LNA current output+Mixer current input.Need matching?Help Me

[wccheng]

Dear all,

The LNA output is current output (output is from the drain of the transistor) and this current output will directly connect to the current input of the mixer (This is the connection point which is connected with the drain of the pmos current source at the upper and the drain of the communtating stage). Actually, I do not do conjungate matching between the LNA and Mixer because it is current conneciton. Now, I don't know how could I explain why do I need not to do conjungate match. PLEASE help me.....

Because traditional LNA is voltage output and it will connect to the gate of the transconductance transistor in the mixer. This is possible to do the matching network between these two blocks and give out the required bias at the same time.

HELP ME, THANKS

wccheng

 

[Mazz]

Re: LNA current output+Mixer current input.Need matching?Hel

You have given the answer!
Match means Power match that allows you to deliver the maximum power to a load. If you want to deliver current to the load, you cannot be power matched! You'll have an output high impedance (approximating a current generator) from your LNA (transconductor) output and a low input impedance (is it a Gibert cell?) in your load. I guess it is an integrated circuit approach, and usually no one is surprised to see voltage or current transfer in IC.
Hope it can help.
Mazz

 

[dsjomo]

Re: LNA current output+Mixer current input.Need matching?Hel

1st,
you have to know, WHY you need a matching network. Why power match? For example, how much is the difference of injected power between reflection gamma = -3dB and -10dB? The answer is, the input power nearly doubles when reflection gamma improves from -3dB to -10dB. And if this is the S11 parameter of your LNA, S11 improves from -3dB to -10dB means that a 3dB improvement in power gain, if the LNA is still unilateral (i.e S12 is small enough).

Well, if 3dB is a enticement to you, than a amplifier with an S22 around -3dB deserves to match. Otherwise, if the power gain is large enough to meet the specification, there is no need to do this interface matching network. Though a matching network boosts gain, lower down noise, it degrades linearity.

And the other reason (Imortant) to apply a matching network to our system, is to assure that the SAW filter has power matched interface both at input and output interfaces. Why we have to ensure these interfaces to be power matched? Because we want SHARP EDGEs and FLAT IN-BAND ATTENUATION.

For a system without SAW filter, and the front-end power gain meets the specification, there's no need to pay more to make a matching network unless you have too much Money.

 

[springf2000]

Re: LNA current output+Mixer current input.Need matching?Hel

conjugate match provide the maxim power deliver