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Gary Pimm self bias CCS 500kHz 'ringing'
Gary Pimm self bias CCS 500kHz 'ringing'
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Old 5th February 2018, 05:42 AM   #1
Hanze Khronye is offline Hanze Khronye  Australia
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Default Gary Pimm self bias CCS 500kHz 'ringing'

Hi,

I've lived with this as a plate load for some while. Low capacitance and high impedance with the 'mu output' for low output impedance, whats not to like?.

Have recently acquired a good Tek analog scope, and I'll let the pics tell the story. I constructed it using perfboard layout as close to the original PCB's and used the recommended parts. Yep, it sounds great.

With mu output there is a voltage gain of 6dB at 500kHz, and square wave rings (at any frequency), screen shot is 1kHz.

Taking output from the plate of the tube (at the 'bottom' of R1), all is clear up until well past 1.5MHz, the stage starts to roll HF off at a tick over 100kHz, and down by 6dB at 250kHz there abouts, no visible distortion of sine wave at all, until it rolls completely off well below 2MHz.

Any thoughts?.. buy a CCS kit that doesnt oscillate, yep.. but the super low capacitance and high impedance is what attracted me to this years ago. Could it be (and I'm not a SS guy), simply a matter of increasing the stopper to 100R or 1K in lieu of the 10R as specified.

Anyway, hope someone finds this useful and any information, suggestion is much appreciated. .. Gary are you still around??

Thanks.
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Last edited by Hanze Khronye; 5th February 2018 at 06:14 AM.
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Old 5th February 2018, 08:33 AM   #2
IDM is offline IDM  United Kingdom
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I would try grid stoppers (1K) on the LND150's and see if that helps.
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Old 6th February 2018, 05:43 AM   #3
Hanze Khronye is offline Hanze Khronye  Australia
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Thanks. Tried but to no avail.

I am looking for a replacement CCS, have emailed a few vendors, one of which replied that he had not tested his CCS (commercial off the shelf offering) beyond 100kHz, or at any stage with a square wave. (really?)

Failing a decent CCS, I'll spring for a 2:1 stepdown transformer, and just mount it to the back of the chassis.

Thanks.
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Old 6th February 2018, 06:05 AM   #4
ErikdeBest is offline ErikdeBest  Switzerland
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Gary Pimm self bias CCS 500kHz 'ringing'
I would increase the value of R7 to some 680R, and as 2nd step decrease te 4k7. I do not know why Gary Pimm used such differing values (being gate stoppers) - still Rod Coleman posted recently that Gate stoppers should best be kept below 1k (for larger FETs even much lower than 1k)
Good luck! Erik
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Old 6th February 2018, 09:39 AM   #5
Rod Coleman is offline Rod Coleman  United Kingdom
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Gary Pimm self bias CCS 500kHz 'ringing'
Quote:
Originally Posted by Hanze Khronye View Post

With mu output there is a voltage gain of 6dB at 500kHz, and square wave rings (at any frequency), screen shot is 1kHz.
If you are measuring square waves from a low impedance source, the ringing you observe on the scope may be caused by the probing method, rather than a misbehaviour of the circuit.

The 'µ output' has a drive impedance of only a few ohms, and if the signal has a fast risetime, feeds a capacitive load, or has a large amplitude, the risk of coupling into the ground-lead of the probe increases, and can show ringing that will disappear with careful probing.

The easy thing to try is a 'zero-length ground lead' made from a length of 1mm solid copper (tin plated). Remove the probe's end-clip, and wrap the copper around the probe to form a tight coil. Then remove this coil and solder it to the ground-plane of the circuit under test. The picture tells the tale. This method prevents external fields from adding a [series] noise voltage in the scope probe return. As well as cleaning up square waves, the noise floor of the scope drops dramatically with this method.

