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Design >> RF Design >> tuned amplifier
https://designers-guide.org/forum/YaBB.pl?num=1402840247

Message started by adelaid on Jun 15^th, 2014, 6:50am

Title: tuned amplifier
Post by adelaid on Jun 15^th, 2014, 6:50am

Hello All,

I am very new to circuit design; so excuse me for my basic questions. I want to design an amplifier for a signal coming from a PLL; the output should be a very nice sinusoidal at the frequency set by the PLL and with some 20dB gain. I was reading in several books what is the best way to implement this, and I came to two directions: either designing a narrowband amplifier with a collector tank, like in LNA, or design a matching network; I would like to understand what is the approach that best suites my need, cause I see both fit to my end purpose. Thank you in advance!

Title: Re: tuned amplifier
Post by aaron_do on Jun 15^th, 2014, 5:40pm

Hi,

first you need to know a few things.

1) do you want voltage gain or power gain?
2) what is the source impedance coming from the PLL? for example, 50 ohm.
3) what is your load impedance?

Power is V²/R. If you don't want power gain, you can still get voltage gain by changing R. This is the matching network approach. When you design your matching network, you will need to use a low-pass filter configuration. However, the matching network voltage gain will be limited by the source and load impedance since the input power must be equal to the output power. Therefore, you may not be able to do the matching network approach anyway.

If you describe your application, you will get a better answer. For example, is your PLL on the same die as your design? Where is it used (a mixer for example)?

regards,
Aaron

Title: Re: tuned amplifier
Post by adelaid on Jun 16^th, 2014, 10:11pm

Hello Aaron,

thank you for your reply. PLL and this amplifier will be integrated, so what I need to do is like a cascade of amplifiers, between them need voltage gain, but in the end I have to deliver it to a load of 50 ohm then I need power gain. What I plan to do is a cascade of amplifiers, the first one a diff pair tuned (with a tank in the colector tuned at the PLL freq) to filter the input a bit and get a better sinusoidal, and then at the output, some matching networks that I have seen to improve the S22. Is that the right approach? Thank you!

Title: Re: tuned amplifier
Post by aaron_do on Jun 17^th, 2014, 12:11am

Hi adelaid,

the approach isn't necessarily wrong, but maybe it could be improved. Unfortunately, its impossible to know without more info. For example, why do you need voltage gain? The PLL usually provides a good signal swing. What kind of output power do you need? Is the output power spec just for testing purposes? What is the operating frequency? Since you say you need a sinusoid, maybe you have a harmonics spec?

When you have multiple stages of amplification, the distortion primarily comes from the later stages. So I would keep my filtering closer to the end of the chain. You could use a matching network to improve S22 if you really need to, but whether you need to or not depends on your application. The matching network can be of the low-pass type so that you can get your sinusoid.

regards,
Aaron

Title: Re: tuned amplifier
Post by loose-electron on Jun 20^th, 2014, 7:44pm

what is the frequency range of your sinusoid?