# Terminating Impedance?

Discussion in 'Wireless & RF Design' started by yoamocuy, Dec 3, 2010.

1. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
Hey, I've been doing some research on IF transformers and have realized that it is important to know what the terminating impedance is so that it can be matched, but I'm not sure how to find the terminating impedance. I've been looking online for some explanation as to what exactly terminating impedance is but all I can find is the reason why it's important. Could someone please explain terminating impedance/impedance matching to me or give me a link to a site that explains it? Thanks.

2. ### BillO Distinguished Member

Nov 24, 2008
993
137
The terminating resistance is just the resistance 'seen' by the particular winding of the transformer.

As an example, if the transformer is designed with a 60KΩ primary and a 600Ω secondary, then the output of the driving stage should be as close as possible to 60KΩ and the input to the driven stage should as close as possible to 600Ω.

They are built this way for historic reasons, but essentially, because they are hi-Q tuned devices, the impedances need to match closely for them to work. Their design makes it easy to match impedances. Generally speaking, they will be driven by a low impedance diver, so a series resistor can be used on the primary to ensure a perfect match. On the secondary an amplifier with fairly high input impedance usually follows them so that matching is easily done using a parallel resistor.

Some knowledge of the inherent impedance of the driving and driven stages is required in order to select the resistors properly. This can usually be determined from the design of, or specification of, the amplifiers used.

What will you be using for inter-stage amplifiers?

3. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
I'm using BJT's for amplification. If I need my input resistance to match my secondary resistance, 600 in your example, then do I just find the total resistance in AC on the input side?

4. ### BillO Distinguished Member

Nov 24, 2008
993
137
Op amps would make this a ton easier, but that's okay.

Yes, find the AC impedence of the amplifier input and then, given that it's higher than 600Ω, figure out the resistor value needed in parrallel to make it 600Ω.

5. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
Ok thanks so much for the insights, that really clears up a lot of confusion for me.