1meg to 50ohm Impedance Matching Amplifier for general instrumentation use

[Insatman]

I've wanted such an amplifier for quite some time.  This would allow testing of probes and other devices designed to drive a 1M scope input on a spectrum or network analyzer.  It would also allow the use of a probe at a distance from the scope longer than it's normal cord distance.   These trans-impedance amplifiers are available.  One 400Mhz rated unit sells for $400.   An older Tektronix version sells on Ebay for various amounts but requires an external supply and dealing with its output connector (GR type).   I elected to try and build a simple one using a buffer op-amp on perf-board in a small aluminum enclosure.   I was careful of the geometry and based the design on a example circuit in the datasheet.   The unit has a negative gain of -6dB nominal due to the 50 ohm series output resistor.   Tests on my spectrum analyzer showed the gain varies from about -7 to -5.8dB over the range of 0-100MHz.   -3dB point is at 125MHz when using fresh 9V batteries.   If you drop the voltage to 5V the -3dB point moves to around 110MHz.

It should be possible to improve the performance with a PCB and better op-amp.   The op-amp used for this first attempt was an OPA633KP.  The present unit is more than good enough to use with my spectrum analyzer.  Simply use the tracking generator and terminate into 50ohms at the tran-impedance amp input.  Connect the amp output to the spectrum analyzer input.   The response shown is the variance of the tracking generator and frequency response of the trans-impedance amplifier.  Normalize the data and replace the 50ohm terminator at the trans-amp input with your device under test you are good to go...at least up to 100MHz and abit beyond

[Someone]

That term has a very specific meaning:
https://en.wikipedia.org/wiki/Transimpedance_amplifier
I think you are describing a more common impedance matching network.

[TheUnnamedNewbie]

A very important spec would be the distortion. You can tune out non-flat gain, but you can't tune out distortion. Any measurements on that?

[Insatman]

That term has a very specific meaning:
https://en.wikipedia.org/wiki/Transimpedance_amplifier
I think you are describing a more common impedance matching network.

Interesting.  For years in the company I worked for over 30 years we used the term trans-impedance amplifier for any amplifier that was designed to match a higher (or lower) source impedance to some other impedance...with the most common being high to low version.  It applied to photomultipliers like in the wiki you linked, but the term also applied to amplifiers like I described.  It could be the term has changed meaning over time.

[Insatman]

A very important spec would be the distortion. You can tune out non-flat gain, but you can't tune out distortion. Any measurements on that?

Hmmm....I'm not sure how I would measure it.   It does phase-shift a bit which is only visible at the higher frequencies.

[Insatman]

A very important spec would be the distortion. You can tune out non-flat gain, but you can't tune out distortion. Any measurements on that?

I found this graph in the buffer op-amp datasheet I used.  My load impedance is 100 ohms net, so this should apply for 1V rms amplitude signals for the lower frequencies shown. 

[TheUnnamedNewbie]

In order to really measure distortion you need a basic understanding of what it is and is not. If phase is important for your application, AM-PM distortion could also be important but that is significantly harder to measure (since that requires coherent measurement).

A very crude and simple way: put in a tone at fixed frequency. Measure the output spectrum. What you will see is that in said output, there is not only a component at your frequency but also at harmonics. As you increase the power of the input tone, you will notice that these harmonics grow faster than your fundamental (why follows from the mathematics involved).

Do this at a few frequencies. As you approach the bandwidth of your amplifier you will find the distortion performance gets better since the limited bandwidth acts as a filter to get rid of these harmonics.

Keep in mind that you need to be sure that what you are seeing is harmonics of your amplifier and not of your source. (there are various techniques to do this but I'm not going to go into them here as they would take too much space.

[Insatman]

A very important spec would be the distortion. You can tune out non-flat gain, but you can't tune out distortion. Any measurements on that?

