Amp output Z

H

Harry Dellamano

Guest
Ok, so I got this amplifier with an internal output resistance of 10 ohms
and we are outside of all feedback loops. It is coupled to the 600 ohm
resistive load thru a 5.6uF capacitor. The output voltage of the amp is
10.0Vrms at 400 Hz with no loads connected. If I measure the Zo with SPICE
using a swept 1 amp current source at the output, I get about 71 ohms which
is the Xc of the coupling cap. If I use a DVM on the real circuit and
measure the output voltage at two different loads, 600 ohms and 300 ohms, I
get about 13.0 ohms using Ro=((E1-E2)/(E1/600)-(E2/300)). I need to measure
the Zo using a DVM and not Ro. By observation it must be 71 ohms. What must
I do??
Harry
 
On Mon, 11 Apr 2005 22:00:13 GMT, "Harry Dellamano"
<harryd@tdsystems.org> wrote:

Ok, so I got this amplifier with an internal output resistance of 10 ohms
and we are outside of all feedback loops. It is coupled to the 600 ohm
resistive load thru a 5.6uF capacitor. The output voltage of the amp is
10.0Vrms at 400 Hz with no loads connected. If I measure the Zo with SPICE
using a swept 1 amp current source at the output,
Click to expand...
"AC" current source?

I get about 71 ohms which
is the Xc of the coupling cap. If I use a DVM on the real circuit and
measure the output voltage at two different loads, 600 ohms and 300 ohms, I
get about 13.0 ohms using Ro=((E1-E2)/(E1/600)-(E2/300)). I need to measure
the Zo using a DVM and not Ro. By observation it must be 71 ohms. What must
I do??
Harry
Click to expand...
Get a new DVM / learn to do complex math ?:)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Harry Dellamano wrote:

Ok, so I got this amplifier with an internal output resistance of 10 ohms
and we are outside of all feedback loops. It is coupled to the 600 ohm
resistive load thru a 5.6uF capacitor. The output voltage of the amp is
10.0Vrms at 400 Hz with no loads connected. If I measure the Zo with SPICE
Click to expand...
*Measure* with SPICE ? You can only *simulate* !

using a swept 1 amp current source at the output, I get about 71 ohms which
is the Xc of the coupling cap. If I use a DVM on the real circuit and
measure the output voltage at two different loads, 600 ohms and 300 ohms, I
get about 13.0 ohms using Ro=((E1-E2)/(E1/600)-(E2/300)). I need to measure
the Zo using a DVM and not Ro. By observation it must be 71 ohms. What must
I do??
Click to expand...
You sound confused. I think you now confused me too.

*Exactly* how are you measuring Zout ? I.e. method, technique etc - detail
please.


Graham
 
"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:425AFD79.DFD5E727@hotmail.com...
Harry Dellamano wrote:

Ok, so I got this amplifier with an internal output resistance of 10
ohms
and we are outside of all feedback loops. It is coupled to the 600 ohm
resistive load thru a 5.6uF capacitor. The output voltage of the amp is
10.0Vrms at 400 Hz with no loads connected. If I measure the Zo with
SPICE

*Measure* with SPICE ? You can only *simulate* !

using a swept 1 amp current source at the output, I get about 71 ohms
which
is the Xc of the coupling cap. If I use a DVM on the real circuit and
measure the output voltage at two different loads, 600 ohms and 300 ohms,
I
get about 13.0 ohms using Ro=((E1-E2)/(E1/600)-(E2/300)). I need to
measure
the Zo using a DVM and not Ro. By observation it must be 71 ohms. What
must
I do??

You sound confused. I think you now confused me too.

*Exactly* how are you measuring Zout ? I.e. method, technique etc -
detail
please.


Graham
Click to expand...
Oops,
I simulate with SPICE using the normal 1 amp AC current source and get 71
ohms at 400Hz which sounds correct.
I measure the actual circuit with a good DMM and using the above formula
calculate about 13 ohms.
So how do I measure the actual circuit to get the proper 71 ohms output
impedance?
thanks,
Harry
 
Hello Harry,

Just a brief comment. I never had much luck with DVMs in such
situations. In our lab I use a true RMS meter for that, in this case a
Rhode&Schwarz UVM which reads exact levels from 10Hz to 15MHz.

