Hi all,
let me add my way to see this ripple modulation matter:
First, let's think tubes. In a tube, cathode current flow depends on the
electric field around the cathode, which is created by the combination
of the voltages of all other elements, relative to the cathode, and
considering their relative distances and gridding densities.
So, for a triode, cathode current is determined both by grid and plate
voltage, with grid voltage having the larger influence. With a tetrode
or pentode, instead, the plate voltage will only have a small effect,
while the cathode current is mostly determined by grid and screen voltages.
Most of the cathode current will flow into the anode. Caution has to be
applied to tetrodes and pentodes in which under some conditions a
significant current can flow into the screen instead.
Any radio-frequent variation of the plate current will translate into RF
output from the amplifier. Given that the plate matching isn't altered,
RF power output depends on the square of the RF component in the plate
current.
Plate RF voltage depends by plate RF current multiplied by the loading
resistance offered to the plate by the matching network and the load
(antenna).
When the above means that the RF voltage on the plate should become
larger than it can be, according to the existing DC plate supply
voltage, the tube will go into saturation. In deep saturation, the
output power will be dependent almost solely on plate voltage.
As a result of all the above, a tetrode or pentode amplifier can be
highly linear, AND INDEPENDENT OF PLATE VOLTAGE RIPPLE, as long as it
never saturates. That means driving it low enough so that even at the
deepest valleys of the ripple waveform on the supply, the plate supply
voltage does not yet significantly influence the output power.
But if this amplifier is overdriven, its saturated output power will be
directly modulated by that ripple.
In simplified, practical words: A tetrode or pentode amplifier will not
produce any significant hum modulation, as long as it is driven low
enough so that it stays out of clipping even in the supply voltage
ripple lows. If it's overdriven, hum modulation will appear on the
peaks. Since voice peaks are usually of higher frequency than the 100 or
120Hz ripple on the supply, the resulting hum on the output will not be
very obvious. It's peceived as an unclean, somewhat raspy sound, but not
really as hum. But in continuous power modes, such as CW or RTTY, hum on
the output will be readily noticeable when the amplifier is driven into
saturation. Instead if operated out of saturation, the output will be
essentially hum free.
The more ripple there is on the supply voltage, the lower the amplifier
needs to be driven, to avoid hum modulation. In the extreme case, that
of having no filter capacitor, the maximum output possible without hum
decreases to zero.
The above assumes that the other voltages, those for the screen and the
grid bias, are free from ripple. Any ripple on the screen will directly
modulate the gain of the tube, and thus introduce hum on the output,
even while not saturating the amplifier. Hum on the grid, instead, is
less critical, because it doesn't cause much change to the tube's gain.
Hum on the grid will just cause a hum component on the plate current,
but the RF signal will remain superimposed on this hum, without being
significantly modulated in its amplitude. So, as long as the total plate
current, including the hum component, doesn't drive anything into
saturation, ripple on the grid bias should not have a big effect. It's
like varying the idle current setting at 100 or 120Hz rate. Depending on
the specific tube, this has only a very small effect on gain.
Now the case of triodes: Since teh plate current of a triode depends
heavily on plate voltage, in addition to grid voltage, a triode used
without strong feedback will be nonlinear (distorting). In such an
amplifier, any ripple on the plate supply will modulate the RF output.
But due to the resulting nonlinearity, triodes are normally never used
in low feedback circuits, when linear amplification is desired. Instead
they are used in some setup that provides a high negative feedback, such
as the grounded grid configuration. A typical triode will have around
10% to 25% of the output voltage fed back to the input, in opposed
phase. This tends to establish the output voltage of the tube as a
certain proportion of input voltage, thus linearizing it to some degree.
This reduces its sensitivity to ripple on the supply, and this is what
allows us to run triodes in grounded grid from simple power supplies
that do have significant ripple.
The exact amount of hum modulation suffered by a specific triode, fed
from a supply with a certain amount of ripple, could be calculated from
the tube's transfer functions and the circuit it is used in. The more
negative feedback it has (the lower its gain), the less sensitive it
will be to ripple in the plate supply.
Now let's think transistors:
Both bipolar transistors and MOSFETs behave rather close to a pentode,
in terms of transfer function. That is, their gain does not depend a lot
on collector or drain voltage, because their collector or drain current
doesn't vary much with voltage, but instead is mostly controlled by base
current or gate voltage. For this reason, amplitude modulation of the
output by ripple on the supply is much as it is for pentodes and
tetrodes, that is, it becomes significant only when the transistor is
driven into saturation.
But there is another problematic effect, which is not a big factor in
tubes: The internal capacitances of transistors change a lot with
applied voltage. Specially in MOSFETs, in which these capacitances are
rather high, this leads not only to some amplitude modulation through
gain changes, but also to phase modulation, causing an FM hum on the
signals, when the supply voltage has ripple on it.
But this effect is small, and depends a lot on the specific devices
used. Some MOSFETs show a significant change of capacitance with
voltage, even at the supply voltage level. Others show this big change
of capacitance only at low voltage levels, say, one fifth of the supply
voltage or so. With such a MOSFET, a small signal will be confined to
voltage swings well out of the area where capacitances change a lot, and
this will be rather unaffected by ripple on the supply. As we go further
toward large signals, close to clipping, a MOSFET will suffer from these
capacitance changes, causing both nonlinear gain reduction and phase
shifting, and at this level it will also show increased sensitivity to
supply ripple modulating the signal both in amplitude and phase.
So, this is how I see it. It's certainly not a complete picture, but I
hope I didn't forget any of the most important effects.
In practice, some ripple on the supply (say, 10%) seems to be just a
minor issue, in most ham amplifiers. On the other hand, with today's
technology of switching regulators it's really easy and inexpensive to
make power supplies that are essentially ripple free. A switching supply
with just a single stage output filter typically has less than 0.1%
ripple at full load, less at partial load, and this small ripple is not
at line frequency, but at its switching frequency of several tens of
kHz. An additional stage of filtering, made with a small inductor and a
modest capacitor, can easily reduce the ripple to 0.01% or even less.
With a switching supply feeding an amplifier it becomes very important
to have very low ripple, because the sidebands introduced by the higher
frequency ripple fall into rather distant channels, where they are far
more noticeable. But since it's so easy to produce very low ripple
output from such a power supply, this isn't a problem at all. It's just
a matter to be aware of during the design phase.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|