How to do impedance matching of a VHF pre-amplifier using ADS simulation ?

Hello,
i'm trying to match the input and the output stage of a VHF pre-amplifier using simulation in ADS. When i match the output stage everything is ok but when i try to match the input, the output is no longer matched, the gain drops and the stability factor also drops below unity. I guess there is a interrelation between the input and output matching, so whats the proper way of doing this? i'm not very familiar with this rf engineering.

Hello,

Yes you are quite right there is a interrelation between input and output. Especially at RF frequencies If you are not familiar with RF engineering then I suggest you look up S-parameters. There is also a book by called Microwave Engineering by David Pozar. This book deals with S-parameters, gain circles and stability circles. This will help if you are doing an "ordinary" gain amplifier. If you are making a power amplifier it is a different matter.

Regards

The relation is defined by S12 and in case of MOSFETs can be said to be because of Cgd which is across input and output.

Try to tune first the input for best gain and noise figure, and second tune the output for best linearity. For both cases keep good stability of the amplifier.

Hi

If you are making LNA match input first for gain/noise trade-off. Then you can look at the drain to see which impedance is seen there and apply conjugate matching. For PA you start by matching output for optimum power output/efficiency and then conjugately match input. Valid for unconditionally stable transistors, if not you can "kill" some gain by resistive loading to make it unconditionally stable.

flyhigh

For flyhigh

I have seen in some designs that they put a resistive load at the emitter (AC bypassed by a capacitor) to low the gain, other put a resistance in parallel in the output port. I guess this is the techniques you mean for doing stable designs, isnt it?

Hi Draytek

You can use either one, series or parallel resistor in input or output. The excellent explanation is given in

Rowan Gilmore, Les Besser – Practical RF Circuit Design for Modern Wireless Systems Volume II - Active Circuits and Systems, Artech House, 2003

More details on, so called "device modification", which also explains resistive, but inductive and capacitive loadings of a transistor can be found in

Pieter L. D. Abrie – Design of RF and Microwave Amplifiers and Oscillators, Artech House, 1999

flyhigh

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