High Efficiency Power Amplifier Design:
What Factors Contribute to the Efficiency of an Amplifier
By: Jerry Kleker, W9QXP MAG PRODUCTS
Over the years, a number of 2 meter and 70 CM amplifier designs have been presented. It seems that these have resolved to a couple of basic approaches, i.e. the K2RIW 1/2 wave stripline and the W6PO center fed 1/2 wave plate circuits. These designs do provide an amplifier that gives high output power in a compact package. However, the efficiency of these designs (power out/power in) is in the 50 to 60 percent range.
The writer will present here the factors which control/contribute to the efficiency of an amplifier so that the reader may better understand what to watch for when building or designing a 2 meter or 70 CM power amplifier. If the guidelines are followed, efficiencies of better than 80% at 2 meters and 70% at 70 CM can be obtained.
GROUND GRID OR GRID DRIVEN AMPLIFIERS
Grounded grid operation has become increasingly popular in recent years:
1. Inherently more stable
2. Physical layout of the tube elements lends itself to grounded grid
3. Lower power gain is no problem because of the "bricks" which are available as drivers
4. Most of the driving power is coupled to the plate circuit In addition to circuit protection which prevents operation without plate voltage but with screen voltage (would cause excessive screen current), it is necessary to insure that no grid drive is applied unless both plate and screen voltages are present. Instead of the driver power coupling thru to the output, it would dissipate in the control grid.
However, the 2 meter amplifier which uses the Penta 4CX1600/B that has a unique internal "crossover" feature which brings the control grid connections below the cathode. This allows the cathode and screen to be bypassed together and to ground. Therefore, the higher performance grid driven approach can be used.
INPUT CIRCUIT CONSIDERATION (GROUNDED GRID)
In a grounded grid amplifier, the RF currents in the input and output circuits are complimentary. Losses in the grid/cathode circuit will directly effect plate circuit efficiency. At 70 CM a 3/4 wave HI Q reentrant coaxial cavity is recommended, keeping the characteristic impedance close to 77 ohms.
OUTPUT (PLATE) CIRCUITRY
The ideal plate configuration for highest efficiency would be a coaxial concentric cavity with a half wave plate line having a characteristics impedance of about 77 ohms.
Coaxial Concentric - This approach insures equal and symmetrical RF current paths within the tube as opposed to a stripline extending off to one side of the tube, which creates unequal RF current paths inside the tube.
Half-wave plate line - If a half wave plate line is used, as opposed to a quarter wave, it is not necessary to have an RF return to ground thru a capacitor, which in turn would generate additional loss. The half wave approach is easily obtainable on 70 CM, but on 2 meters would result in a plate circuit that could be over 30 inches in length. The 2 meter amplifier pictured here does use a quarter wave plate line but the bypass capacitor is of high Q and high capacitance. Another alternative, on two meters, is to use a "push pull" amplifier configuration. In push pull there is no RF return to ground other than thru the tubes themselves.
77 ohms characteristic impedance - Due to the relationship of conductor surface resistance and distributed reactance, this number is identified as the best value for highest unloaded Q in a coaxial cavity. The net efficiency of the plate circuit is dependent upon the ratio of unloaded Q to loaded Q. Because of practicality in size, compromise to no less than 50 ohms. It is suggested that the diameter of the plate line be the same as the external anode of the tube.
Another important point is to keep the plate circuit tuning capacitance as low as possible. Large "flappers" which are close to the plate line have high RF currents and therefore induce additional loss. Keep the plate line as long as possible and use a small screw type disk tuning capacitor. Make the coaxial line length such that it resonates with a 1-1/2" to 2-1/2" disk halfway between the cavity line and the cavity wall.
MECHANICAL SUPPORT OF PLATE LINE
If possible, avoid using support posts for the plate line. Attach/clamp the coaxial plate line to the anode with a 2" wide piece of .015 copper or aluminum sheet. Do not use finger stock. Make sure the tube is socketed very tightly, but avoid excessive stress on the tube seals.
The placement of the cooling air inlet is important. The inlet should be placed at the highest voltage point of the plate cavity (lowest current point). The hole can be covered with 1/4" mesh expanded aluminum. GROUND BYPASSING OF TUBE ELEMENTS In any amplifier design it is necessary to provide RF bypassing of some of the tube elements. This is an important factor, as inadequate bypassing can induce additional losses as a result of RF phase shift, feedback, and I2R losses from the extremely high RF currents that occur at the bypass points. If possible, keep the reactance less than .4 ohm and preferably as low as .2 ohm. Use teflon or ruby mica for capacitor dielectrics.
Most amplifiers are designed for class B or AB operation to provide linear SSB operation. Much of our DXing is done on cw, so it is a good idea to take advantage of the higher efficiency that Class C operation offers. Many of the amplifier designs actually ground the control grid or the screen grid to overcome some of the "bypassing" problems mentioned earlier. These approaches generally make it difficult to easily vary the control grid bias. The writer prefers to actually DC ground the cathode of all amplifier designs, making it a simple matter of switching in different values of low power zener diodes in the grid bias power supply to change from class C to a linear class of tube operation (class B or AB).
I have tried to highlight the ways and means of obtaining high efficiency in power amplifier designs. At 1500 watts output, 50% efficiency results in 1500 watts of loss (heat), 75% efficiency is 500 watts of loss (heat).
Here is the checklist:
1. Use a 1/2 wave coaxial or push pull quarter wave plate circuit.
2. Coaxial cavity impedance of about 77 ohms is most desirable (no less than 50 ohms).
3. Have a high L/C ratio in the plate circuit. Use minimal tuning capacitance.
4. Screen and control grid (and plate if used) bypass capacitors must have very low reactance (less than .4 ohm).
5. The input circuit of a grounded grid stage should have very High Q (low loss).
6. Use Class C operation when operating CW. 7. Use teflon for bypasses (AVOID MYLAR).
High efficiency power amplification insures longer tube life, is environmentally good practice, reduces HV power supply requirements (less expensive), offers cooler and greater stability of operation, and besides, we really don't need the heat here in Arizona.
**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Penta Laboratories does not guarantee the usefulness or marketability of products based on this material.