Class D & E amplifiers
In a class D amplifier, a pair of transistors switch on and off, out of phase across an O/P transformer.
Efficiency of 75% can be expected but class D amplifiers are relativley complex.
Class E amplifiers are about 90% efficient.
Because of the low loss no cooling fan is needed, no TR switch is nedded, the received signal is piped through the amplifier itself.
The Class D & E amplifiers were invented and patented by Nathan Sokal and Alan Sokal (Father/Son) in 1975
They minimize heat loss by having as little overlap as possible between voltage and current.
In class D and E amplifiers,the devices act as switches, half time completely on, half time completely off.
But transistors are not perfect switches; Mosfets have a resistance of 1 ohm (on) and several hundred Pf when off.
Losses are greatly reduced in switching amplifiers but the penalty that the O/P power no longer depends on the drive power, but rather on the supply voltage.
This means that switching amplifiers are not linear amplifiers and therefore not suitable for SSB without aditional limiting and modulating circuits.
However they are fine for CW, FSK and FM.
From designs cited in May 1997 QST
Class 'E' transmitter for 1.8MHz (2 design's) - "click to enlarge"
TELEVISION LINE OUTPUT STAGES AND THE EVOLUTION OF THE DAMPER DIODE OR “ENERGY RECOVERY DIODE”
The following is reproduced from "evolution of the damper diode" in television systems
an original work by Dr Hugo Holden (see www.worldphaco.com)
IMHO it surpasses any other textbook explanation of how this type of circuitry really works.
The first person to suggest the use of a "damper" diode in magnetic deflection scanning, in 1936,
was Alan Dower Blumlein, the "inventor" of stereo audio.
He patented "binaural" audio recording in 1931.
Alan Blumlein was killed in a plane crash in 1942 while testing radar for the war effort.
This loss was described by Winston Churchill as a national tragedy.
Following this, damper diode function was well examined by RCA laboratories presumably during wartime and in the immediate two years thereafter.
RCA produced a series of review articles and in "Magnetic Deflection Circuits for Cathode-Ray Tubes" Otto. H. Schade Volume V 1947-1948 Pg 105, RCA Labs, reference is made to Blumlein's original 1936 patent, and damper diode technology is thoroughly explained.
The circuits have been reduced to their basic forms without linearity or width controls so as to show their basic configuration.
Coupling the yoke to the line output tube by a transformer is shown in Fig 1.
At flyback the tube is cut-off and the magnetic field in the transformer and yoke collapses and resonates due to the self-inductance and distributed capacitance of these structures.
There are oscillatory voltages and currents representing relatively undamped oscillations.
These oscillations, which are visible in the scanning raster, decay away, and become damped out when the line output tube is again driven into conduction by the drive voltage.
These oscillations must be eliminated for a satisfactory scanning raster.
Fig 2 shows resistive damping.
In this case the damping occurs across the entire duration of the sawtooth current scanning waveform on both the positive and negative excursions of current, so it can be called bidirectional damping.
This is wasteful of energy, lengthens the flyback period, and reduces the opportunity to utilize the positive going high voltage spikes generated at the line output tube’s anode, or via an OVERWIND coil to generate EHT.
Fig 3 shows an improvement to resistive damping.
This technique is used in the HMV Marconi 904 (1939).
The RC network is frequency selective, damping the parts of the waveform with the highest rates of change.
This reduces the oscillations of currents (shown in red) however, because the flyback period contains high frequency (Fourier) components, this is also damped.
Again this wastes energy and lengthens the flyback period.
Fig 4 Shows what might appear to be the introduction of an efficiency diode in the RCA TRK9 (and TRK 12) but is in fact, not.
This circuit has the damper conducting only over flyback time alone, and is really a spike suppressor.
A true efficiency diode conducts during the active scan time on the left hand side of the scanning raster and recovers energy from the magnetic field of the yoke and line output transformer.
The recovered energy is stored in the magnetic field at the end of scan time at the right side of the raster.
The circuit of fig 4 damps the flyback voltage oscillations and absorbs energy when the output tube is cut off.
This arrangement can’t be used in a system to generate EHT from the flyback voltage spike. In 1938 the Baird/Bush TV and radio company in the United Kingdom were using the circuit shown in Fig 5:
(Provided by Mr Victor Barker (VK2BTV AUSTRALIA)
This is probably one of the first examples of energy recovery scanning.