I decided to try a harmonics measurement at 10Mhz because that's where I can generate a fairly clean source using my old HP signal generator and a 10Mhz low-pass filter in series.  The output was adjusted to 0dBm,  and the results shown on the spectrum analyzer as "TA10Mhz Source".   The Amplifier was then driven by the source (with 50ohm terminator) and the output of the amplifier attached to the spectrum analyzer as "TA10MHz Measured".   As you can see the amplitude is down by a bit over 6dB and no harmonics can be seen...even the small ones in the source measurement...presumably because they are also down 6dB.   Am I doing something wrong?   I didn't expect it to be this good.

[TheUnnamedNewbie]

It is possible that your performance is good. Feedback lowers distortion and that is what you will also be seeing. I would suggest going up in power and observe how it changes. I don't know what power you can achieve?

A similar property is the compression. As you increase power you will see that gain decreases. The point at which you see a decrease of 1 dB is called the 1dB compression point. You will find this spec expressed either as OP1dB or IP1dB, which simply refers to what port we are refering this power leven to (input or output: Ie, it compresses 1 dB when we input x dBm or when the output is delivering y dBm. The difference will be one dB less than the gain since ofcourse at this point the gain hass compressed by one dB).

Also lowering the RBW of the spectrum to lower the noise floor if possible so you can see the harmonics.

[Insatman]

It is possible that your performance is good. Feedback lowers distortion and that is what you will also be seeing. I would suggest going up in power and observe how it changes. I don't know what power you can achieve?

A similar property is the compression. As you increase power you will see that gain decreases. The point at which you see a decrease of 1 dB is called the 1dB compression point. You will find this spec expressed either as OP1dB or IP1dB, which simply refers to what port we are refering this power leven to (input or output: Ie, it compresses 1 dB when we input x dBm or when the output is delivering y dBm. The difference will be one dB less than the gain since ofcourse at this point the gain hass compressed by one dB).

Also lowering the RBW of the spectrum to lower the noise floor if possible so you can see the harmonics.

The max power I can generate from my 10MHz source is about 5dB higher than shown.  When I do that I get the first harmonic spike just appearing.  I will try increasing the RBW in future.  I already employed averaging just to get the noise as low as you are seeing.  Thanks for the insight and prodding me in a good direction.  Today I ordered some filters so I can generate several "pure-ish" frequencies for future work.

[tggzzz]

The max power I can generate from my 10MHz source is about 5dB higher than shown.  When I do that I get the first harmonic spike just appearing. 

The "first harmonic" is 10MHz, so I would hope you could see it!

Perhaps you mean 2nd harmonic or third harmonic?

[Insatman]

The max power I can generate from my 10MHz source is about 5dB higher than shown.  When I do that I get the first harmonic spike just appearing. 

The "first harmonic" is 10MHz, so I would hope you could see it!

Perhaps you mean 2nd harmonic or third harmonic?

Yes...I meant the 2nd harmonic. 

[MasterT]

I recently built similar device, using VCA821. It has better freq. performance, low THD and variable gain. I see the price is lower on mouser, almost half compare to OPA633.

[Insatman]

I recently built similar device, using VCA821. It has better freq. performance, low THD and variable gain. I see the price is lower on mouser, almost half compare to OPA633.

I checked on Digikey and the VCA821 is about a $1 cheaper than the OPA633.  It's in a SOIC14 package whereas the OPA633 is DIP.   I will buy a couple of VCA821 for experiments.

[Insatman]

I recently designed and had fabricated a PCB for the amplifier I built on perf board earlier.   A few circuit tweaks were implemented.   The results are a bit better but generally similar.   Response is within +/-1dB  from DC to ~100Mhz.   The nominal gain is -6.2dB.   The -3dB (plus nominal -6.2dB) is ~125MHz.   Crude checks for Harmonic distortion were made at 10MHz and 100MHz.  No harmonics were observed at 10MHz.  At 100MHz the 2nd harmonic is about 40dB down and the 3rd harmonic is over 50dB down.   The PCB uses all through hole components except for some optional SMD capacitors on the back side.  They were installed for these tests.   The PCB can accommodate a variety of coaxial IN/OUT connectors.  BNC straight or right-angle or SMA types.  Various screen shots from my SA, photos and the schematic are shown.  The circuit is intended to be powered by a pair of 9V batteries, but any decent supply could be used.

