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Re: 6 m BITX ??

Ron
 

Dear Heinz,
I really appreciate your summurization of all those good sources!
Another proof that IRF (610) can be used on the 'magic' 6 m band.
Up till now I have noticed 6m PA-stages with multiple IRF's only, no
QRP (single IRF) 6m PAs. Maybe qty of MOSFET must be increased in
order to obtain a reasonable RF output because of reduced efficiency
at higher frequencies of IRF family (or just coincidence).
Bye for now, danke sch?n,
Ron
PA2RF


--- In BITX20@..., Heinz Schnait <oe5eep@q...> wrote:
Hello Ron,

There is a design for a 50W V-MOSFET Linear for 50MHz in Pat
Hawkers, G3VA,
"Technical Topics 2000 - 2004", pages 32 - 33, which might be
interesting
for you. This book is a collection of reprints of a monthly column
in the
RSGB Radcom. This article was published in Radcom of July 2000.

The design goes back to a publication of Dieter Mergner, DJ9FG,
originally
appearing in the German "Funkamateur" 12/1999 and 01/2000. It uses
two
IRF610 in push-pull with a 50V power supply. I was surprised to see
IRF
MOSFETS at 50MHz. Some years ago I have build some single ended as
well as
push-pull designs myself, but I couldn't get a reasonable output
above 14
MHz. I worked with a maximum supply voltage of 30V. I have a better
power
supply now. So maybe I should try MOSFETS again...

The book of Pat Hawker is worth reading every line of it!

73 Heinz, OE5EEP

Am Mittwoch, 29. Juni 2005 12:10 schrieb Ron:
Hi fellow BITX-homebrewer,
Yesterday I was browsing through some old "Electrons" (Dutch Ham
magazine) and found an interesting article in which it was told
that
our famous MOSFET IRF-family (in particular IRF510 and IRF540)
could
work on 50 MHz ! I am surprised by this. The article did not show
fixed designs, so maybe it is just wishfull thinking...
According to the article, achievable power levels should less
lower
than at, let's say 7 MHz ( 2 Watts versus 12 Watts RF).
Anybody experience in this field ?
Re: 6 m BITX ??

Ron
 

Sure Chris,

SBL have a bi-directional nature.
Signetics NE612/602 not. Additional switching required! Or 2 times
NE6..?
Thanks for advice,
Ron

--- In BITX20@..., "vdberghak" <vdberghak@z...> wrote:
Indeed, there you have your RF input circuitry.
Not the output. Sinse BITX is a transceiver, the SBL solution looks
more relevant to me. In fact, you can build double balanced mixers
yourself like the ones in the current BITX20.
Regards,
Chris.

Have a look at and
look for the 6 m converter. Here is your RF input circuitry !
ON6MU has described this fb 50MHz mixer with the NE6...
Re: 6 m BITX ??

Ron
 

Tnx for your input Rahul,

Yes impressive. OK this proves IRF family MOSFET can be used on 6m.
(in spite of relative high input capacity of several pF)
This is non-QRP for sure!
Interesting URLs you supplied Rahul.
Bye
Ron


--- In BITX20@..., Rahul Srivastava <vu3wjm@y...> wrote:
Hi!

I had been interested in 6mtr and related equipment. Once again as
evident by BITX concept we had no 6mtr stuff available in VU. I was
much impressed with this 50mhz mega amp using IRF510.



Sorry this is no QRP stuff but just goes on to show what can be
achieved using common garden variety components.

In due course I also came across some design having very high IF ie
up conversion ie 40mhz range. Now with such high IFs it is simple
matter to to hook up a 6mtr equipment using conventional low freq VFO.



Most of the multiband design here can be tailored for 6mtr
operations.

Lately we had some serious discussions on using microwave and high
Ft devices I am sure getting onto 6 would be no problem.

73

Rahul VU3WJM







Ron <pa2rf@y...> wrote:
Hi fellow BITX-homebrewer,
Yesterday I was browsing through some old "Electrons" (Dutch Ham
magazine) and found an interesting article in which it was told
that
our famous MOSFET IRF-family (in particular IRF510 and IRF540)
could
work on 50 MHz ! I am surprised by this. The article did not show
fixed designs, so maybe it is just wishfull thinking...
According to the article, achievable power levels should less lower
than at, let's say 7 MHz ( 2 Watts versus 12 Watts RF).
Anybody experience in this field ?