For comparative measurements in the audio frequency range it is possible
that even the sound card in your PC could be superior to a regular DVM.

Regards, Joerg

http://www.analogconsultants.com
 
"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:2RD6e.573$J12.150@newssvr14.news.prodigy.com...
Hello Harry,

Just a brief comment. I never had much luck with DVMs in such situations.
In our lab I use a true RMS meter for that, in this case a Rhode&Schwarz
UVM which reads exact levels from 10Hz to 15MHz.

For comparative measurements in the audio frequency range it is possible
that even the sound card in your PC could be superior to a regular DVM.

Regards, Joerg

http://www.analogconsultants.com
Click to expand...
Hi Joerg, thanks for your response.
But I am only dealing with pure sine waves, THD<1% and my DMM has excellent
response at 400Hzs.
Thanks,
Harry
 
Harry Dellamano wrote:

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:425AFD79.DFD5E727@hotmail.com...

Harry Dellamano wrote:

Ok, so I got this amplifier with an internal output resistance of 10
ohms
and we are outside of all feedback loops. It is coupled to the 600 ohm
resistive load thru a 5.6uF capacitor. The output voltage of the amp is
10.0Vrms at 400 Hz with no loads connected. If I measure the Zo with
SPICE

*Measure* with SPICE ? You can only *simulate* !

using a swept 1 amp current source at the output, I get about 71 ohms
which
is the Xc of the coupling cap. If I use a DVM on the real circuit and
measure the output voltage at two different loads, 600 ohms and 300 ohms,
I
get about 13.0 ohms using Ro=((E1-E2)/(E1/600)-(E2/300)). I need to
measure
the Zo using a DVM and not Ro. By observation it must be 71 ohms. What
must
I do??

You sound confused. I think you now confused me too.

*Exactly* how are you measuring Zout ? I.e. method, technique etc -
detail
please.


Graham


Oops,
I simulate with SPICE using the normal 1 amp AC current source and get 71
ohms at 400Hz which sounds correct.
I measure the actual circuit with a good DMM and using the above formula
calculate about 13 ohms.
So how do I measure the actual circuit to get the proper 71 ohms output
impedance?
Click to expand...
I didn't check your formula in detail. Are you sure it's right ? Maybe the
change in load from 600 to 300 R is too small to get a meaningful result ? Don't
forget that the output Z will be 10R-71j.

Maybe the complex part is causing the trouble ? Is the load purely resistive ?


Graham
 
On Mon, 11 Apr 2005 23:41:47 GMT, "Harry Dellamano"
<harryd@tdsystems.org> wrote:

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:2RD6e.573$J12.150@newssvr14.news.prodigy.com...
Hello Harry,

Just a brief comment. I never had much luck with DVMs in such situations.
In our lab I use a true RMS meter for that, in this case a Rhode&Schwarz
UVM which reads exact levels from 10Hz to 15MHz.

For comparative measurements in the audio frequency range it is possible
that even the sound card in your PC could be superior to a regular DVM.

Regards, Joerg

http://www.analogconsultants.com

Hi Joerg, thanks for your response.
But I am only dealing with pure sine waves, THD<1% and my DMM has excellent
response at 400Hzs.
Thanks,
Harry
Click to expand...
But you're missing the phase angle difference at the two differing
loads.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
On Mon, 11 Apr 2005 23:02:49 GMT, "Harry Dellamano"
<harryd@tdsystems.org> wrote:


Oops,
I simulate with SPICE using the normal 1 amp AC current source and get 71
ohms at 400Hz which sounds correct.
I measure the actual circuit with a good DMM and using the above formula
calculate about 13 ohms.
So how do I measure the actual circuit to get the proper 71 ohms output
impedance?
Click to expand...
---
Here's your circuit:



+---[10R]--> >----+
| |
| [5.6ľF]
| |
[GEN] +
| |
| [600R]
| |
+----------> >----+
| |
GND GND

What you know for sure is that you have an AC voltage source (the
generator) in series with 10 ohms and that before the generator's
connected to the load, {5.6ľF in series with 600 ohms) you're getting
10V at 400Hz between the uncommitted end of the resistor and ground.