I assume you're using x10 probing, and have trimmed the x10 probe for correct response, using the scope's squarewave generator.
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Old 6th February 2018, 10:04 AM   #6
Rod Coleman is offline Rod Coleman  United Kingdom
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Gary Pimm self bias CCS 500kHz 'ringing'
About the FET gate stoppers:

- depletion FETs. Usually, connecting the depletion FETs as current sources, even cascodes, means that the gate wiring can be very short, and the loop area around gate to source is correspondingly small. In this case, the danger of creating a Hartley oscillator (Drain wiring too long) or Colpitts oscillator (source-follower circuits) should not be too great.
Oscillation stems from coupling from drain to gate, and if the circuit wiring is well-implemented (easy with a depletion FET CCS), the coupling is mostly from the drain-gate capacitance. In this situation, a low gate-source impedance is helpful, since it suppresses signals arriving via this D-G capacitance. Adding a high-value gate stopper can allow the gate signals to increase, and this makes the oscillation worse - not the result we may have been expecting.

When the external gate wiring can't be made short (less than about 30mm) a stopper may be needed, but don't get carried away with the value. If the circuit is stable over a wide range of signal amplitudes using a 100Ω stopper, leave it at that.

In the circuit of post №1, the lack of stoppers for LND150 is probably fine, if there are no long wires leading to the gate(s). And Q1's 10Ω is also likely to be perfect, too, because the G→S circuit is suppressed by a large capacitor. Increasing the 10Ω may well lead to problems here.

The 4.7K in the gate of the other IRF820 seems too high to me, and this FET also sees very high impedance from gate to source. Noise pulses coupling through the D→G capacitance can prevent the channel from fully turning OFF, and the breakdown voltage may appear to be lowered. I wouldn't operate this circuit anywhere near the 500V rating of the IRF820.

The µ output has no series output resistor, which may be desirable for low output impedance, but the stability of the FET can be compromised by reactive loads acting as (example) an unintended Colpitts oscillator. If the system can stand some series resistance, a 50Ω resistor will help ensure stability.
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Old 6th February 2018, 09:35 PM   #7
dhsettim is offline dhsettim  New Zealand
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Although this post by Gary relates to the Swenson/Pimm regulator it does give some important information on stoppers.

Look for high-voltage regulator projects

Ale Moglia over at his excellent blog has posted his finding that sometimes the gate stopper needs reducing;not increasing.

And always follow Rod's advice.

tim
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Old 7th February 2018, 03:32 AM   #8
Hanze Khronye is offline Hanze Khronye  Australia
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Thanks Rod, I will investigate this further.. really appreciate your comprehensive reply.

Also thanks to Tim, as this link provides some useful information as to why the values where selected;

R15, the resistor on Q2 is more than a just a gate stopper. It is actually tuning the speed of the MOSFET cascode circuit. If the value of R15 is too high, the circuit will oscillate in the 150Khz to 250Khz range with what looks like a relaxation oscillator. If the value of R15 is too low the circuit breaks into a chaotic high frequency oscillation that my Tektronix TDS460 scope can't trigger on well enough to get a frequency measurement.

This behavior has been constant with the CCSs, the amplifier modules, and the regulator circuit. The value is chosen to be at the approximate mid point between the 2 modes of oscillation.

In the self bias and battery bias CCS circuits when using the IRF820a parts the Q2 gate stopper was 2.2K. When the IRF820B came out the value had to be increased to 4.7K as the IRF820B is a faster part
.

Last edited by Hanze Khronye; 7th February 2018 at 03:35 AM.
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Old 7th February 2018, 04:03 AM   #9
Ultima Thule is offline Ultima Thule  Europe
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A too large gate resistor may have a diminishing effect as a gate stopper, R15 seems large.

Is C5 supposed to be connected to Q1 source, and what is the value of R1?

Also, as already mentioned, probes themselves can interact with the circuit and one thing you could try out is attaching something like a 1k resistor in series with the probe tip.

Last edited by Ultima Thule; 7th February 2018 at 04:21 AM.
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Old 7th February 2018, 05:27 AM   #10
Hanze Khronye is offline Hanze Khronye  Australia
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The reasoning behind the value for R15 gate resistor is given by the designer of the cct as indicated above.

I do not know where C5 could otherwise be connected, the guy who designed it was an engineer who worked for Tektronix - I went with the schematic.

Will also try the 1k series resistor as you suggest and report back.

R1 = 1k2.

Thank you.

Last edited by Hanze Khronye; 7th February 2018 at 05:38 AM.
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