Interesting idea, by the way, to make the BITX useable for 6m.

However, with an IF of 10MHz, VFO freq of 40MHz might be too high
to
get a (simple) rock-stable VFO.

Since I do have ham equipment for 70 cm, 2m, HF but not for 6 m,
this
might be another nice (winter) project.

Anybody ideas for homebrew 6m transceiver (50...52 MHz) on the www
according to the KISS principle (Keep It Simple Stupid) ??

73 de Ron
PA2RF




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First QSO BITX-15 with PLL

 

I make my first QSO today with 3W, a dipole in the band allocated to
apprentice hams in spain, my call sign EC7ALV in HF bands.

I worked hard with my Autooscilating Power Amplifier.

The QSO was with my fellow hams of my own city, the band is somewhat
difficult for propagation.

I make a PLL circuit. I will post the schematic and source code in C
(2000 lines of C in HI-TECH compiler) as soon as possible. It make
work the VCO in 31MHz, rock stable, the FI is the original in 10MHz.
It has 3.2KHz PLL step with 100Hz resolution by the aid of D/A
converter of five bits uC controled. It work flawesly. The
components of PLL are 16F628 PIC uC, MC145170 serial PLL, CD4094
CMOS shift register, MAX232 future for PC comunication and remote
control.

The software has:

- Add the FI value.
- Show the frecuency in correct format.
- Show a S-Meter with 16 levels of resolution.
- Remember last settings including frecuency by the internal EEPROM.

To Do:

- Chanells
- Remote control
- Repeater Shift
- Multimode/multiband conmutation

Sorry my bad english. You have some photos in the files section.
I am very happy.
Greetings
Manuel
EB7HPM/EC7ALV
Re: 6 m BITX ??

Rahul Srivastava
 

Hi!
?
I had been interested in 6mtr and related equipment. Once again as evident by BITX concept we had no 6mtr stuff available in VU. I was much impressed with this 50mhz mega amp using IRF510.
?
?
Sorry this is no QRP stuff but just goes on to show what can be achieved using common garden variety components.
?
In due course I also came across some design having very high IF ie up conversion ie 40mhz range. Now with such high IFs it is simple matter to to hook up a 6mtr equipment using conventional low freq VFO.
?
?
Most of the multiband design here? can be tailored for 6mtr operations.
?
Lately we had some serious discussions on using microwave and high Ft devices I am sure getting onto 6 would be no problem.
?
73
?
Rahul VU3WJM
?
?
?
?
?
?

Ron wrote:
Hi fellow BITX-homebrewer,
Yesterday I was browsing through some old "Electrons" (Dutch Ham
magazine) and found an interesting article in which it was told that
our famous MOSFET IRF-family (in particular IRF510 and IRF540) could
work on 50 MHz ! I am surprised by this. The article did not show
fixed designs, so maybe it is just wishfull thinking...
According to the article, achievable power levels should less lower
than at, let's say 7 MHz ( 2 Watts versus 12 Watts RF).
Anybody experience in this field ?

Interesting idea, by the way, to make the BITX useable for 6m.

However, with an IF of 10MHz, VFO freq of 40MHz might be too high to
get a (simple) rock-stable VFO.

Since I do have ham equipment for 70 cm, 2m, HF but not for 6 m, this
might be another nice (winter) project.

Anybody ideas for homebrew 6m transceiver (50...52 MHz) on the www
according to the KISS principle (Keep It Simple Stupid) ??

73 de Ron
PA2RF


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Re: 6 m BITX ??

 

Indeed, there you have your RF input circuitry.
Not the output. Sinse BITX is a transceiver, the SBL solution looks
more relevant to me. In fact, you can build double balanced mixers
yourself like the ones in the current BITX20.
Regards,
Chris.

Have a look at and
look for the 6 m converter. Here is your RF input circuitry !
ON6MU has described this fb 50MHz mixer with the NE6...
Re: 6 m BITX ??

Heinz Schnait
 

Hello Ron,

There is a design for a 50W V-MOSFET Linear for 50MHz in Pat Hawkers, G3VA,
"Technical Topics 2000 - 2004", pages 32 - 33, which might be interesting
for you. This book is a collection of reprints of a monthly column in the
RSGB Radcom. This article was published in Radcom of July 2000.