What you want to know, you say, is what to measure so that the "output
impedance" will be 71 ohms.

I'm confused. There is no 71 ohm output impedance anywhere in the
circuit. There _is_ a 71 ohm reactance which the capacitor will
exhibit at 400Hz, and which will influence the impedance of the
circuit, but all you need to know to determine that is the output
frequency of the generator and the capacitance of the capacitor.

Likewise, you don't need to know anything to determine the impedance
of the load except the resistance of the resistor and the reactance of
the cap. Knowing that, you can write:



Z = sqrt (R˛ + X˛)


and plugging in what we have so far, determine the load impedance:


Z = sqrt (600˛ + 71˛) ~ 602 ohms


So I don't understand what you're trying to do.

--
John Fields
Professional Circuit Designer
 
Hello John,

I believe Harry wants to verify the output impedance of the amp.
Considering that it's in the 10ohm range and the load is a few hundred
the required accuracy is a bit of a stretch for the AC circuitry of many
DVMs.

Regards, Joerg

http://www.analogconsultants.com
 
"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:z0F6e.1871$dT4.115@newssvr13.news.prodigy.com...
Hello John,

I believe Harry wants to verify the output impedance of the amp.
Considering that it's in the 10ohm range and the load is a few hundred the
required accuracy is a bit of a stretch for the AC circuitry of many DVMs.

Regards, Joerg

http://www.analogconsultants.com
Click to expand...
We are trying to measure the output Z across the 600 ohm resistive load at
400HZ. We measure about 13 ohms using the DVM method and the formula given
previously. No complex math is necessary because we are looking across a
stepped resistive load. We step the 600 ohm load to 300 ohms to obtain delta
Eo.
By inspection the output Z is 600 ohms resistance in parallel with a 70
ohm reactance. Is that not about 60 ohms impedance?
What am I missing?
Harry
 
Hello Harry,

By inspection the output Z is 600 ohms resistance in parallel with a 70
ohm reactance. Is that not about 60 ohms impedance?
What am I missing?
Click to expand...
Assuming the 13ohms are correct these would add to the Z of the cap. I
don't see anything missing. But all that is only good at 400Hz.

Regards, Joerg

http://www.analogconsultants.com
 
On Tue, 12 Apr 2005 01:24:06 GMT, "Harry Dellamano"
<harryd@tdsystems.org> wrote:

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:z0F6e.1871$dT4.115@newssvr13.news.prodigy.com...
Hello John,

I believe Harry wants to verify the output impedance of the amp.
Considering that it's in the 10ohm range and the load is a few hundred the
required accuracy is a bit of a stretch for the AC circuitry of many DVMs.

Regards, Joerg

http://www.analogconsultants.com

We are trying to measure the output Z across the 600 ohm resistive load at
400HZ. We measure about 13 ohms using the DVM method and the formula given
previously. No complex math is necessary because we are looking across a
stepped resistive load. We step the 600 ohm load to 300 ohms to obtain delta
Eo.
By inspection the output Z is 600 ohms resistance in parallel with a 70
ohm reactance. Is that not about 60 ohms impedance?
What am I missing?
Click to expand...

---
Dunno, but here's are your circuits:


+---[10R]---------+------E1A
| |
| [5.6ľF]
| |
[GEN] +------E2A
| |
| [600R]
| |
+-----------------+
| |
GND GND



+---[10R]---------+------E1B
| |
| [5.6ľF]
| |
[GEN] +------E2B
| |
| [300R]
| |
+-----------------+
| |
GND GND

How about replacing E1A, E1B, E2A, and E2B with the voltages you
actually measured at those points?


--
John Fields
Professional Circuit Designer
 
On Tue, 12 Apr 2005 01:24:06 GMT, the renowned "Harry Dellamano"
<harryd@tdsystems.org> wrote:

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:z0F6e.1871$dT4.115@newssvr13.news.prodigy.com...
Hello John,

I believe Harry wants to verify the output impedance of the amp.
Considering that it's in the 10ohm range and the load is a few hundred the
required accuracy is a bit of a stretch for the AC circuitry of many DVMs.

