Impedance Matching (A Match made in heaven)

[Kevin Doyle]

Hi all,
I'm browned off from reading unhelpful literatuire about impedance matching
networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a CC
Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and propper RF
energy.

Q2. How do you know if the input or output impedance has reactive components
in it.
If the CE amplifier above has capacitor on the output does its Zout become
576-j1/WC ohms?

Regards,
Kevin.

PS the avtice devices I'm using are the mps 5179 npn Transistor ft=900Mhz.

 

[Andrew Holme]

Kevin Doyle wrote:

Hi all,
I'm browned off from reading unhelpful literatuire about impedance
matching networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a
CC Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and
propper RF energy.

You don't always need to. Impedance matching is typically required between:
1. antennas and recivers;
2. transmitters and antennas;
3. high-power stages in a transmitter

See the recent thread "How come its OK to mix impedances in a radio
system??" in s.e.b.

Judging by the impedance levels you mention, those are low-power stages.
They can be voltage driven. Between higher power stages, the matching
network is designed to present the driver with just the right dynamic
collector load to suck out the correct amount of power needed to drive the
following stage. You've probably seen examples using P = Vpk^2/2R to
calculate collector load.

At the other end of the power range - in receivers - impedance matching is
important for totally different reasons: to preserve precious pico-watts of
energy, maximising the signal-to-noise ratio. At low and intermediate power
levels, it doesn't matter.

You can work out how much drive a stage needs by subtracting the transistor
manufacturer's stated gain from the desired output power. Or you can do it
by trial and error. It's good to have a way to adjust drive.

Q2. How do you know if the input or output impedance has reactive
components in it.

If the manufacturer's datasheet doesn't tell you, you can always include
trimmers in your circuit to allow for some latitude of adjustment.

If the CE amplifier above has capacitor on the output does its Zout
become 576-j1/WC ohms?

Not if the capacitor and resistor are (effectively) in parallel.

 

[colin]

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsqbDe.2669$R5.543@news.indigo.ie...

Hi all,
I'm browned off from reading unhelpful literatuire about impedance
matching
networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a CC
Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and propper RF
energy.

Theres been a few of these questions lately.

With Zout << Zin the power going into the CE stage will be Vout/Zin.
If you increase the impedance of Zout by adding a series resistor to match
Zin you will see a reduction in the power transfered.
If you decrease zin by adding a parallel resistor to match zout you will
also see a reduction in power transfer.
In both cases you will see a reduction in voltage at the cc amp what is
probably not what you want.

So impedance matching does not necessarily mean you get full and proper
power transfer.
I dont know why they seem to teach this as if its always necessary.

although you might get maximum transfer if zout = zin for high power systems
you wil also get maximum smoke !

as has been said before if you have a multi killowat transmiter with an
internal resistance of 50ohm it will melt, it will usualy have much less.

Impedance matching is usualy only necesary at the receiveing end of
transmision lines or filter stages to ensure they perform properly, you have
no such thing here that needs to be matched.


However you can use a rf transformer to increase the output voltage and
power transfer wich will at the same time increase the Zout by a factor of
n^2.

Q2. How do you know if the input or output impedance has reactive
components
in it.
If the CE amplifier above has capacitor on the output does its Zout become
576-j1/WC ohms?

If its a largish decoupling capacitor in series with the output it can
probably be ignored, the stray capacitances, inductances and transister
charecteristics become an issue at high rf frequencies, like a capacitor in
parallel with the Rc it will add a reactive component.

Colin =^.^=

 

[Kevin Doyle]

Thanks for that guys!
Kevin.
"colin" <no.spam.for.me@ntlworld.com> wrote in message
news:kDcDe.6180$Ox6.5465@newsfe1-win.ntli.net...

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsqbDe.2669$R5.543@news.indigo.ie...
Hi all,
I'm browned off from reading unhelpful literatuire about impedance
matching
networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a CC
Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and propper
RF
energy.

Theres been a few of these questions lately.

