Impedance Matching

[SeanGerard]

Hi, I have a question about impedance matching. Excuse my laziness; I could probably figure this out, but I was wondering if anyone knew the answer off-hand.

When you are trying to deliver power from a source, say, Rs = 200 Ohms, and Rl = 50 Ohms, you can match at a particular frequency to obtain maximum power transfer. I get this.

What I don't understand, is, for example, why you impedance match from a source, say Rs 200 Ohms, to the impedance seen between the base and emitter (or 'Re', if there is emitter feedback etc.) of a common emitter amplifier. Since a BJT is essentially a current controlled device, why are you trying to deliver maximum power to it? Does delivering maximum power at the 'input' of a single BJT stage guarantee maximum power at the output? I would think you'd want to maximize Ib, or something like that.

Hopefully this is clear.

Ok, I get it now, somewhat. I wrote the above about an hour ago, and haven't submitted it yet. It turns out that delivering maximum power to the input does guarantee maximum power gain as long as the output is matched as well. I think I still need to think about this more, so I'll post it anyway, haha. Happy New Year.

 

[Nigel Goodwin]

You need to consider why you're messing with impedance matching at all - it's only VERY rarely required, and in almost all cases would be a VERY bad thing.

 

[caster.cp]

I agree with Nigel Goodwin.

Unless you're dealing with RF (and VHF, and UHF, and so on), in which case impedance matching is of EXTREME importance.

In RF designs, if the impedance of the load doesn't match with the characteristic impedance of the "line" (the line can be thought of the circuit itself, its wires and patches and stuff), there'll be reflections across the circuit that can ruin everything.

In transistor circuits and so on, it's more important that the signal doesn't get attenuated along its path. So, for voltage signals, the input impedance of your "amplifier" (understand amplifier as "signal conditioning system") should be much higher than your source impedance. Conversely, the output impedance of the amplifier should be much lower than the load impedance, so that most of the signal stays on the load, and not on the amplifier.

For current signals, everything is exactly the opposite.



edit: Unless, of course, you're interested in power transfer. If you want to transfer power from one system to another, their impedances must be matched (didn't said that before because I think you already know this)

 

[crutschow]

edit: Unless, of course, you're interested in power transfer. If you want to transfer power from one system to another, their impedances must be matched (didn't said that before because I think you already know this)

It depends upon whether you are interested in maximum power transfer or efficiency.

In an RF circuit you are usually interested in maximum power transfer so you match impedances (50% of the power is then dissipated in the source and 50% in the load).

For a power line, battery, generator, etc., you are interested in efficiency so you want the source impedance to be as small as possible and much less than the load, so that most of the power is dissipated in the load.