When the magnetic field in the line output transformer collapses, the diode conducts on the first negative half cycle of voltage on the diode’s cathode, to produce a more linear rate of change of current.
This damps the oscillations and also returns energy to the power supply.
As can be seen this was the precursor of the typical transistorised line output stage that appeared in early transistor televisions in the early 1960’s.
Returning to this later, let's look at this Bush circuit in the following three equivalents: Rather than returning the anode to ground (zero volts), it can be returned to B+ provided B+ is cancelled to zero volts (or close) by another “-B+” supply as seen in Fig 5A. This added supply can then be replaced with an RC network, as seen in Fig 5B, which charges to a value Y, say close to the value of B+ but in practice is a little less as the line opt tube anode voltage doesn’t go completely to zero during active scan time.
Then simply this diode and RC network is placed on a secondary
winding, not the primary, and the position of the diode and RC network reversed
as they’re in a series circuit then you end up with the following seen in Fig
This same basic circuit, with the diode and RC network is also shown in Fink’s Principles of Television Engineering 1940 page 152, fig 87, placed in the primary circuit
When the line output tube is cut-off at flyback, the first half
cycle of voltage oscillation takes the damper anode negative (cutting it off
Other variations of damper diode circuits which have occurred in the post war period include a triode pair used as a controlled damper diode, which gives additional control over the linearity of the saw-tooth scanning current. See fig 10 below:
LINEARITY AND DEFLECTION OUTPUT STAGE CIRCUIT DESIGN
In general, in the line output stage, every effort is made to keep the resistances of the yoke and output transformer windings as low as possible.
In the case of transistor output stages (as shown in fig 5), where the working impedances are lower (lower ratio if dynamic voltages to dynamic currents), the nature of the line output transformer is such that the inductance dominates and the resistances of the coils are very low. This enables the output stage to operate as a saturated switch.
The transistor is driven with a step function, or rectangular wave, to cause it to switch on for 2/3 or more of the active scan time leading to the right side of the raster.
The rise in current in the yoke when the transistor is switched on is linear because the rate of change of current dI/dt is close to a constant for a period after the voltage is initially applied. The fundamental differential equation for an inductor is V = -L.dI/dt, or voltage equals a constant (the inductance L) multiplied by the rate of change of current with time. So if a rectangular voltage is applied to an inductor, the initial rise in current is a saw-tooth, ideal for scanning.
The negative sign in the equation indicates that the EMF of inductance, is reactive to, or opposes the applied EMF.
Of course the current cannot rise linearly indefinitely in an RL circuit, either it will become non-linear as the transformer core starts to saturate, or exponentially taper off to a value given by the applied voltage divided by the resistance.
The proportions of inductance, applied voltage and winding resistances are chosen so over the time interval of the scan when, the transistor is turned on, the rise in current is substantially linear.
At the end of scan, the transistor switched off and the magnetic field collapses for a half cycle of operation, this then forces the semiconductor damper diode into conduction, where it effectively acts like a switch and results in a substantially linear scan on the left hand side of the raster.
The transistor line output circuit, however, is therefore such that the transistor, acting as a switch, is very efficient, but the transistor, and its drive waveform can have little effect over the linearity of the scan it generates (unlike a tube line output stage as will be explained).
To gain linearity control, in the transistor line output stage, usually a capacitor is inserted in series with the yoke (sometimes called an "S" correction capacitor), or an inductance in apposition to a permanent magnet is placed in series with the yoke.
In contrast the impedances in a tube line output stage (ratios of dynamic voltages to dynamic currents) are higher than in the transistor case.
These lower currents (and higher voltages) in the primary circuits require that there are more turns in the output transformer at least, and usually the yoke too.
Horizontal yoke winding resistances are in the order of 10 to 60 ohms for tube work, but very low, less than 1 ohm sometimes in portable transistor TV’s with 12-volt supply rails.
Overall though, in a tube set, the line yoke coils are transformed to the anode of the line output tube as a substantially inductive load, and the anode voltage wave, with the saw-tooth grid drive is substantially rectangular in character.
However, in the case of the tube line output stage, the nature of the grid drive (horizontal drive), unlike the transistor case, is able to influence the linearity, especially on the middle right hand side of the raster.
(The line yoke can also be driven directly from the tube anode; in this case the yoke has a higher inductance and higher resistance than in the transformer coupled case)
In general linearity controls in tube line output stages are introduced as an inductor in series with the B+ boost supply to the primary winding, with varying amounts of capacitive filtering around this for the B+ boost voltage.
