Load Impedance Matching Examples - Stanford Research Systems SIM954 Operation And Service Manual

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3.5 Load Impedance Matching Examples

Blocking DC currents protects both the amplifier as well as RF trans-
formers which can be damaged by the amplifier's 1 A output cur-
rent capability (especially wideband RF transformers which are often
wound with very thin wires on small cores).
While series capacitors may also be used as DC blocks on the output,
care must be taken that they do not form high Q series resonance
circuits with the transformer's winding inductance. The better way
to avoid DC voltages is to connect both amplifier inputs and outputs
directly to transformers. This adds the least number of poles to the
circuit's transfer function and will lead to a benign and well defined
frequency response.
In this case, the low DC input o set voltage will lead to an output
o set of no more than 5 mV to 10 mV, and the built-in 3.3
resistance will limit DC output currents to a few milliamps – a value
which all but the smallest RF transformers can handle safely and
without signal degradation.
The main advantage of transformer coupling is the added possibility
of load impedance matching and bridge operation which allow the
use of the SIM954 as a small RF power amplifier.
The SIM954 is designed to generate up to 1 A output current into
low impedances and up to 10 V output voltage into 50 . Because
of its low output impedance of 3.3 , however, the amplifier cannot
fully drive into a 50
10V
approximately 200 mA, a factor of five shy of the amplifier's
50
output current limit.
The actual amplifier (without series resistors) will be able to gener-
ate 10.6 V before the overload detection circuit indicates an invalid
operating state. The most power is available at the output when the
actual amplifier produces its highest output voltage and 1 A output
current simultaneously.
This is equivalent to a power matched load resistance of 10.6 . By
subtracting the internal series resistance of 3.3
we arrive at an ideal external load of 7.3 . The most power that can
be extracted from a single SIM954 channel using a 7.3
7.3 W .
peak
To match the ideal load to a 50
a voltage ratio of 50 7 3
can be easily achieved with wideband RF transformers which can
only have a few turns on either primary and secondary side are:
SIM954 300 MHz Dual Inverting Driver Amplifier
load directly, which would limit the current to
system, an output transformer with
2 62 is required. The closest ratios that
Application notes
output
from this ideal load,
load is then