Communication Concepts AN762 Application Note - Manual (Page 4)

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PRODUCT TRANSFERRED T
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AN762
4 RF Application Reports
A. B.
Figure 3. Two Variations of the Input and Output Transformers (T1 and T3)
Both transformers are loaded with ferrite material to
provide sufficient low-frequency response. The minimum
required inductance in the one turn winding can be calculated
as:
2 Πf
R
L =
where L = Inductance in µH
R = Base-to-Base or Collector-to-Collector
Impedance
f = Lowest Frequency in MHz
For example, in the 180 Watt version the input transformer
is of 16:1 impedance ratio, making the secondary impedance
3.13
Ω with a 50 Ω interface.
Then:
= 0.31 µH.
6.28 x 1.6
3.13
L =
For the output transformer having a 25:1 impedance ratio to
a 50
Ω interface,
= 0.20 µH.
6.28 x 1.6
2
L =
It should be noted that in the lower power versions, where
the input and output impedances are higher and the
transformers have lower impedance ratios, the required
minimum inductances are also higher.
T2, the collector choke supplying the dc to each collector,
also provides an artificial center tap for T3. This combination
functions as a real center tapped transformer with even
harmonic cancellation. T2 provides a convenient low
impedance source for the negative feedback voltage, which
is derived from a separate one turn winding.
T3 alone does not have a true ac center tap, as there
is virtually no magnetic coupling between its two halves. If
the collector dc feed is done through point E (Figure 1)
without T2, the IMD or power gain is not affected, but the
even harmonic suppression may be reduced by as much
as 10 dB at the lower frequencies.
The characteristic impedance of ac and bd (T2) should
equal one half the collector-to-collector impedance but is not
critical, and for physical convenience a bifilar winding is
recommended.
The center tap of T2 is actually bc (Figure 1), but for
stabilization purposes, b and c are separated by RF chokes
by-passed individually by C8 and C9.
GENERAL DESIGN CONSIDERATIONS
As the primary and secondary windings of T3 are
electrically isolated, the collector dc blocking capacitors
(which may also function as low-frequency compensation
elements) have been omitted. This decreases the loss in RF
voltage between the collectors and the transformer primary,
where every 100 mV amounts to approximately 2 W in output
power at 180 W level. The RF currents at the collectors
operating into a 2
Ω load are extremely high, e.g.:
2.0
180
I
RF
= = 9.5 A, or peak
0.707
9.5
= 13.45 A.
Similarly, the resistive losses in the collector dc voltage
path should be minimized. From the layout diagram of the
lower side of the circuit board (Figure 4), V
CC
is brought
through two 1/4 ″ wide runs, one on each side of the board.
With the standard 1.0 oz. Iaminate, the copper thickness is
1.4 thousands of an inch, and their combined cross sectional
area would be equivalent to AWG #20 wire. This is not
adequate to carry the dc collector current which under worst
case conditions can be over 25 A. Therefore, the high power
version of this design requires 2 oz. or heavier copper
laminate, or these runs should be reinforced with parallel
wires of sufficient gauge.