Is matching source and amplifier Zin always necessary?

[arebee]

I understand that matching the output impedance of an amplifier to the load will supply the most power to the load.

But is it always necessary to match the input impedance of an RF amplifier to the source impedance? Even when reflections aren't an issue?

Won't more power be delivered to the amplifier's load if the input impedance of the amplifier is much greater than the source impedance?


Regards,
arebee

 

[BigBoss]

It depends on amplifier...
If the amplifier is a high power amplifier at RF frequencies, "optimum load impedance" should be found and matching is done with this impedance.

Or if the amplifier is low power or small signal amlifier, matching should be done with "conjugate matching technique" to obtain highest power in according with " max. power transfer principal".

 

[arebee]

For a small signal amp, don't you want maximum voltage rather than maximum power at the input for maximum power output (e.g. a common gate stage)?

 

[flatulent]

There is also the matter of noise. The source impedance for lowest noise is rarely the value for highest gain.

 

[Lindberg]

arebee,

Conjugate matching the amplifier input will give you the largest voltage swing at the input, and therefore the largest input power. If you have a large input impedance, the maximum voltage swing you will get is equal to the sourve voltage (if the source resistance is low). However, if you use the conjugate impedance, the voltage swing can be much higher than the source voltage.

Best regards,
Peter

 

[arebee]

"Conjugate matching the amplifier input will give you the largest voltage swing at the input, and therefore the largest input power."

Why would you want the largest input power in say an common gate amplifier? Don't you want the largest input voltage in that case?

 

[khouly]

the input matching is related to the amplifier nature
if it is a LNA , then optimum noise matching in input is required

but if it is just a gain stage or driver , so u need the gain so u make a matching for max power

khouly

 

[arebee]

khouly,

I understand matching in a LNA, but for a gain stage don't you want maximum input voltage (as opposed to power)?

Imagine if the source impedance Rs is purely real and the amplifier input impedance Rin is also real.

If Rin=Rs then you "waste" half the input voltage and the amplifier sees a smaller input.
If Rin>>Rs then the amplifier sees the full input voltage, and given a larger output.

 

[Lindberg]

arebee,

If you want maximum voltage swing at the input (or, equivalently, maximum input power) you have to conjugate match the input.

If Rin>>Rs, then the maximum voltage swing at the input of the amplifier is Vs. BUT, if you conjugate match the input, the voltage swing will be MUCH HIGHER than Vs (depending on how large the un-matched Rin is). One example:

Vs=1 V, Rs=50 Ohm. Rin=5000 Ohm (unmatched)
By using e.g. a 10:1 transformer for matching, the voltage swing at the input of Rin will be 10 V (compared to ~1 V in the unmatched case).

Regards,
Peter