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3 – 8
3.4 Transformers
3.4.1 Input Side Transformer
3.4.2 Power Splitter and Bridge operation
Transformers are inductive loads which are of great importance in
practical applications. The SIM954 has excellent properties in trans-
former circuits.
Transformers can be connected to both the input and the output of the
SIM954, and in many applications such a topology is advantageous.
An input transformer to the SIM954 can, but does not have to be
isolated. Autotransformers and wideband transmission line trans-
formers are equally well suited to drive the module.
An input side transformer without a series capacitor will present a
DC short to the SIM954. Because of the small input o set current,
the additional DC error will be less than 1 mV and is acceptable for
most applications. This circuit has the advantage that it guarantees
that the output is DC free, which is important if the module has to
also drive an output transformer.
The transformer's inductance will form an LC high pass filter with
the 50
input impedance of the module. For an RF transformer with
1 H secondary winding inductance, the 3 dB corner frequency will
be at 7.96 MHz.
It follows that a practical input transformer that covers a lower corner
frequency f should have a secondary winding inductance of at least
f
8 H
. The primary inductance will then be determined by the
MHz
square of the winding ratios.
An input transfomer is often used as a 180 power splitter circuit. The
two outputs of such a splitter can drive the two SIM954 amplifiers in
one module di erentially and 180 out of phase.
The two amplifiers will act as a di erential driver which has twice
the output power of a single channel.
A 180 splitter as shown in figure 3.7 uses a transformer with a
single primary and a split secondary winding with a winding ratio
of
2 : 1. At this ratio it matches both input and output impedances
to 50 . However, the naive transformer circuit omitting R1 would
not isolate the two output ports from each other, which can lead to
crosstalk and unwanted feedback. Wilkinson
1
E.J. Wilkinson, "An N-way Power Divider", IRE Trans. on Microwave Theory &
Techniques, v8, p116-118 (1960).
SIM954 300 MHz Dual Inverting Driver Amplifier
Application notes
1
proposed the shown
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