Designing matching network for high collector/drain Zout

[arebee]

Hi.

I am designing a tuned common emitter RF amplifier at about 100 MHz.

How does one match the high collector output impedance to a low load resistance?

I did a simulation and the collector impedance is rather large, about 100 kohm. How do you match this to say a 50 ohm load?

Is it necessary really to match the interstage impedances in a multistage amplifier? Will reflection be a problem between stages (on the same PCB)?

Regards,
RB

 

[flatulent]

Something is wrong with your simulation. You should get an impedance several orders of magnitude less at 100 MHz.

 

[arebee]

Sorry, I meant to say common-base.

(With a common emitter and Rs=50 ohms I get around |Zout|=400 ohms, but a common base gives me |Zout|=120k ohms)

 

[flatulent]

Still with common base, the shunt capacitance such as Cbc (which should be a few pF) should reduce the impedance. Anyway, there are classical LC networks that can have values which transform any impedance to any other impedance. The problem is the bandwidth. Once you want a wider bandwidth, the load impedance transformed to the transistor side will have a smaller real part.

Why are you operating at common base? This is good for stability, but bad for gain.

 

[arebee]

Yes, I was using a common base for stability. I did some testing and a common base seems to have a higher y21 than a cascode but a slighly higher y12. The trade off of reduced y21 for reduced y12 versus a common emitter seemed reasonable.

I did try a common emitter before but alignability and stability were giving me problems and I had to heavily load the input and output ports to fix it.