impedance matching in a circuit

[rakesh.kumar78910]

i have designed my circuit, but i am confused about impedance matching. i know we can use a resistor for matching, but is it possible to use a capacitor or inductor for matching the impedance in a circuit.

 

[keith1200rs]

For RF circuits the impedance matching is usually with L/C circuits to minimise losses. That may not be appropriate for very wide band signals.

Keith

 

[Los Frijoles]

For impedence matching you can also use balun transformers (they also come in very small surface mount sizes in addition to through hole). They come in different ratios to balance the impedance on one side vs. the impedance on another (for example a 1:2 balun takes a 50ohm impedance line on one side and a 100ohm impedance line on the other).

 

[rakesh.kumar78910]

thanks keith. i am using a RF signal as input and load is a laser diode, which has typical impedance value of 25Ω. so how to connect capacitor and inductor in between then for impedance matching. The working frequency is 2.6GHz.

 

[BradtheRad]

Do you want to obtain resonance at 2.6 GHz and 25 ohms?

If so then you're talking tiny values for the capacitor and inductor. A few pF for the cap.

About 1/1000 of a milliHenry for the inductor. Not very workable.

Or do you intend to amplify the incoming rf? In that case typical radio amplification circuitry could work.

Do you intend to pulse the laser at 2.5 GHz? To modulate its output? Some more components must be added.

 

[keith1200rs]

About 1/1000 of a milliHenry for the inductor. Not very workable.

I would have expected values considerably lower than that - a few nano Henries. However, values like that are perfectly workable and in common use for RF circuits.

Keith

 

[BradtheRad]

I would have expected values considerably lower than that - a few nano Henries. However, values like that are perfectly workable and in common use for RF circuits.

Keith

Yes I should have typed microHenry, not milliHenry.

I used an online crossover calculator to find the values. The units being in milliHenries, and there being so many zeroes after the decimal point...

That makes 1 billionth of a Henry all told. Very unexpected.

I didn't realize they managed to make an inductor that low. Or rather I guess it would be one or two loops of wire.

 

[rakesh.kumar78910]

the input impedance value is 50Ω and the output or load is 25Ω typically . so how to match impedance between those two resistor value and the important thing is , use only capacitor and inductor in the circuit. the input signal is RF signal and the output is a laser diode.the frequency range is 2.6 GHz. could you please show me the maths calculations also for it .

 

[thylacine1975]

Heya rakesh.kumar78910 - when you say the frequency "range" is 2.6 GHz, do you mean you are interested in coupling frequencies from just above DC - 2.6 GHz to your diode, or just a narrow band of frequencies about 2.6 GHz?

The reason for the distinction is it strongly influences the topology of your matching network. The latter case (narrow range of frequencies about 2.6 GHz) is the simplest, although at these frequencies the agreement between calculated lumped element solutions and reality are going to be pretty underwhelming. At these frequencies, transmission line approaches (e.g. microstrip or baluns bent out of semi-rigid coax) are more predictable and useful, although mini circuits do have a few transformers/baluns that might fit your need - e.g. https://www.minicircuits.com/pdfs/NCS2-ED12817-34B23_SPEC.pdf

(Just remember to include the impedance of the DC blocking capacitor between your RF input and the diode bias path as part of the impedance being transformed, since it's possible it's significant at 2.6 GHz.)

(Hyper expensive) software like Agilent's ADS is one way to design transmission line matching networks with a minimum of design/build iterations, although there is an abundance of practical advice on the net at sites like Microwaves101.com - A practical resource covering the fundamental principles of microwave design and Radio, Radar, RF, Microwave, & Wireless Engineering Resources - RF Cafe.

For specific design theory (and worked axamples with discrete Ls and Cs), you can't go past Chris Bowick's awesome "RF circuit design", chapter 4! (RF circuit design - Chris Bowick - Google Books)

Broadband matching (i.e. DC - 2.6 GHz) is quite difficult, and the lossy resistor approach might indeed be the best option if you really need a good match and don't care quite so much about efficiency. Again, transformers might be worth looking into. Remember - short lead lengths and surface mount R's and C's [stripline/small wire loops for L] will be essential whichever path you choose!

 

[ElectronicsRookie]

Impedance matching is used to optimize power/signal transfer between two independent circuits
Try this,
Impedance Matching Network Designer

 

[Syncopator]

..... balun transformers ... balance the impedance on one side vs. the impedance on another ...

A balun (balanced to unbalanced) transformer changes a balanced feed to an unbalanced one, or vice versa.

It may have a 1:1 ratio, a 2:1 ratio a 4:1 ratio or some other figure.