Regards, Joerg

http://www.analogconsultants.com

We are trying to measure the output Z across the 600 ohm resistive load at
400HZ. We measure about 13 ohms using the DVM method and the formula given
previously. No complex math is necessary because we are looking across a
stepped resistive load. We step the 600 ohm load to 300 ohms to obtain delta
Eo.
By inspection the output Z is 600 ohms resistance in parallel with a 70
ohm reactance. Is that not about 60 ohms impedance?
Click to expand...
Mmm.. I think more like 69.5 ohms..


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:1lam5154fnuhm080jnc1690dk88prbjaah@4ax.com...
On Tue, 12 Apr 2005 01:24:06 GMT, the renowned "Harry Dellamano"
harryd@tdsystems.org> wrote:


"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:z0F6e.1871$dT4.115@newssvr13.news.prodigy.com...
Hello John,

I believe Harry wants to verify the output impedance of the amp.
Considering that it's in the 10ohm range and the load is a few hundred
the
required accuracy is a bit of a stretch for the AC circuitry of many
DVMs.

Regards, Joerg

http://www.analogconsultants.com

We are trying to measure the output Z across the 600 ohm resistive load
at
400HZ. We measure about 13 ohms using the DVM method and the formula given
previously. No complex math is necessary because we are looking across a
stepped resistive load. We step the 600 ohm load to 300 ohms to obtain
delta
Eo.
By inspection the output Z is 600 ohms resistance in parallel with a 70
ohm reactance. Is that not about 60 ohms impedance?

Mmm.. I think more like 69.5 ohms..


Best regards,
Spehro Pefhany
--
Click to expand...
Hey Spehro,
If it is 69.5 ohms how do I measure/calculate this given only the load to
vary and measure with a DMM. I get 13 ohms when I measure/calculate. Do I
need other equipment and/or the node on the other side of the coupling cap?
I do like your 69.5 ohms but can't measure it.
How is your cooking show doing?
Harry
 
On Mon, 11 Apr 2005 22:00:13 GMT, "Harry Dellamano"
<harryd@tdsystems.org> wrote:

Ok, so I got this amplifier with an internal output resistance of 10 ohms
and we are outside of all feedback loops. It is coupled to the 600 ohm
resistive load thru a 5.6uF capacitor. The output voltage of the amp is
10.0Vrms at 400 Hz with no loads connected. If I measure the Zo with SPICE
using a swept 1 amp current source at the output, I get about 71 ohms which
is the Xc of the coupling cap. If I use a DVM on the real circuit and
measure the output voltage at two different loads, 600 ohms and 300 ohms, I
get about 13.0 ohms using Ro=((E1-E2)/(E1/600)-(E2/300)). I need to measure
the Zo using a DVM and not Ro. By observation it must be 71 ohms. What must
I do??
Click to expand...
Harry,
Few things are suspect.
Your equation has some sign and parentheses problems.
It might be Ro=(E1-E2)/((E2/300)-(E1/600))

Your equation is for pure resistance. This is a complex number
problem. Use Spice to measure the output amplitude like you would with
a DMM. I'll bet you end up with the same wrong answer.

You need to deal with complex numbers since your series capacitor
introduces a significant phase angle at 400 Hz. At 400 Hz your output
impedance is 10-j71 Ohms.

Mark
 
On Mon, 11 Apr 2005 20:43:14 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Tue, 12 Apr 2005 01:24:06 GMT, "Harry Dellamano"
harryd@tdsystems.org> wrote:


"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:z0F6e.1871$dT4.115@newssvr13.news.prodigy.com...
Hello John,

I believe Harry wants to verify the output impedance of the amp.
Considering that it's in the 10ohm range and the load is a few hundred the
required accuracy is a bit of a stretch for the AC circuitry of many DVMs.