The design goes back to a publication of Dieter Mergner, DJ9FG, originally
appearing in the German "Funkamateur" 12/1999 and 01/2000. It uses two
IRF610 in push-pull with a 50V power supply. I was surprised to see IRF
MOSFETS at 50MHz. Some years ago I have build some single ended as well as
push-pull designs myself, but I couldn't get a reasonable output above 14
MHz. I worked with a maximum supply voltage of 30V. I have a better power
supply now. So maybe I should try MOSFETS again...

The book of Pat Hawker is worth reading every line of it!

73 Heinz, OE5EEP

Am Mittwoch, 29. Juni 2005 12:10 schrieb Ron:

Hi fellow BITX-homebrewer,
Yesterday I was browsing through some old "Electrons" (Dutch Ham
magazine) and found an interesting article in which it was told that
our famous MOSFET IRF-family (in particular IRF510 and IRF540) could
work on 50 MHz ! I am surprised by this. The article did not show
fixed designs, so maybe it is just wishfull thinking...
According to the article, achievable power levels should less lower
than at, let's say 7 MHz ( 2 Watts versus 12 Watts RF).
Anybody experience in this field ?
New file uploaded to BITX20

[email protected]
 

Hello,

This email message is a notification to let you know that
a file has been uploaded to the Files area of the BITX20
group.

File : /irf510(IR_Datasheet).pdf
Uploaded by : k7hkl_arv <arvevans@...>
Description : International Rectifier's Datasheet for IRF510 device

You can access this file at the URL:


To learn more about file sharing for your group, please visit:


Regards,

k7hkl_arv <arvevans@...>
Re: 6 m BITX ??

Arv Evans
 

Ron

The IRF510 device has an input capacitance of 180 pf, and an output
capacitance if 81 pf (from the International Rectifier product datasheet).
With this high input capacitance your 52 MHz driver stage impedance will have
to be very low. If you can drive this low impedance then it might be
possible.

But, there are other devices that could be used as the driver and output
stages with a high probability of success.

I wonder if anybody has cataloged all the bands for which BITX units have been
built? Obviously, 20M, 40M, 17M, and I think 30M have been done. Any
others?

Arv K7HKL
_._

On Wednesday 29 June 2005 04:10 am, Ron wrote:
Hi fellow BITX-homebrewer,
Yesterday I was browsing through some old "Electrons" (Dutch Ham
magazine) and found an interesting article in which it was told that
our famous MOSFET IRF-family (in particular IRF510 and IRF540) could
work on 50 MHz ! I am surprised by this. The article did not show
fixed designs, so maybe it is just wishfull thinking...
According to the article, achievable power levels should less lower
than at, let's say 7 MHz ( 2 Watts versus 12 Watts RF).
Anybody experience in this field ?

Interesting idea, by the way, to make the BITX useable for 6m.

However, with an IF of 10MHz, VFO freq of 40MHz might be too high to
get a (simple) rock-stable VFO.

Since I do have ham equipment for 70 cm, 2m, HF but not for 6 m, this
might be another nice (winter) project.

Anybody ideas for homebrew 6m transceiver (50...52 MHz) on the www
according to the KISS principle (Keep It Simple Stupid) ??

73 de Ron
PA2RF




YAHOO! GROUPS LINKS


?Visit your group "BITX20" on the web.
?
?To unsubscribe from this group, send an email to:
?BITX20-unsubscribe@...
?
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Re: 6 m BITX ??

Ron Brink
 

Dear David,
That's an interesting idea. Just add a mixer ! Sure the SBL-1 is well capable of handling 50 MHz. Famous Signetics/Philips NE612 or NE602 could be used as well (Fmax appr. 200 MHz). This ic contains an internal VFO which can be X-tal controlled and should be running on 36MHz without problems!
?
Have a look at ?and look for the 6 m converter. Here is your RF input circuitry !
ON6MU has described?this fb 50MHz mixer with the NE6...
?
How do you think about using the IRF510 endstage for 6 m...??? I do not feel comfortable with it.
73 from Holland
Ron

"Pedley David (South Devon Health Care NHS Trust)" wrote:
Hi Ron and all BITX'ers
? You have have been reading my thoughts, my BITX20 is still
at the building stage due to an Open University course which
finishes in October. My thoughts were to use the basic
20metre transceiver without PA stage, feed this into an SBL-1
mixer with a 36MHz crystal oscillator. The output/input of
the mixer will be at 50MHz. This is then fed into a 50MHz PA
stage based on the existing design. This would give a
transceiver capable of working on 14 and 50MHZ.
72/73's
David G8EMA
???