I intend to experiment with other op-amps suggested on this thread and those efforts are in progress.   

[kony]

Why do you call this trandimpedance amp? It is not.

[tggzzz]

Why do you call this trandimpedance amp? It is not.

Correct.

That has already been pointed out to the OP: https://www.eevblog.com/forum/projects/1meg-to-50ohm-trans-impedance-amplifier-for-general-instrumentation-use/msg1431533/#msg1431533

While it isn't unreasonable to make a mistake and then correct it (e.g. see comments about harmonics), it really doesn't look good for someone to repeat a mistake after it has been pointed out to them.

As I taught my daughter, "it is OK to make new mistakes"!

Hint for the OP: it is a transimpedance amplifier if and only if the input is a current (I) and the output is a voltage (V). Then the gain is V/I, which has units of impedance - and hence the name transimpedance amplifier.

[David Hess]

That term has a very specific meaning:
https://en.wikipedia.org/wiki/Transimpedance_amplifier
I think you are describing a more common impedance matching network.

Interesting.  For years in the company I worked for over 30 years we used the term trans-impedance amplifier for any amplifier that was designed to match a higher (or lower) source impedance to some other impedance...with the most common being high to low version.  It applied to photomultipliers like in the wiki you linked, but the term also applied to amplifiers like I described.  It could be the term has changed meaning over time.

Terms do change.  For instance in the 1960s Tektronix referred to differential circuits as push-pull which leads to some confusion now if reading a circuit description without a schematic.  Modern confusion with the terms difference amplifier, differential amplifier, and instrumentation amplifier all sometimes referring to the same thing (in my opinion incorrectly thanks to Texas Instruments) so this is hardly new.

I have only heard the term transimpedance applied to this function mistakenly for the reason others identify; a transimpedance amplifier is a current to voltage converter.  The oldest oscilloscope documentation I regularly use from the 1960s just calls these circuits cathode or source followers which is not very descriptive of the actual function.  Tektronix started using the term buffer amplifier in the 1980s but this is still not very descriptive.

Steve Roach who worked for HP in the 1990s on oscilloscope front ends and wrote a chapter in Jim Williams' "The Art and Science of Analog Circuit Design" about signal conditioning in oscilloscopes (1) called this function an impedance converter but to me that implies a passive device so I tend to use the term impedance buffer.  Of the terms you could use though, transimpedance amplifier is just going to lead to confusion.

(1) You might want to look this up for some ideas.  Also consider the design of the Tektronix P6202A active probe which provides for a gain of 2 to drive a double terminated transmission line.  I would keep the discrete impedance buffer but replace the transconductance amplifier with a gain of 2 current feedback operational amplifier for simplicity.

[tggzzz]

Terms do indeed sometimes change, but all too often individual companies:

• screw up, e.g. Tek measuring time in units of conductance (S)
• deliberately obfuscate, to try to gain a marketing advantage

The latter is particularly frequent in the software arena, and isn't unknown in the hardware area

As is frequently the case, Lewis Carroll characterised the problem...

"I don't know what you mean by 'glory,'?" Alice said.
Humpty Dumpty smiled contemptuously. "Of course you don't—till I tell you. I meant 'there's a nice knock-down argument for you!'?"
"But 'glory' doesn't mean 'a nice knock-down argument'," Alice objected.
"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean—neither more nor less."
"The question is," said Alice, "whether you can make words mean so many different things."
"The question is," said Humpty Dumpty, "which is to be master—that's all.”
? Lewis Carroll, "Alice in Wonderlnd".

[David Hess]

Terms do indeed sometimes change, but all too often individual companies:

screw up, e.g. Tek measuring time in units of conductance (S)

Where was this?  I give them some slack for using upper case S for seconds in CRT readouts for increased legibility.  There have been complaints about the SI system increasing confusion by replacing the upside down omega with upper case S.

