transformer as balun - even mode behavior
the picture shows an ideal transformer balun.
Normally it shows short circuit behavior to even mode excitation.
Is it possible to model open circuit behavior by putting an inifinitely high resistor between the center tap and ground.
I am actually wondering because this would be the same as using a transformer without center tap, and the latter does also show
short circuit behavior to even mode excitation.
Thanks for your comments.
-e
I really wonder what you mean by "even mode excitation" of a single ended input? Can you clarify the question in terms of current and voltage specifications?
Hello, the transformer acts as a BALUN.
The single ended (ground referenced) output from a source at the left side, is transformed to a differential signal at the left side.
In case of a symmetric load connected to the left side, no common (even) mode signal will appear in the circuit. In case of an arbitrary load, the sum of
i1 and i2, which is the unbalanced current (common mode current, even mode current) is unequal zero.
For example i1 and i2 are assumed as currents flowing from the transformer to the right side on the upper and lower path.
I agree with your explanation of balun operation. It's doesn't answer the question what even mode excitation of the left (single ended) side means.
In my view, the only way to see a short (with an ideal transformer) on the left side is to connect a differential short on the right side.
Please see the following picture.
If "v" is generated by a single ended signal source, then "Nv" is the balanced counterpart on the right side. "i1-i2" is the balanced current on the right side.
If we connect an arbitrary load (non-symmetric), then "i1+i2" will no longer be zero. This current flows along the center tap to ground (in case of an ideal transformer).
The question was, if we put an OPEN load to the center tap: Is it the same as a transformer that has open circuit behavior to an unbalanced signal on the right side. UNBALANCED SIGNAL, current = i1+i2, voltage = (v1+v2)/2, with v1=Nv/2 and v2=Nv/2