David Pedley
Senior Medical Devices Librarian
Torbay Hospital
tel 01803-655873


This e-mail is confidential and privileged. If you are not the intended
recipient please accept our apologies; please do not disclose, copy or
distribute information in this e-mail or take any action in reliance on its
contents: to do so is strictly prohibited and may be unlawful. Please
inform us that this message has gone astray before deleting it. Thank you
for your co-operation.


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Re: 6 m BITX ??

Pedley David (South Devon Health Care NHS Trust)
 

Hi Ron and all BITX'ers
You have have been reading my thoughts, my BITX20 is still
at the building stage due to an Open University course which
finishes in October. My thoughts were to use the basic
20metre transceiver without PA stage, feed this into an SBL-1
mixer with a 36MHz crystal oscillator. The output/input of
the mixer will be at 50MHz. This is then fed into a 50MHz PA
stage based on the existing design. This would give a
transceiver capable of working on 14 and 50MHZ.
72/73's
David G8EMA


David Pedley
Senior Medical Devices Librarian
Torbay Hospital
tel 01803-655873


This e-mail is confidential and privileged. If you are not the intended
recipient please accept our apologies; please do not disclose, copy or
distribute information in this e-mail or take any action in reliance on its
contents: to do so is strictly prohibited and may be unlawful. Please
inform us that this message has gone astray before deleting it. Thank you
for your co-operation.
6 m BITX ??

Ron
 

Hi fellow BITX-homebrewer,
Yesterday I was browsing through some old "Electrons" (Dutch Ham
magazine) and found an interesting article in which it was told that
our famous MOSFET IRF-family (in particular IRF510 and IRF540) could
work on 50 MHz ! I am surprised by this. The article did not show
fixed designs, so maybe it is just wishfull thinking...
According to the article, achievable power levels should less lower
than at, let's say 7 MHz ( 2 Watts versus 12 Watts RF).
Anybody experience in this field ?

Interesting idea, by the way, to make the BITX useable for 6m.

However, with an IF of 10MHz, VFO freq of 40MHz might be too high to
get a (simple) rock-stable VFO.

Since I do have ham equipment for 70 cm, 2m, HF but not for 6 m, this
might be another nice (winter) project.

Anybody ideas for homebrew 6m transceiver (50...52 MHz) on the www
according to the KISS principle (Keep It Simple Stupid) ??

73 de Ron
PA2RF
Re: need help for bitx VFO & BFO

Wijaya, J.
 

I've been reading JF1OZL article, which i myself sometimes questioned about BFO setting for USB or LSB operation, Arv K7HKL also provide me the same information about it, and this is what JF1OZL wrote:

----Please remember that "when the mixer generates the difference of two input signals, the USB changes LSB, LSB changes USB", and, " when the mixer generates the sum of the two input signals , USB became from USB, LSB became from LSB".----

rgds
julius


* REPLY SEPARATOR *

On 6/28/2005 at 7:26 PM vdberghak wrote:

Wise words from Julius.
Several postings are related to this subject.
It can be measured with a tone generator connected to the microphone
connection or with the sound card of the PC.
Dig in de messages, the photos and links directories and you will find
more issues that will help you getting up your rig and to make
understand 'why' and 'how'.
Best regards,
Chris.

there are method to measure your ladder filter using VFO, BFO, and
AF Stage, pls read back to the mailing list archive.

rgds
julius





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Re: need help for bitx VFO & BFO

 

Wise words from Julius.
Several postings are related to this subject.
It can be measured with a tone generator connected to the microphone
connection or with the sound card of the PC.
Dig in de messages, the photos and links directories and you will find
more issues that will help you getting up your rig and to make
understand 'why' and 'how'.
Best regards,
Chris.

there are method to measure your ladder filter using VFO, BFO, and
AF Stage, pls read back to the mailing list archive.

rgds
julius
Re: need help for bitx VFO & BFO

Wijaya, J.
 

Hi Kumar,

in my experience building a VFO, you may need to adjust number of turns on your coils to get the desired tuning range when you turn the trimmer.

