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A long long video on building feedback amplifiers

 

Gang,
I have recorded a video on how to build feedback amplifiers. It is little?long. Do take a look if you are into homebrewing. I explain what a feedback amplifier is, how to choose the component values, how to build it and how to measure all the aspects like gain, input and output impedances and IIP3.

- f
 

开云体育

Thanks!!


On Jun 20, 2020, at 11:20, Ashhar Farhan <farhanbox@...> wrote:

?
Gang,
I have recorded a video on how to build feedback amplifiers. It is little?long. Do take a look if you are into homebrewing. I explain what a feedback amplifier is, how to choose the component values, how to build it and how to measure all the aspects like gain, input and output impedances and IIP3.

- f
 

Farhan - it's still an excellent video and calculator - thank you!
?
Are there any rules of thumb about selecting the isolation resistor for the power input? I've seen from 10 to 220 Ohms. Is it calculated like a current limiter, or??
?
I've been experimenting with connecting multiple stages of amplifiers together.? I was surprised at the interaction between transformer coupled stages designed for 50 Ohms in and out. After tweaking a W8DIZ 2SC5551A amp for 17 dB of gain, gain of the first stage jumped about 5 dB after connecting? the second 17 dB stage. Does that sound normal?
?
There's art and science in this RF stuff.? Thanks for your work, and for easing the transition to W7ZOI'S feedback amp models.
?
73, Andy
?
On Sat, Jun 20, 2020 at 10:20 AM, Ashhar Farhan wrote:

Gang,
I have recorded a video on how to build feedback amplifiers. It is little?long. Do take a look if you are into homebrewing. I explain what a feedback amplifier is, how to choose the component values, how to build it and how to measure all the aspects like gain, input and output impedances and IIP3.
- f
 

Andy,
It is a pleasant surprise that you sat through it all!
These amplifiers need 'strong' terminations at both ends. If two amps cascade, this is usually lost. A trick is to add a 6dB attenuator between the stages. However, I havent noticed such a drastic drop of gain. There is a catch though.
Are you sure the second stage is not saturating? This is where a step attenuator is really effective tool to have. Reduce the input power by 10 dB and check?
The isolation resistors are chosen for a compromise between sufficient isolation and the current draw.
If your amplifier is drawing 20mA current, then a 220 ohms resistor will show 4 volts of drop! I usually gey away with 10 ohms unless the amp will draw more than 50mA. Usually with higher current, you have to increase the bypass capacitance and decrease the isolation resistance to keep the RF from leaking in and out of the DC line.
A more elaborate technique, often used with power stages is to use an inductor in place of the resistor. This is not a casual choice. You must not use the cheap molded chokes. They have a self resonance frequency well below the RF frequencies of our operations. This means that they are behaving more like capacitors rather than inductors! Instead, choose an inductance that has around 50 ohms reactance at the lowest operating frequency and wind it on a ferrite toroid. Remeber to bypass both ends of the inductor.


On Sun, Sep 22, 2024, 5:07 AM AndyH via <awhecker=[email protected]> wrote:
Farhan - it's still an excellent video and calculator - thank you!
?
Are there any rules of thumb about selecting the isolation resistor for the power input? I've seen from 10 to 220 Ohms. Is it calculated like a current limiter, or??
?
I've been experimenting with connecting multiple stages of amplifiers together.? I was surprised at the interaction between transformer coupled stages designed for 50 Ohms in and out. After tweaking a W8DIZ 2SC5551A amp for 17 dB of gain, gain of the first stage jumped about 5 dB after connecting? the second 17 dB stage. Does that sound normal?
?
There's art and science in this RF stuff.? Thanks for your work, and for easing the transition to W7ZOI'S feedback amp models.
?
73, Andy
?
On Sat, Jun 20, 2020 at 10:20 AM, Ashhar Farhan wrote:
Gang,
I have recorded a video on how to build feedback amplifiers. It is little?long. Do take a look if you are into homebrewing. I explain what a feedback amplifier is, how to choose the component values, how to build it and how to measure all the aspects like gain, input and output impedances and IIP3.
- f
 

