Output Impedance
 

One thing that puzzled me for years was that the output impedance of
an amplifying stage could be much higher than the voltage drop
across the active element divided by the current through it.

Now, clearly as current sources, this would be true, and it could be
measured.,
by changing the loads.

But, how could you calculate such a value up front?

Output Impedance
 

On 16/05/14 20:44, gareth wrote:
One thing that puzzled me for years

You seem to have spent quite a lot of your time puzzled by some, very
basic, problems; have you ever thought that you may have chosen a route
that wasn't 'right' for you?

Output Impedance
 

"gareth" wrote in message
...
One thing that puzzled me for years was
why do I keep posting this crap?

Output Impedance
 

On 16/05/14 20:49, Osama Bean Laden wrote:

One thing that puzzled me for years

You seem to have spent quite a lot of your time puzzled by some, very
basic, problems; have you ever thought that you may have chosen a route
that wasn't 'right' for you?

It isn't a good advent for his old Uni.

Nor will it inspire confidence in anyone accepting his offers of assistance.

Output Impedance
 

"Brian Reay" wrote in message
...

It isn't a good advent for his old Uni.
Nor will it inspire confidence in anyone accepting his offers of
assistance.

39

Output Impedance
 

"gareth" wrote in message
...
One thing that puzzled me for years was that the output impedance of
an amplifying stage could be much higher than the voltage drop
across the active element divided by the current through it.

Now, clearly as current sources, this would be true, and it could be
measured.,
by changing the loads.

But, how could you calculate such a value up front?

Interesting how the great blusterers display their ignorance, either
by not entering the discussion by the device of side-stepping with abusive
remarks*****, or trumpeting complete nonsense ....

Consider the case of your homebrewed 1.5kW linear being set up with QRP for
safety.

Let's say 6W output on a 12V supply. Some claim that this means an
impedance of 24 ohms that has to be matched to the 50 ohm antenna.

So, having done your matching, you now crank up the output to 1.44kW
by increasing the drive, and those some claimers will now say that the
ouput impedance is now 0.1 ohm.

Hang on there! By their argument, you will now be in danger of severely
damaging your pride and joy by driving into what you say is a complete
mismatch, set, not for 0.1 ohm but for 24 ohms.



***** And there are some who are simply too pig-ignorant even to bluster but
they tailgate with glee any abusive remarks in order to justify their own
personality defects.

Output Impedance
 

"gareth" wrote in message
...
"gareth" wrote in message
...
One thing that puzzled me for years was that the output impedance of
an amplifying stage could be much higher than the voltage drop
across the active element divided by the current through it.

Now, clearly as current sources, this would be true, and it could be
measured.,
by changing the loads.

But, how could you calculate such a value up front?

Interesting how the great blusterers display their ignorance, either
by not entering the discussion by the device of side-stepping with abusive
remarks*****, or trumpeting complete nonsense ....

Consider the case of your homebrewed 1.5kW linear being set up with QRP
for safety.

Let's say 6W output on a 12V supply. Some claim that this means an
impedance of 24 ohms that has to be matched to the 50 ohm antenna.

So, having done your matching, you now crank up the output to 1.44kW
by increasing the drive, and those some claimers will now say that the
ouput impedance is now 0.1 ohm.

Hang on there! By their argument, you will now be in danger of severely
damaging your pride and joy by driving into what you say is a complete
mismatch, set, not for 0.1 ohm but for 24 ohms.

You would need to increase the supply volts to at least 200V to give
headroom, unless you want a square wave output.
--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk

Output Impedance
 

"gareth" wrote in message
...
One thing that puzzled me for years was that the output impedance of
an amplifying stage could be much higher than the voltage drop
across the active element divided by the current through it.

Now, clearly as current sources, this would be true, and it could be
measured.,
by changing the loads.

But, how could you calculate such a value up front?

Consider for a moment the equivalent circuit of a current generator
with a shunt impedance feeding a load.

The voltage across the load is the same as that across the current
generator,
and measuring the current coming out of the generator at any one time will
tell you
about the load impedance, but not the shunt impedance.

However, by changing the load impedance and again measuring the voltage
and the current, then you will ahve sufficient information to calcualte the
source impedance.

Output Impedance
 

FranK Turner-Smith G3VKI wrote:
"gareth" wrote in message
...
"gareth" wrote in message
...
One thing that puzzled me for years was that the output impedance of
an amplifying stage could be much higher than the voltage drop
across the active element divided by the current through it.

Now, clearly as current sources, this would be true, and it could be
measured.,
by changing the loads.

But, how could you calculate such a value up front?

Interesting how the great blusterers display their ignorance, either
by not entering the discussion by the device of side-stepping with abusive
remarks*****, or trumpeting complete nonsense ....

Consider the case of your homebrewed 1.5kW linear being set up with QRP
for safety.

Let's say 6W output on a 12V supply. Some claim that this means an
impedance of 24 ohms that has to be matched to the 50 ohm antenna.

So, having done your matching, you now crank up the output to 1.44kW
by increasing the drive, and those some claimers will now say that the
ouput impedance is now 0.1 ohm.

Hang on there! By their argument, you will now be in danger of severely
damaging your pride and joy by driving into what you say is a complete
mismatch, set, not for 0.1 ohm but for 24 ohms.

You would need to increase the supply volts to at least 200V to give
headroom, unless you want a square wave output.

It is a widely distributed misunderstanding that a linear that has
been designed to delever its output in a 50 ohm load always has a
50 ohm output impedance. In fact it almost never has.

Output Impedance
 

On 17/05/2014 07:07, gareth wrote:

So, having done your matching, you now crank up the output to 1.44kW
by increasing the drive, and those some claimers will now say that the
ouput impedance is now 0.1 ohm.

Hang on there! By their argument, you will now be in danger of severely
damaging your pride and joy by driving into what you say is a complete
mismatch, set, not for 0.1 ohm but for 24 ohms.

And this is why valved PAs in older radios (FT-101Es, for example) had
variable impedance matching (the Pi Tank circuit), and why it's good
practice to tweak that matching after increasing the drive to full power
(say 250mA anode current) having first matched at a lower power (100mA
anode current).