With Zout << Zin the power going into the CE stage will be Vout/Zin.
If you increase the impedance of Zout by adding a series resistor to match
Zin you will see a reduction in the power transfered.
If you decrease zin by adding a parallel resistor to match zout you will
also see a reduction in power transfer.
In both cases you will see a reduction in voltage at the cc amp what is
probably not what you want.

So impedance matching does not necessarily mean you get full and proper
power transfer.
I dont know why they seem to teach this as if its always necessary.

although you might get maximum transfer if zout = zin for high power
systems
you wil also get maximum smoke !

as has been said before if you have a multi killowat transmiter with an
internal resistance of 50ohm it will melt, it will usualy have much less.

Impedance matching is usualy only necesary at the receiveing end of
transmision lines or filter stages to ensure they perform properly, you
have
no such thing here that needs to be matched.


However you can use a rf transformer to increase the output voltage and
power transfer wich will at the same time increase the Zout by a factor of
n^2.

Q2. How do you know if the input or output impedance has reactive
components
in it.
If the CE amplifier above has capacitor on the output does its Zout
become
576-j1/WC ohms?


If its a largish decoupling capacitor in series with the output it can
probably be ignored, the stray capacitances, inductances and transister
charecteristics become an issue at high rf frequencies, like a capacitor
in
parallel with the Rc it will add a reactive component.

Colin =^.^=

 

[John Larkin]

On Tue, 19 Jul 2005 19:15:21 +0100, "Kevin Doyle"
<irishdude79@REMOVEyahoo.ie> wrote:

Hi all,
I'm browned off from reading unhelpful literatuire about impedance matching
networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a CC
Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and propper RF
energy.

Q2. How do you know if the input or output impedance has reactive components
in it.
If the CE amplifier above has capacitor on the output does its Zout become
576-j1/WC ohms?

Regards,
Kevin.

PS the avtice devices I'm using are the mps 5179 npn Transistor ft=900Mhz.

What frequency are you working at? What are the input and load like?
Ordinary low-freq transistor designs, like resistive-loaded
common-emitter amps and emitter followers, tend to not work at high
frequencies.

John

 

[Roger Lascelles]

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsqbDe.2669$R5.543@news.indigo.ie...

Hi all,
I'm browned off from reading unhelpful literatuire about impedance
matching
networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a CC
Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and propper RF
energy.

Q2. How do you know if the input or output impedance has reactive
components
in it.
If the CE amplifier above has capacitor on the output does its Zout become
576-j1/WC ohms?

Regards,
Kevin.

PS the avtice devices I'm using are the mps 5179 npn Transistor ft=900Mhz.

Kevin.

Time to get a book or two.


Solid State Design for the Radio Amateur - published by ARRL

RF Circuit Design - Chris Bowick - published by SAMS.

Introduction to RF Design - Wes Haywood - Prentice Hall


Flipping thru Bowick's book I see a chapter on Small Signal RF Amplifiers
and one on Power Amplifiers - nice discussion on the difference. Also a
chapter on Impedance Matching". Clear worked examples. You can build
transmitters with this information! Recommended.

Perhaps this thought will help. Mostly, we are not impedance *matching* -
we are impedance *transforming*. The same networks and maths is used for
both. Mostly, we transform the impedance of the load into the impedance
required by the amplifier. The impedance required by an amplifier depends
on its output voltage swing and the power it can safely deliver. In the
rarer cases where we do impedance match, we use transform the load to the
correct value - often 50 ohms - stamped on the amplifier output etc.

Roger

 

[Kevin Aylward]

colin wrote:

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsqbDe.2669$R5.543@news.indigo.ie...
Hi all,
I'm browned off from reading unhelpful literatuire about impedance
matching
networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a
CC Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and
propper RF energy.

Theres been a few of these questions lately.

With Zout << Zin the power going into the CE stage will be Vout/Zin.
If you increase the impedance of Zout by adding a series resistor to
match Zin you will see a reduction in the power transfered.
If you decrease zin by adding a parallel resistor to match zout you
will also see a reduction in power transfer.
In both cases you will see a reduction in voltage at the cc amp what
is probably not what you want.

