Output impedance matching cascode amplifier

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megaknaller

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Hi,

I´m designing an output matching section for a high-frequency cascode amplifier. The simulation shows that the S22 of the amplifier is almost 0 dB (very high output impedance typical for a cascode) and now i have to design a matching section for a 50 Ohm load.

How could i do this? Is it possible to match an almost open circuit to a real impedance (50 Ohms)? Or should i first convert the current output of the cascode to a voltage output through an additional stage like a CE stage? Or use maybe a buffer follower emitter stage before designing the output section?

In advance thanks for any idea!

Regards
 

Output Impedance of Cascode configuration is normally high and a LC combination for narrowband applications is used or simply a resistance may also be used for wideband applications.Whichever the application is, matching is not always necessary because cascode topology is especially used for wideband amplification and matching can still not be maintaned over a wideband.A simple emitter follower after the cascode will complete the amplifier.Nevertheless, there are some different application for that purpose and configuration should be chosen regarding to specific application.If this cascode is an integrated circuit, there is no sense to talk about the matching.
 

Hi BigBoss,

The cascode is designed as a first stage of a multi-stage LNA IC around 140 GHz, therefore, input/output matching is necessary even on the IC and I will eventually need some interstage matching which will be evaluated or optimized once the designed amplifier is at least working in the desired frequency range and delivering some output power around the design goal.
Adding an additional transistor is still an option but it would degrade the noise figure of the amp more than if I could design a matching stage with low-loss components (TL, inductors, capacitors) but I still don´t know if it is possible to match an OC(imaginary impedance) to a relatively small resistance (50 Ohm, real impedance) by using L,C or TL components.

I´ll be thankful for any feedback. Hope my problem is a bit better described.

Regards
 

megaknaller said:
I still don´t know if it is possible to match an OC(imaginary impedance) to a relatively small resistance (50 Ohm, real impedance) by using L,C or TL components.
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Different topologies of L, C, and TL can be used to match almost any imaginary impedance to a small resistance load, but 140GHz frequency is beyond the normal theory for impedance matching.
So when you go searching for related information have to check only applications near this frequency range.
 


Since this cascode amplifier will be the first stage of an LNA chain, the interstage matching won't be very difficult because the input impedance of the second stage will also be vicinity of the output impedance.So impedance matching may be feasible within this configuration.
Why do you intend to use cascode ??? Your bandwidth is so high ?? If it isn't, you don't need to...just a remark.
 

To match a certain resistance you can select custom L and C values.

To obtain 50 ohms inductive reactance, you need a Henry value in the area of 30 to 90 pH.

To obtain 50 ohms capacitive reactance, you need a value in the area of .01 to .07 pF.

Maybe you know if such small components can be fabricated on an IC. I suppose this circuit will be microscopic, since the wavelength of 140 GHz is .084 inches.
 
The intended bandwidth should be as high as possible, the first goal is to get at least 10 GHz and I would be happy if I manage to get something around 20 GHz. Anything beyond that will be of course a win. The cascode configuration is chosen because it´s one of the most simple ones that still can provide "high gain" and stability at this frequency range (I´ve simulated the MAG and it lies around 15 dB but I´m pointing to extract from each stage at least 8-10 dB, maybe more, over the whole BW and end up with a total 35-40 dB gain), which would mean less stages --> less transistors --> less noise.
Btw, the LNA is just the very first part of the whole system (basically a receiver) and for verification purposes I need to test it as a single component at the lab in which all the equipment (power meters and VNAs) have a typical 50 Ohm real impedance, reactive/susceptive impedances cannot help me here.
 

The cascode configuration is not a preferred one at that frequency.Instead, Distributed Amplifier scheme is almost common.Because matching high to lowor low to high is pretty difficult at those frequencies.
Distributed Amplifier configuration is much better for high-end microwave frequencies.
 

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