dec 12, 2017 6:26 am

Just my 2 cents but wouldn't you want some small (1K-ish or less) series resistance with the input to reduce RF rectification and protect the gate?
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Re: Incorporating LPF in the FET input stage

Post by JR. » Tue Dec 12, 2017 10:25 am

mediatechnology wrote:
Tue Dec 12, 2017 6:26 am
Just my 2 cents but wouldn't you want some small (1K-ish or less) series resistance with the input to reduce RF rectification and protect the gate?
Probably good practice but won't make a well defined R for a LPF since source impedance will add to that 1k.

JR

Note: placing the RC at the output buildout R will also interact with load capacitance (like cable capacitance) but that should be small wrt expected C value.
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Re: Incorporating LPF in the FET input stage

Post by ilya » Tue Dec 12, 2017 1:33 pm

Tried several things and cap on the output worked the best. No traces of high frequency garbage that was otherwise getting inside the circuit (even with the cap on the input). Thanks gents! I'll insert the 1K protecting resistor at the input as well.

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Re: Incorporating LPF in the FET input stage

Post by mediatechnology » Tue Dec 12, 2017 2:18 pm

Probably good practice but won't make a well defined R for a LPF since source impedance will add to that 1k.
That suggestion wasn't intended to make it a LPF - just some R in series with the gate for HF and VHF RFI.
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Re: Incorporating LPF in the FET input stage

Post by JR. » Tue Dec 12, 2017 4:19 pm

mediatechnology wrote:
Tue Dec 12, 2017 2:18 pm
Probably good practice but won't make a well defined R for a LPF since source impedance will add to that 1k.
That suggestion wasn't intended to make it a LPF - just some R in series with the gate for HF and VHF RFI.
You mentioned preventing rectification... There will be a LPF from stray capacitance (and 1k) but a modest C to ground would be more reliable.

For a one off, do what works...

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Re: Incorporating RFI LPF in the FET input stage

Post by mediatechnology » Wed Feb 13, 2019 7:30 am

A few articles about adding series resistance between the input and transistor base (or FET gate) to provide "base stoppers" for emitter followers.

I've found that a small amount of series resistance added to instrumentation amp inputs also reduces rectification and appears to improve HF CM rejection. It seems to work for op amp based INAs as well as current-feedback topologies.

In a mic preamp, the required series resistance for phantom protection can be split in two, with protection diodes and differential capacitance in the middle, to provide both phantom protection and base-stopper/Q-reduction without adding unnecessary source impedance.

(I think the OP should read LPF BTW so I changed it.)

http://theradioboard.com/rb/viewtopic.php?t=7710

http://sound.whsites.net/amp_design.htm#s12

Image
Figure 3 http://sound.whsites.net/ampd-f3.png
This approach might work if PCB track lengths in that part of the circuit are very short, ensuring minimal inductance. This is not always the case, and some layouts may include more than enough track length to not only act as an inductor, but as an antenna as well. Then things can get really sneaky, such as when the levels of RF energy are so high that some amount manages to get through anyway. I once had a workshop/lab which was triangulated by three TV transmission towers - very nasty. RF interference was a fact of life there.

The traditional method not only did not work, but often made matters worse by ensuring that the transistor base was fed from a very low impedance (from an RF perspective) because of C1. A vast number of commercial amplifiers and other equipment which I worked on in that time picked up quite unacceptable amounts of TV frame buzz, caused by the detection of the 50Hz vertical synchronisation pulses in the TV signal. As the picture component of analogue TV is (or was - it's almost completely digital now) amplitude modulated RF, this was readily converted into audio - of the most objectionable kind. (emphasis added)
I particularly agree with the emphasized part.

Image
Figure 4 http://sound.whsites.net/ampd-f4.png
Figure 4 shows the remedy - but to be effective the R2 must be as close as possible to the base, or the performance is degraded. How does this work? Simple, the base-emitter junction of a transistor is a diode, and even when conducting it will retain non-linearities. These are often sufficient to enable the input stage to act as a crude AM detector, which will be quite effective with high-level TV or CB radio signals. Adding the external resistance again swamps the internal non-linearities, reducing the diode effect to negligible levels. This is not to say that it will entirely eliminate the problem where strong RF fields are present, but will at least reduce it to 'nuisance' rather than 'intolerable' levels.

UPDATE: I have been advised by a reader who works in a transmitting station that connecting the capacitor directly between base and emitter (in conjunction with the stopper resistor) is very effective. He too found that the traditional method was useless, but that when high strength fields are encountered, the simple stopper is not enough.
John Dunn also writes about base stopper resistance providing Q-reduction of the parasitic tuned circuit formed by input inductance and stray capacitances. https://www.edn.com/electronics-blogs/l ... cillators-

There is also Jensen's paper: viewtopic.php?f=12&t=932

We also ponder the role of an op amp input series resistor here with a response by Dennis Bohn: viewtopic.php?f=6&t=497&p=5696

Self writes about base-stoppers in Small Signal Design as well.
EMITTER-FOLLOWER STABILITY. In preparation.

The emitter-follower is about as simple as an amplifier gets, and it seems highly unlikely that it could suffer from obscure stability problems. However, it can. Emitter-followers are liable to RF oscillation when fed from an inductive source impedances. This oscillation may well be in the VHF region and invisible on the average oscilloscope; however a sure sign is unusually high distortion that varies strongly when the transistor is touched with a probing finger. The standard way to stop this is to put a "base-stopper" resistor in series with the base. This should
come after the bias resistor to minimise loss of gain. Depending on the circuit conditions, the resistor may be as low as 100 Ohms or as high as 2K. The higher values can be an inconvenient source of Johnson noise in low-noise circuitry. The instability mechanism(s) are not easy to explain quickly. The best source I know for more information on this is a book by Feucht, "The Handbook of Analog Circuit Design" published by Academic Press, 1990.
TI show them in the INA217 datasheet.
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