Impedance matching of a multistage RF power amplifier - Page 1
Forum section: electronics / rf-microwave

author: polcad
author_rank: Contributor
timestamp: April 06, 2025, 09:18:53 am
content: I have designed many RF amplifiers in the past, but there is something that has puzzled me for a long time.Why is interstage matching in multistage (two-stage) RF power amplifiers always done to 50 ohms? For instance, the output impedance of the first-stage transistor might be 10 ohms, and the input impedance of the second stage ldmos might be 3 ohms. However, every design I've seen follows this approach: the output of the first stage is matched from 10 to 50 ohms, and then the input of the second stage is matched from 50 to 3 ohms. The stages are then connected together. Why is it never done directly from 10 to 3 ohms?The impedance step (10:3) is much lower, and it should be easier to match this way. Additionally, the bandwidth would be larger.The only reason I would think of is when one wants to test each stage separately, but ever practical pcb I saw does not provide a connection testpoint at interstage point.

author: szoftveres
author_rank: Regular Contributor
timestamp: April 06, 2025, 03:47:22 pm
content: The perception ofevery2 stage PAs being done this way could come from the overwhelming number of textbook examples vs. real world designs (which rarely get published) - obviously the educative examples are there to demonstrate the concept of impedance matching. Other purpose of introducing an intermediate impedance could be harmonic suppression of the first stage; e.g. a T match made of L-C-L components inherently introduces an intermediate impedance (which even could be 50 ohms) and heavily suppresses upper harmonics as a side effect, which could result in better linear performance.Quote from: polcad on April 06, 2025, 09:18:53 amThis could also be a very good reason, characterizing a 10ohm output or 3 ohm input amp is somewhat hard.

author: polcad
author_rank: Contributor
timestamp: April 06, 2025, 05:29:38 pm
content: Quote from: szoftveres on April 06, 2025, 03:47:22 pmI have been designing RF power amps for base stations for several years and every single one has been done with 50 ohm intermediate impedance. I still do not understand why.QuoteAgreed. But in tha case of a power amplifier those intermediate impedances are very often in the order of a single ohm. Why not to pick the intermediate impedance as for example 12.5 ohm? This has many advantages overs 50 ohm impedance (for example loss) and a simple 1:4 transformer makes the characterisation easy. Although, I have never seen anyone trying to probe the interstage point. We always were tuning the amplifier a whole thing.

author: radiolistener
author_rank: Super Contributor
timestamp: April 06, 2025, 06:10:53 pm
content: I think the reason is that testing it with 50Ω is more easy. It's hard to find coax cable with 10Ω impedance to connect test load.

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2025, 06:35:24 pm
content: Also, RF test equipment almost always has an input impedance of 50 ohms, from HF up to microwave frequencies.For example, power meters, network analyzers, and spectrum analyzers.

author: rf-fil
author_rank: Regular Contributor
timestamp: April 07, 2025, 01:00:58 am
content: I have designed a few of RF PAs, and - yes - I matched the stages to each other without a 50-ohm intermediate point. These designs are with my (ex) employer. Not published...

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 07, 2025, 01:56:54 pm
content: Quote from: polcad on April 06, 2025, 09:18:53 amIMO this is probably the major reason. There's a couple other contributors I could imagine:1. Working at 50 ohms might make component selection easier at extremely high/low frequencies.2. Makes analysis of the design easier. For example when using scattering parameters, having different Z0 at different ports can make analysis far more of a headache IME.

author: polcad
author_rank: Contributor
timestamp: April 08, 2025, 08:25:08 am
content: Quote from: rf-fil on April 07, 2025, 01:00:58 amDid you match complex Z to complex Z or did you go throu some real intermediate impedance, say 5 ohm?For example, assume two stage amp and both stages are MRF186 would you match it directly like below?930MHz , Zin=2.5 + j6.9 , Zout=4.3 + j1.2

author: szoftveres
author_rank: Regular Contributor
timestamp: April 08, 2025, 01:40:31 pm
content: Quote from: polcad on April 08, 2025, 08:25:08 amIn this case I'd just put a resistor in between them, say 1.5 ohm. It's lossy (good for stability) and the worst thing that could happen is that some minimal driving power will be lost on the resistor. Losing even 3dB within the driver stages has no real consequence (the worst thing that could happen is a slight drop on PAE), and there's no real requirement for a perfect match between PA stages, the SNR is pretty high, this is not an LNA where you want to push down NF to the minimum.Since you gave identical S11 and S22 for both stages, I assume that you bias them identically, which means that the driver stage will have plenty of built-in driver capability to compensate for some resistive loss (overall efficiency will be quite low though).

author: tszaboo
author_rank: Super Contributor
timestamp: April 08, 2025, 09:10:43 pm
content: Quote from: TimFox on April 06, 2025, 06:35:24 pmNot only the testing equipment, but also the cables. How do you connect your DUT to any sort of RF testing equipment if you don't have the right impedance cable?

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 09, 2025, 12:51:53 pm
content: If measuring impedance with VNA is it perfectly ok to measure using 50 Ohm cables or any else cable impedance. If correctly calibrated at cable tip is VNA then showing correct impedance at that tip. Direct matching in a single step is what I always do in multistage circuits, both passive and active. Passive can be SAW-filters which by design often need matching.Say that it is needed for testing reasons to temporally replace internal signal source with an 50 Ohm external signal generator for correctly measuring actual gain, then is a temporary impedance match constructed to match signal generator impedance. Impedance match is sometimes not critical if some power is lost in a resistor network but if noise level is critical or DC power consumption is critical, as it often is in headsets and similar thing with small batteries must resistors be avoided and matching do often have double functions to also intentionally mismatch harmonics and then is resitor network less effective. If resistor loss is in range less then a mW is it still adding to total battery consumption so if possible are reactive components always to prefer for matching.A problem measuring non 50 Ohm impedance, especially for small ground-plane is that measuring cable braid can add to ground plane size and affect measured result so that a matching circuit becomes unmatched when measuring cable is desoldered.Then can it be an simplification to do a impedance match in two steps, to 50 Ohm and back again.It have several drawbacks. Costs more components and more losses.Better to solder measuring cable-braid in a non-hot part of ground and using ferrite-tubes or similar to isolate braid from acting an extension of ground plane.

author: CaptDon
author_rank: Super Contributor
timestamp: April 09, 2025, 01:06:17 pm
content: Maybe your '50 ohm' examples have a small amount of validity in solid state RF power amplifiers but not in the tube world of commercial broadcast transmitters. There were a few examples of commercial transmitters built from increasingly bigger power stages with 50 ohm matching between stages and often also included 'interstage' VSWR meters. This was more typical in grounded grid cathode driven stages. In grid driven power stages usually the plate tank of the driving stage provided optimum plate load impedance while also providing optimum grid drive impedance. You could usually find 'I.P.A. Plate Tune' followed by 'P.A. Grid Tune'. The manufacturer usually gave 'ballpark' dial indications for various frequencies, otherwise getting it right by trial and error could destroy the tubes.