As for your BFO, you may need to know your xtal filter center freq, and then tune your BFO for USB or LSB operation, on LSB, BFO should be on the upper edge of your xtal filter bandwidth, on USB, at lower edge of xtal filter. I test my xtal filter (10MHz xtal) with a signal generator and oscilosope, i tune the signal generator from 9.995MHz to 10.005MHz and watch the output on my osciloscope, i marked the freq when it first shows an output to the scope, again marked the higest output and the lowest until its gone, from the result, i get 9.996MHz at the lower edge, and 9.998MHz at upper edge, i operate my BITX at 40m, so i tune the BFO for LSB operation which is 9.998MHz at the BFO.

there are method to measure your ladder filter using VFO, BFO, and AF Stage, pls read back to the mailing list archive.

rgds
julius





* REPLY SEPARATOR *

On 6/25/2005 at 8:58 AM kumar gautam wrote:

Hello,
In my bitx20 BFO shows 9.995 MHz. and in VFO section when i add 2j gang
capacitor my VFO shows 4.000 to 4.180 MHz(when trimmer shifted to
4.000MHz) and after tuning Trimmer(shifted to 4.150MHz) it shows 4.180 to
4.410Mhz.
After trying various type of variable capacitors i coul'd not find entire
range i.e. 4.000 to 4.400MHz.
Kindly help me in this regard i.e.
1. what is the exact frequency of BFO.
2. What modification should i do for correcting the VFO.
also could i connect T1,T2 and T3 legs in any direction or it is coneected
in any direction.
with 73's
de kumar gautam
fromgautam@...



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New file uploaded to BITX20

[email protected]
 

Hello,

This email message is a notification to let you know that
a file has been uploaded to the Files area of the BITX20
group.

File : /BITX 15 + PLL/Complete QRP.JPG
Uploaded by : eb7hpm <msraya@...>
Description : My QRP

You can access this file at the URL:


To learn more about file sharing for your group, please visit:


Regards,

eb7hpm <msraya@...>
need help for bitx VFO & BFO

kumar gautam
 

Hello,
In my bitx20 BFO shows 9.995 MHz. and in VFO section when i add 2j gang capacitor my VFO shows 4.000 to 4.180 MHz(when trimmer shifted to 4.000MHz)?and after tuning Trimmer(shifted to 4.150MHz) it shows 4.180 to 4.410Mhz.
After trying various type of variable capacitors i coul'd not find entire range i.e. 4.000 to 4.400MHz.
Kindly help me in this regard i.e.
1. what is the exact frequency of BFO.
2. What modification should i do for correcting the VFO.
also could i connect T1,T2 and T3 legs in any direction or it is coneected in any direction.
with 73's
de kumar gautam
?

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Re: Transistors for the BITX20....

 

Hi Rob,
the termination of the filter is not in accordance with Fahrans design
50 ohm. Between the mixer and the filter is a bi-amp. The mixer is
likely 50 ohm so the other side of the bi-amp is about 200 ohm.
I have a 4.9 MHz filter my self, you can make the impedance higher by
adding two small transformers. See BITX17 in the photos directory.

About the transistors: I hear everyone complaining about BC547. I made
the BITX17 with BC107 and it works fine. A frind of me made it with
BC547 and he had also rather poor results. It should not make much
difference between these two types? How to explain?
Best regards,
Chris.

I had some surplus 6 MHz crystals here, and so I'm using that as an
IF. If I use the 50 ohm termination impedance that Fahran has set
up, the filter ripple is more than 10 dB in the passband.
I may
have to go looking in my junk box some more for some more microwave
transistors.

73,

Rob, KL7NA
--
Re: Rahul's on your two transistor shunt circuit....

Rob Frohne
 

Hi Rahul, et. al.,

Interesting that the circuit appeared in Solid State Design.? I should have been familiar with it then, but it has been a while since I read that book.??
On Jun 23, 2005, at 8:37 AM, Rahul Srivastava wrote:

Hello! Prof Rob,
?
Thank you for the indepth analysis on the ckt. The 2 transistor? configration goes under title of shunt feedback RF buffer amplifier.
First mention of the circuit Icame across in Solid State designs. It?has been used not only as buffer for VFOs?but as a high gain?audio preamplifier after mixer in DC Rx.
Lately its application as IF amplifier was noticed in Norcal 20 transceiver. A google on its schematics or Red Hot radio schematics will give a practical application.
?A breif mention can be found here:
?
?
?
I was also amused at its simplicity and high gain, near about 40dbs. My first BITX prototype lacked gain in IF stages so I looked for alternatives. The shunt feedback had a well defined and easily setable? impedances , a 220E resistor in input provided good match to the Xtal filter and the 50ohm output to the PD. Secondly it was a design with RC topology muck like rest of the BITX .
?
From your analysis? I come to conclusion that the first device needs to changed to a lower HFe maybe a BF494 will be more suitable.
?
I was astonished seeing this high noise figure , this needs to be looked into. Unfortunately for all its mention I have not yet found any noise figures for this config. Thanks.
?
I have also encountred high input capacitances , one of the set of BPFs that normaly tuned with 47pf cap tuned with 33pf in BITX. Possibly use of BC547 was also a reason for high input cap a 2N3904 would have been a better choice. I am yet to try them in Tx chain.
You can change in input reactance by adjusting the series 220 ohm resistor.? If I recall right, lowering it makes it more capacitive, and raising it makes it more inductive.

In case anyone wants to play with the spice analysis, I uploaded the spice file for the V3 type amplifier to the BITX 20 V1,2, and 3 directory.? I also uploaded the spice file for the regular case to the root directory.? These files probably have tweaked values of resistors, etc. but the general thing is there and with nutmeg it should do the plots of the input impedance, etc.

73,

Rob

?
I am thinking of discreet implimentation of MMICs and giving them a try or a BITX all with 2SC2570 devices. Thanks for the guidance.
?
73
?
Rahul VU3WJM
?
PS: I am also working on a new TRX design using these high Ft transistors in Norton noiseless preamp configration biased at 16mA. The results have been very encouraging.?
?
?
?
?


Rob Frohne <frohro@...> wrote:
Hi Rahul,

Your two transistor shunt circuit is quite interesting to me.? It isn't one I have seen before, and so I did a little spice analysis of it and have been thinking about how it was dreamed up.? I hope you can shed some light on that.? Here is what I have observed:

1)? Spice simulation:
a)? Input impedance is about 50 ohms for low frequencies.? (Surprising to me at first because I was thinking all the feedback was negative.)
b)? Gain is about 40 dB at low frequencies, and drops to perhaps 30 dB at 10 MHz.
c)? Noise figure was about 25 dB.? (The original circuit is about 14 dB.)
d)? Input impedance varies from 50 ohms at hfe = 255 to about 300 ohms at hfe = 55.
e)? Gain varies from 40 dB at hfe = 255 to 25 dB at hfe = 55.

2)? Feedback Analysis
a)? There are two types of feedback (similar to Fahran's circuit), but instead of using parallel-voltage and series-current both negative feedback, it is series-current negative feedback on the input transistor, and series-voltage positive feedback on the combination.?
b)? Normally, we use only negative feedback, because of its advantages for stability, and reducing the sensitivity of the circuit to variations in circuit parameters (like the current gain of the BJT, hfe), and because it degrades the noise figure.? We have to pay for these advantages with gain though.? So seeing positive feedback is very surprising to me.? In case my terminology isn't familiar to you, I'll go through the thoughts that I had to come up with the conclusion that it really is positive feedback.? The feedback network is the voltage divider with the 10K and the 47K resistors.? The voltage on the output is sampled (it is the input the voltage divider).? The output of the voltage divider is added in series with the input voltage, so I call it series-voltage feedback.? When the input voltage is increased the voltage at the collector of the input transistor goes down (the gain of the first stage is negative).? The seco nd stage is a common collector (or emitter follower) amplifier and its voltage gain is slightly less than one.? (Its output is approximately .7 volts DC less than its input.)? So the output of the voltage divider feedback network goes down in response to a positive input.? This makes the base emitter voltage on the input transistor bigger, and so we have positive feedback.? The spice results are in agreement with this analysis in the following ways.??
??? i)? The noise figure is fairly high.
?? ?ii)? The input impedance is low.? Note that for negative series feedback, the input impedance always increases, but this has an input impedance that is very much lower (50 ohms) than the rough estimate of (hfe + 1) 10K? of the input impedance without this positive feedback.? Since negative feedback increases the input impedance, positive feedback decreases it.
??? iii)? The gain is quite high, and that is the result of positive feedback.
??? iv)? The variation in gain and input impedance as a result of changing hfe is significant.? That would probably preclude building this circuit commercially, because the transistors would have to be hand selected to get uniform results.