Regards, Joerg

http://www.analogconsultants.com

We are trying to measure the output Z across the 600 ohm resistive load at
400HZ. We measure about 13 ohms using the DVM method and the formula given
previously. No complex math is necessary because we are looking across a
stepped resistive load. We step the 600 ohm load to 300 ohms to obtain delta
Eo.
By inspection the output Z is 600 ohms resistance in parallel with a 70
ohm reactance. Is that not about 60 ohms impedance?
What am I missing?


---
Dunno, but here's are your circuits:


+---[10R]---------+------E1A
| |
| [5.6ľF]
| |
[GEN] +------E2A
| |
| [600R]
| |
+-----------------+
| |
GND GND



+---[10R]---------+------E1B
| |
| [5.6ľF]
| |
[GEN] +------E2B
| |
| [300R]
| |
+-----------------+
| |
GND GND

How about replacing E1A, E1B, E2A, and E2B with the voltages you
actually measured at those points?
Click to expand...
Harry, If I understand correctly, you are measuring E2 and your ro is
everything back from there including the Xc.

I get this equation:

ro = r1r2(e2-e1)/(e1r2-e2r1)

r1 and r2 are 300 and 600 for your test and e1 and e2 are the voltage
across it. Hope I got the math right.
I don't understand why you have Xc in parallel with something in your
explanation.
 
Hello Spehro,

Mmm.. I think more like 69.5 ohms..
Click to expand...
Once I pulled a little prank when the financials were discussed. Asked
about what my confidence level would be for the margin I replied "plus
minus 3dB". Some jaws dropped so I quickly explained that this was a
joke and promised to myself never to do that again.

Regards, Joerg

http://www.analogconsultants.com
 
"qrk" <SpamTrap@reson.com> wrote in message
news:fgam51dbabvmr86kfrnfcpie1rev9bsrqm@4ax.com...
On Mon, 11 Apr 2005 22:00:13 GMT, "Harry Dellamano"
harryd@tdsystems.org> wrote:

Ok, so I got this amplifier with an internal output resistance of 10
ohms
and we are outside of all feedback loops. It is coupled to the 600 ohm
resistive load thru a 5.6uF capacitor. The output voltage of the amp is
10.0Vrms at 400 Hz with no loads connected. If I measure the Zo with SPICE
using a swept 1 amp current source at the output, I get about 71 ohms
which
is the Xc of the coupling cap. If I use a DVM on the real circuit and
measure the output voltage at two different loads, 600 ohms and 300 ohms,
I
get about 13.0 ohms using Ro=((E1-E2)/(E1/600)-(E2/300)). I need to
measure
the Zo using a DVM and not Ro. By observation it must be 71 ohms. What
must
I do??

Harry,
Few things are suspect.
Your equation has some sign and parentheses problems.
It might be Ro=(E1-E2)/((E2/300)-(E1/600))

Your equation is for pure resistance. This is a complex number
problem. Use Spice to measure the output amplitude like you would with
a DMM. I'll bet you end up with the same wrong answer.

You need to deal with complex numbers since your series capacitor
introduces a significant phase angle at 400 Hz. At 400 Hz your output
impedance is 10-j71 Ohms.

Mark
Hi Mark,
Click to expand...
If I can only measure and vary the 600 ohm resistive load then all voltages
and currents that I can measure are in phase and complex math is useless.
This yields Zo=13 ohms but it sounds wrong. I like your 70 ohms above but
don't know how to go about measuring it.
OT Got to get a new SPICE puter. You were a big help the last time so I
may bug you again.
Thanks,
Harry
 
On Tue, 12 Apr 2005 02:13:16 GMT, rex <notat@hotmail.invalid> wrote:

Harry, If I understand correctly, you are measuring E2 and your ro is
everything back from there including the Xc.

I get this equation:

ro = r1r2(e2-e1)/(e1r2-e2r1)

r1 and r2 are 300 and 600 for your test and e1 and e2 are the voltage
across it. Hope I got the math right.
I don't understand why you have Xc in parallel with something in your
explanation.
Click to expand...
Oops. That is treating ro as if it is real, which it isn't. Changing the
external load resistance will rotate the phasor, so I over simplified
the problem.
 
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