The ripple voltage generated alters the output tube’s working load and variations in linearity can be obtained that way, which tends to vary the linearity near the picture centre. In contrast, due to the much lower working frequency in the frame circuits, transformers with larger inductances (and also resistances) are required and the load presented to the vertical output tube is a combination resistive and inductive.
This requires an overall drive voltage which is a combination of a rectangular wave and saw-tooth (trapezoid) to generate a saw-tooth current is a circuit with series elements of L and R.
The design of tube or transistor vertical output stages very much resembles their audio output stage counterparts, and the exact shape of the trapezoidal waveform and the operating conditions of the output device, has a substantial effect on the vertical linearity.
This is why one common form of linearity control in a vertical output stage consists of a variable resistor in the cathode of the vertical output tube.
Fighting (side effects of) Macrovision
This is an account of some modifications I did to an ancient (circa 1980) Sanyo TV of ours to fix an annoying side effect of the so-called "Macrovision" protection of rental VHS video tapes.
Okay - back to the story of our old Sanyo!Vertical Blocking Oscillator Tfmr Replacement B & W TV
This old warrior has been kept in service over the past 22 years for the
simple reason that I fitted a nice big new 63cm (25") tube back in 1985 after
the green cathode of the original had gone low in emmision.
Anyway, in June of 2003, this old Sanyo had gone "bang" and stopped working
An old original electrolytic capacitor (4.7uF 350 volt) had shorted out, plus
incinerating a 1 ohm series resistor which obviously hadn't enjoyed having 120
volts across it as a result (see Fig.2).
An 8uF 450 volt electro and a 1 ohm 5 watt resistor soldered in, and sound and picture returned as good as new.
Okay - so far, pretty straightfoward.
At this point, I wondered to myself
Finally, I decided to have a shot at getting rid of the
Sure enough - there was no evidence of any CRT-retrace blanking circuitry at all! One could only assume that more recent (or better designed) receivers must include such blanking, because our old chassis was one of the few on which I'd ever noticed this effect. So - how to get rid of the problem?
It seemed obvious that what was needed here was some additional circuitry
that would clamp the video signal down to black (or below) during the vertical
(field) retrace interval.
I eventually managed to get this single transistor switch system working to a
greater or lesser extent. I could clean out a certain amount of the effect - but
it was still disappointingly short of the mark.
At this point, the idea of some digital logic went through my mind.
Over the following weekend, I put the CRO on the set once more and checked out the waveform and gave it some more thought. "Why not a bistable flip-flop? That should allow some control over the pulse length, if nothing else, and it'll be absolutely "on" or "off", ie: a nice simple, predictable switch. Why not give that a try?"
So once again it was out with the pen and paper, now drawing up a bistable. I then wired it up and tried it, and this time things looked much more promising. Initially it would turn off too early, but this was soon traced to my poor choice of sampling point (on the vertical output circuit) and a coupling capacitor which at 470pf was too low. So I fixed that, moved the input to the actual vertical oscillator circuit (which is an 18 volt square wave instead the 120 volt sawtooth wave I'd been sampling) - and now the timing was much closer to what I wanted. Spot on, in fact.
My only other problem now was that there seemed to be no convenient spot in the circuit to which I connect the output of this little rat's nest circuit to completely blank the video. I could reduce it, sure - but not completely kill it. It was frustrating in the extreme. All I wanted was a spot where I could ground the video signal or pull it up to 18 volts for black, and yet there seemed to be no such point! I could hardly believe it - every obvious output connection point produced problems of one sort or another. So once again I seemed to be going around in circles.
It occurred to me that I should just try running the flip-flop output into a
high voltage NPN transistor and use that to ground the CRT control grid.
I checked the effect of grounding the grid circuit, and going that extra 100 volts negative certainly blacked things out. So I grabbed an MJE340, wired it up to the CRT board, and connected the output of the flip-flop to it. And hey - bingo - success at long last!
I finally had reason to feel pleased with myself.
Here's a few pictures.
(The protection diode on the RHS is uneccessary, of course - I only included
it because I was feeding signal to that side in one of my earlier
In my experiences with restoring valve television sets, particularly ones
from the 1950's, one of the most common faults is the vertical blocking
oscillator transformer failure.