It is a very interesting circuit.? After thinking about it, personally I like the original circuit with two types of negative feedback better.? It has all the advantages of negative feedback, and the input impedance is adjustable by changing the ratio of the two types of feedback.? (Series feedback increases the input impedance, and parallel feedback decreases it.)? The disadvantage is that you get less gain, and that is especially significant at high frequencies where Cmu is providing parallel voltage feedback in addition to the 2.2K resistors.? So the original circuit needs?higher frequency transistors?though, as you originally pointed out. ?I'm glad they aren't too expensive in India. ?I just hope I can find some more in my desk drawer or somewhere. ?:-) ?

On Jun 22, 2005, at 9:27 PM, Rahul Srivastava wrote:

Hi!
?
I feel?transistors used in cable TV industry are more suited , cheap and universaly available. I would recommend BFR96, BFR91, MPS571, 2SC3359 (7Ghz) and 2SC2570A (5Ghz) devices. The later is very cheap abt 6-8cents each here in VU.
?
For slightly more power or better IMD performance BFW16.
?
73
?
?
Rahul VU3WJM

I concur that any of these transistors would be good.? Anything with an fT of greater than 1GHz should work.? The 2N2222 has an fT of about 300 MHz and that isn't quite enough.? (fT is the frequency at which the current gain, beta drops to unity.)? I am using a couple BFR90's for the preamplifier and hope I can find some more in my drawer at school for the IF amplifiers too.

Another comment on using high frequency resistors:? It appears to me that you need to do it both on the receive and transmit side of the bidirectional amplifiers, because the Cmu (collector to base capacitance) of the transmit transistor is in parallel with the transistor for receive.? (That is why Fahran used 2.2K resistors instead of 1.1K ones.)

My kids are getting impatient with my analysis, and want to get back to soldering parts on the board, so I'm probably going to have to quit fiddling around here, but it has been fun.? Thanks for providing the food for thought in the way of this interesting circuit.

73,

Rob, KL7NA
?
?
?


?

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Re: Rahul's on your two transistor shunt circuit....

Rahul Srivastava
 

Hello! Prof Rob,
?
Thank you for the indepth analysis on the ckt. The 2 transistor? configration goes under title of shunt feedback RF buffer amplifier.
First mention of the circuit Icame across in Solid State designs. It?has been used not only as buffer for VFOs?but as a high gain?audio preamplifier after mixer in DC Rx.
Lately its application as IF amplifier was noticed in Norcal 20 transceiver. A google on its schematics or Red Hot radio schematics will give a practical application.
?A breif mention can be found here:
?
?
?
I was also amused at its simplicity and high gain, near about 40dbs. My first BITX prototype lacked gain in IF stages so I looked for alternatives. The shunt feedback had a well defined and easily setable? impedances , a 220E resistor in input provided good match to the Xtal filter and the 50ohm output to the PD. Secondly it was a design with RC topology muck like rest of the BITX .
?
From your analysis? I come to conclusion that the first device needs to changed to a lower HFe maybe a BF494 will be more suitable.
?
I was astonished seeing this high noise figure , this needs to be looked into. Unfortunately for all its mention I have not yet found any noise figures for this config. Thanks.
?
I have also encountred high input capacitances , one of the set of BPFs that normaly tuned with 47pf cap tuned with 33pf in BITX. Possibly use of BC547 was also a reason for high input cap a 2N3904 would have been a better choice. I am yet to try them in Tx chain.
?
I am thinking of discreet implimentation of MMICs and giving them a try or a BITX all with 2SC2570 devices. Thanks for the guidance.
?
73
?
Rahul VU3WJM
?
PS: I am also working on a new TRX design using these high Ft transistors in Norton noiseless preamp configration biased at 16mA. The results have been very encouraging.?
?
?
?
?