On Sat, Sep 21, 2024 at 04:37 PM, AndyH wrote:
2SC5551A
Terminations as farhan says is important!
?
Also Rf (feedback resistors) vs bias...?
Sometimes that resistor can be juggled to be the bias value.? Often
its too low for that so series resistance bypassed with suitable value
cap gives a valid DC bias with correct feedback.
?
Additionally many devices the FT and Beta vary with emitter current.
Some like the 5551A show curves that improve with increased current
also same for 2n2219A.? Others like the frequently used 2n3904 actually
decreases above about 12-15ma (best peak is 10ma).? Searching for
detailed datasheets helps with getting that information. Some vendors
have notably sparse datasheets making that harder to know.? Looking
at the same device from a different vendor often results in a more
detailed datasheet.
?
One thing seen with very high FT devices is oscillation at vhf or uhf
(usually well above frequency of interest) and usually in multi stage
amplifiers that often shows as reduced gain or less than expected
power.? For example the FT for the 2sc5551A 3.5ghz!? ?With a gain
bandwidth product layout and construction is a factor.? Sloppy long
leads and poor grounds will bite back.
?
At the other extreme the usual 2n3904 the FT is 300mhz which varies
from 250 to 380mhz or more.? The problem with that is; build a gain
of 17DB for use at 28mhz...? it does not net the correct gain in practice
due to low FT, it has more than enough beta.? The test is
FT/frequency-max. so for a gain of 20 (13db) at 28mhz the FT must
be more than 560mhz (500mhz will likely do)!? ?Conversely the best
a 300mhz part can do?across many samples is about 10 (10db!).
?
Resistive feedback can also be used with jfets and mosfets as well
The calculations are different due to the gain nature of the devices.
FETs are voltage to current so the calculations need some math
translation.? Generally it works the same and FETs (mos or junction)
do not for the most part exhibit FT.
?
Building wideband amplifies is actually more difficult than the
basic calculations from W1ZOI suggest as they hold well for
small signal at below VHF.? ?As we go up the reactive properties
and unintended feedback become a more significant factor.
?
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
 

Farhan,
?
? ?Your feedback amp page lives in a tan on my phone that's been open for more than a year now. Truly - thank you for the vid and calculator.
?
? ?Thank you for your observations - I will add them to my notebook and work to internalize them!
?
? ?Best,
? ? Andy
 

Minus seven for the pre-release QC fail.? The page lives in an open tab.
?
I'll add that the more about amplifiers I understand, the more fun they're becoming. Fun is good!
?
73
 

That was a very informative video. I'm primed to try some ugly construction for the first time (more or less) since I build the Ugly Weekender back around 1999.
?
I wonder if anyone happens to have the manual for the Antduino, assuming there was one. If so, I'd love to have a copy of it.
?
73
?
Nick, WA5BDU
?
 

Nick, See this ?


On Wed, Sep 25, 2024, 12:24 AM Nick Kennedy via <kennnick=[email protected]> wrote:
That was a very informative video. I'm primed to try some ugly construction for the first time (more or less) since I build the Ugly Weekender back around 1999.
?
I wonder if anyone happens to have the manual for the Antduino, assuming there was one. If so, I'd love to have a copy of it.
?
73
?
Nick, WA5BDU
?
 

Ashar when you discontinue a device you might want to put the gerbers in the github where those who want to get one and build it can get the boards made.


On Tue, Sep 24, 2024 at 11:26?PM Ashhar Farhan via <farhanbox=[email protected]> wrote:

Nick, See this ?


On Wed, Sep 25, 2024, 12:24 AM Nick Kennedy via <kennnick=[email protected]> wrote:
That was a very informative video. I'm primed to try some ugly construction for the first time (more or less) since I build the Ugly Weekender back around 1999.
?
I wonder if anyone happens to have the manual for the Antduino, assuming there was one. If so, I'd love to have a copy of it.
?
73
?
Nick, WA5BDU
?
 

Thanks, Farhan. I was hoping an assembly manual might give info on shielding requirements and so on, plus more explanation of how it works. I was somewhat perplexed as to how square wave excitation could work without use of DSP, as in the nanoVNA for example. But I think I get it now as far as the superhet design with sharp filtering to reject the harmonic components.? It's very clever!

On the ugly FBA - I got inspired enough to give it a shot and managed to create a functional amplifier in an hour or so. I have those somewhat stringy equations from EMRFD in my "everything" excel ham workbook to assist in designing these things.

73

Nick, WA5BDU

On Tue, Sep 24, 2024 at 10:26?PM Ashhar Farhan via <farhanbox=[email protected]> wrote:

Nick, See this ?


On Wed, Sep 25, 2024, 12:24 AM Nick Kennedy via <kennnick=[email protected]> wrote:
That was a very informative video. I'm primed to try some ugly construction for the first time (more or less) since I build the Ugly Weekender back around 1999.
?
I wonder if anyone happens to have the manual for the Antduino, assuming there was one. If so, I'd love to have a copy of it.
?
73
?
Nick, WA5BDU
?

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