# Impedance matching questions?

Discussion in 'General Electronics Chat' started by electronice123, Apr 30, 2014.

1. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
309
0
I have a question on impedance matching.

If I use a transformer as the source then the secondary coil's XL and it's resistance is the source impedance-correct?

If the load is a parallel RC circuit what would be the best way to obtain an impedance match between the source and the load?

Could I insert an inductor between the source and load to match the impedance?

Last edited: Apr 30, 2014
2. ### DC_Kid Distinguished Member

Feb 25, 2008
706
11
do you have a schematic?
impedance is not resistance. Z is a vector of CLR

3. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
309
0

Here's the circuit.
From my calculations at 5kHz the 100mH inductor (source inductance) will have 3.14kΩ of impedance, while the RC load has an impedance of .226Ω. If I used an inductor between the two as an impedance matching device I calculated it would require an impedance of Zl=√3140 x .226 = 26.6Ω. At 5kHz this would require the inductor to be 850μH.

This is my first time dealing with impedance matching so I'm sure I am not considering everything.

4. ### DC_Kid Distinguished Member

Feb 25, 2008
706
11
i suspect the source will have its own CLR, then you are adding in your own RC, etc.

do you know what the Z is of the source @5kHz? you mentioned transformer.

5. ### PRS Well-Known Member

Aug 24, 2008
989
36
The impedance at the transformer secondary will be the impedance at the primary reflected through to the secondary. The primary impedance will be that of the primary XL in parallel with any resistance it is looking at. Say the transformer is being driven by a common emitter with an Rc of 2.2k. Then 2.2k in parallel with the collector resistance of about 100k will be reflected through the transformer to the secondary by the square of the turns ratio. If it is a step down transformer with a 2:1 turns ratio and we ignore XL this resistance will be reduced by a factor of 4 and by guesstimation will be somewhere around 500 ohms. But also present is the capacitance across the secondary which might be somewhere around 30pF. If you need an exact number you'll have to find it experimentally. To model the impedance at the secondary you would draw a source with a resistance of 500 ohms and a capacitor of 30pF to ground and this constitutes a low pass filter. To match this to a load you would make it look like a resistor with 500 ohms impedance and an inductor whose XL cancelled the XC of the 30pF capacitance.

Last edited: Apr 30, 2014
6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
It would be quite unusual for the transformer to have such a high series (leakage) inductance. I could understand the transformer magnetizing inductance being of that magnitude.