Rob Frohne wrote:
Hi Rahul,

Your two transistor shunt circuit is quite interesting to me.? It isn't one I have seen before, and so I did a little spice analysis of it and have been thinking about how it was dreamed up.? I hope you can shed some light on that.? Here is what I have observed:

1)? Spice simulation:
a)? Input impedance is about 50 ohms for low frequencies.? (Surprising to me at first because I was thinking all the feedback was negative.)
b)? Gain is about 40 dB at low frequencies, and drops to perhaps 30 dB at 10 MHz.
c)? Noise figure was about 25 dB.? (The original circuit is about 14 dB.)
d)? Input impedance varies from 50 ohms at hfe = 255 to about 300 ohms at hfe = 55.
e)? Gain varies from 40 dB at hfe = 255 to 25 dB at hfe = 55.

2)? Feedback Analysis
a)? There are two types of feedback (similar to Fahran's circuit), but instead of using parallel-voltage and series-current both negative feedback, it is series-current negative feedback on the input transistor, and series-voltage positive feedback on the combination.?
b)? Normally, we use only negative feedback, because of its advantages for stability, and reducing the sensitivity of the circuit to variations in circuit parameters (like the current gain of the BJT, hfe), and because it degrades the noise figure.? We have to pay for these advantages with gain though.? So seeing positive feedback is very surprising to me.? In case my terminology isn't familiar to you, I'll go through the thoughts that I had to come up with the conclusion that it really is positive feedback.? The feedback network is the voltage divider with the 10K and the 47K resistors.? The voltage on the output is sampled (it is the input the voltage divider).? The output of the voltage divider is added in series with the input voltage, so I call it series-voltage feedback.? When the input voltage is increased the voltage at the collector of the input transistor goes down (the gain of the first stage is negative).? The second stage is a common collector (or emitter follower) amplifier and its voltage gain is slightly less than one.? (Its output is approximately .7 volts DC less than its input.)? So the output of the voltage divider feedback network goes down in response to a positive input.? This makes the base emitter voltage on the input transistor bigger, and so we have positive feedback.? The spice results are in agreement with this analysis in the following ways.??
??? i)? The noise figure is fairly high.
?? ?ii)? The input impedance is low.? Note that for negative series feedback, the input impedance always increases, but this has an input impedance that is very much lower (50 ohms) than the rough estimate of (hfe + 1) 10K? of the input impedance without this positive feedback.? Since negative feedback increases the input impedance, positive feedback decreases it.
??? iii)? The gain is quite high, and that is the result of positive feedback.
??? iv)? The variation in gain and input impedance as a result of changing hfe is significant.? That would probably preclude building this circuit commercially, because the transistors would have to be hand selected to get uniform results.

It is a very interesting circuit.? After thinking about it, personally I like the original circuit with two types of negative feedback better.? It has all the advantages of negative feedback, and the input impedance is adjustable by changing the ratio of the two types of feedback.? (Series feedback increases the input impedance, and parallel feedback decreases it.)? The disadvantage is that you get less gain, and that is especially significant at high frequencies where Cmu is providing parallel voltage feedback in addition to the 2.2K resistors.? So the original circuit needs?higher frequency transistors?though, as you originally pointed out. ?I'm glad they aren't too expensive in India. ?I just hope I can find some more in my desk drawer or somewhere. ?:-) ?

On Jun 22, 2005, at 9:27 PM, Rahul Srivastava wrote:

Hi!
?
I feel?transistors used in cable TV industry are more suited , cheap and universaly available. I would recommend BFR96, BFR91, MPS571, 2SC3359 (7Ghz) and 2SC2570A (5Ghz) devices. The later is very cheap abt 6-8cents each here in VU.
?
For slightly more power or better IMD performance BFW16.
?
73
?
?
Rahul VU3WJM

I concur that any of these transistors would be good.? Anything with an fT of greater than 1GHz should work.? The 2N2222 has an fT of about 300 MHz and that isn't quite enough.? (fT is the frequency at which the current gain, beta drops to unity.)? I am using a couple BFR90's for the preamplifier and hope I can find some more in my drawer at school for the IF amplifiers too.

Another comment on using high frequency resistors:? It appears to me that you need to do it both on the receive and transmit side of the bidirectional amplifiers, because the Cmu (collector to base capacitance) of the transmit transistor is in parallel with the transistor for receive.? (That is why Fahran used 2.2K resistors instead of 1.1K ones.)

My kids are getting impatient with my analysis, and want to get back to soldering parts on the board, so I'm probably going to have to quit fiddling around here, but it has been fun.? Thanks for providing the food for thought in the way of this interesting circuit.

73,

Rob, KL7NA
?
?
?


?

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