So impedance matching does not necessarily mean you get full and
proper power transfer.
I dont know why they seem to teach this as if its always necessary.

I dont think they do. Its usally a case of a smart arse proffessor
trying to be clever with a bit of calculas. You know, the bit about
diferentiating and setting to zero. Then the dumb shit students pay no
attention at all as to the conditions of the derivation.

The max power theorem is when, given a *fixed* source impedance, and the
load is varied, then value of load gives max power when RL=RS. If the
load is fixed and we want to find the max power into it, by varying Rs,
the max occurs when Rs=0!

Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[colin]

"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:8ilDe.2139$Pf3.450@fe2.news.blueyonder.co.uk...

colin wrote:
"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsqbDe.2669$R5.543@news.indigo.ie...
Hi all,
I'm browned off from reading unhelpful literatuire about impedance
matching
networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a
CC Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and
propper RF energy.

Theres been a few of these questions lately.

With Zout << Zin the power going into the CE stage will be Vout/Zin.
If you increase the impedance of Zout by adding a series resistor to
match Zin you will see a reduction in the power transfered.
If you decrease zin by adding a parallel resistor to match zout you
will also see a reduction in power transfer.
In both cases you will see a reduction in voltage at the cc amp what
is probably not what you want.

So impedance matching does not necessarily mean you get full and
proper power transfer.
I dont know why they seem to teach this as if its always necessary.

I dont think they do. Its usally a case of a smart arse proffessor
trying to be clever with a bit of calculas. You know, the bit about
diferentiating and setting to zero. Then the dumb shit students pay no
attention at all as to the conditions of the derivation.

Maybe, just seemed to me there does seem to be a surprising number of people
who think this though.
I cant remember how they taught it at uni, maybe I wasnt paying attention !

The max power theorem is when, given a *fixed* source impedance, and the
load is varied, then value of load gives max power when RL=RS. If the
load is fixed and we want to find the max power into it, by varying Rs,
the max occurs when Rs=0!

yes thats a better way of explaining the situation actualy, youl get least
smoke too !

Colin =^.^=

 

[Kevin Doyle]

"Roger Lascelles" <despam_rklasl@aanet.com.au> wrote in message
news:1121824279.6fc618f759faee6d5ba76252aee2835c@teranews...

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsqbDe.2669$R5.543@news.indigo.ie...
Hi all,
I'm browned off from reading unhelpful literatuire about impedance
matching
networks.

I have two questions that need answering if possible.
Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a CC
Buffer with Zin=15Kohms.
How do I match these two circuits for transferring the full and propper
RF
energy.

Q2. How do you know if the input or output impedance has reactive
components
in it.
If the CE amplifier above has capacitor on the output does its Zout
become
576-j1/WC ohms?

Regards,
Kevin.

PS the avtice devices I'm using are the mps 5179 npn Transistor
ft=900Mhz.


Kevin.

Time to get a book or two.


Solid State Design for the Radio Amateur - published by ARRL

RF Circuit Design - Chris Bowick - published by SAMS.

Introduction to RF Design - Wes Haywood - Prentice Hall


Flipping thru Bowick's book I see a chapter on Small Signal RF Amplifiers
and one on Power Amplifiers - nice discussion on the difference. Also a
chapter on Impedance Matching". Clear worked examples. You can build
transmitters with this information! Recommended.

Perhaps this thought will help. Mostly, we are not impedance *matching* -
we are impedance *transforming*. The same networks and maths is used for
both. Mostly, we transform the impedance of the load into the impedance
required by the amplifier. The impedance required by an amplifier depends
on its output voltage swing and the power it can safely deliver. In the
rarer cases where we do impedance match, we use transform the load to the
correct value - often 50 ohms - stamped on the amplifier output etc.

Roger

Hi there,
The circuit I have is in the FM Band 88-108.
Yes I think I need to get a good book on the topic.
I remember my lecturer in college using the system of transfer of max power
for every
bit of theory we did "Thanks Michael!" It looked great and and clean cut on
the board but
now I'm finding it doesn't work in the real work. I am getting some good
help here though, thanks!

Kevin.