My general solution is to write out Sec or Seconds and use the archaic mhos instead of Siemens.  I would rather be thought archaic but understood rather than modern and misunderstood.  I also put slashes through my zeros and dashes through my 7s and Zs.

[Insatman]

Why do you call this trandimpedance amp? It is not.

Correct.

That has already been pointed out to the OP: https://www.eevblog.com/forum/projects/1meg-to-50ohm-trans-impedance-amplifier-for-general-instrumentation-use/msg1431533/#msg1431533

While it isn't unreasonable to make a mistake and then correct it (e.g. see comments about harmonics), it really doesn't look good for someone to repeat a mistake after it has been pointed out to them.

As I taught my daughter, "it is OK to make new mistakes"!

Hint for the OP: it is a transimpedance amplifier if and only if the input is a current (I) and the output is a voltage (V). Then the gain is V/I, which has units of impedance - and hence the name transimpedance amplifier.

I get it...I did after the first discussion of this subject.  If you noticed my latest post never used the aforementioned forbidden term.  I did not want to start a new thread however because it contains useful info and I'd likely get criticized for doing that was well.   So the thread name will likely cause nit-pickers to beat this dead horse into a fine paste eventually.

I'm NOT going to make a new run of PCBs to remove the forbidden name on the silkscreen.

Unfortunately, all this nomenclature fussiness has distracted from any real technical discussion of the; Impedance changing amplifier, Delta-Z Buffer or thing-a-ma-jig in question.  I do find this item quite useful.   It can be put into a spectrum analyzer - tracking gen loop to allow for characterization of scope probes or other 1Meg output impedance items.  It is also useful for extending the cable on such items in certain test situations.   

Can we get past this BS?  I get it I really do...but the thread bears the "forbidden" name...so deal with it.

[ahbushnell]

I like your logo.  Is that a ceramic thyratron?

[David Hess]

Unfortunately, all this nomenclature fussiness has distracted from any real technical discussion of the; Impedance changing amplifier, Delta-Z Buffer or thing-a-ma-jig in question.  I do find this item quite useful.   It can be put into a spectrum analyzer - tracking gen loop to allow for characterization of scope probes or other 1Meg output impedance items.  It is also useful for extending the cable on such items in certain test situations.   

Impedance converters or buffers are very handy.  Another place they can be used is to drive a 50 ohm frequency counter with a standard high impedance passive prove.  I have seen them used for low frequency network analysis.

With an adapter that includes enough shunt capacitance, an active probe can be used as an impedance converter.

[Insatman]

I like your logo.  Is that a ceramic thyratron?

Yes, it is.  I was my companies expert on hydrogen thyratrons and ignitrons.   Check out the datasheet for one of the tubes I worked with frequently.   It cost upwards of $50K each when I retired.

[tggzzz]

Terms do indeed sometimes change, but all too often individual companies:

screw up, e.g. Tek measuring time in units of conductance (S)

Where was this?  I give them some slack for using upper case S for seconds in CRT readouts for increased legibility.  There have been complaints about the SI system increasing confusion by replacing the upside down omega with upper case S.

Look at any of their scope front panels before the 24xx series. The timebase has units of mS.

I don't cut them any slack for being legibly wrong - because the lower case letters were perfectly legible.

There have been many complaints about the SI system; I've even seen some claim some twats claim SI was the devil trying to replace the "natural" pounds/ounces system.

My general solution is to write out Sec or Seconds and use the archaic mhos instead of Siemens.  I would rather be thought archaic but understood rather than modern and misunderstood.  I also put slashes through my zeros and dashes through my 7s and Zs.

I would rather not appear to be ignorant by using the wrong units. Where appropriate I can give other indications that it is conductance. I too have crossed my 7s and zs since university - but that is principally so I don't misread my (increasingly illegible) scrawl!