Is it possible to use this PA in AM mode???

[IW3FZQ]

Hello!

I bought this PA as kit some months ago:

http://www.qrpproject.de./UK/qrppa2008.html


It uses two VMOS device in push-pull configuration.
Is it possible to use it for AM mode? I would like to put audio in the final stage with a suitable transformer but after some test I obtained negative results. Perhaps I used various transformes but don't suitable for this purpose...

Comments??

Enrico, IW3FZQ

[Ed-VA3ES]

It looks like  a linear amplifier, which cannot be modulated.  You will need a small AM exciter to drive it.  You will not get more than about 3W output.   Alternately, bias it for class C and add an audio amp with suitable impedance transformation  in series with the Vcc supply to pin 2 of  Tr2.

[IW3FZQ]

It looks like  a linear amplifier, which cannot be modulated.  You will need a small AM exciter to drive it.  You will not get more than about 3W output.   Alternately, bias it for class C and add an audio amp with suitable impedance transformation  in series with the Vcc supply to pin 2 of  Tr2.

Hi Ed and thanks for have replied! I already have an exciter, see the link below

http://www.northcountryradio.com/Kitpages/am88.htm

but if I use the exciter with its audio input for AM mode, plus the PA, it works not very good. The PA is a 10-15 Watt SSB/CW wide band amplifier. If I use it with a fitted AM sugnal in its input (by my exciter) I should keep the output power approximatively around 3-4 Watts and the modulation is very low ! I think if I use the AM transmitter only like medium wave or 160 mt generator and I add audio in the final stage the total result will be better, or no? Eventually which feature needs the audio transformer??

Enrico, IW3FZQ

[AB2EZ]

Enrico

Hi!


I have a very similar amplifier that I am using as a linear amplifier with my Flex radio SDR-1000 (low power version) on AM. Mine is made by a different manufacturer, but it has a very similar design/schematic, and a similar output power. It works very well as a linear amplifier for an AM signal (excellent audio, due to very good linear tracking of output v. input). Therefore, I think your amplifier should also work well as a QRP linear amplifier for AM, if it is working as designed.

Some suggestions

1. Do you know if the QRP transmitter is delivering a good AM signal to the QRP amplfier? I.e. have you listened to the output of the QRP transmitter with an off-air monitor in your shack.

2. Make sure that all three of the transistors in your QRP amplifier a biased properly, as per the instructions that came with the kit. In particular, you want to make sure that the driver transistor's bias is high enough so that the driver is not cut off on negative modulation peaks

3. Check the linearity of the QRP amplifier by modulating the QRP transmitter on AM with a 1 kHz sine wave, and looking at the output of your off-air monitor with a scope. Alternatively, put a 3 dB attenuator between the QRP transceiver and the QRP amplifier, and check to see if the output power of the QRP amplifier drops by 3dB (not more).

If you want to modulate the amplifier directly, then remember that the modulation impedance of the qrp amplifier will be about 20 volts/0.2 amps =  100 ohms.  The transformer that you use as a modulation transformer, between the center tap of the rf output transformer and the 20 volt B+ supply, must be capable of supporting about 0.2 amps of DC without saturating. Make sure that the audio amplifier + modulation transformer that you are using can drive a 100 ohm load, with about 20 volts of output modulation voltage (peak), and that the transformer can handle at least 0.2 amps of DC in its output winding.

Caution: This QRP amplifier does not have a low pass RF filter at its output. If you change the biasing to Class C, it will generate a lot of harmonics of the fundamental r.f. frequency. You will then need to include some kind of an output filter to reduce these harmonics to legal levels. Therefore, it is best to continue to run it in Class B (or AB) push-pull mode.
Good luck
Stu

[Gito]

Hi Enrico

  It's not clear how to set the operating condition of this PA ,but reading the speck of it ,it works from 1.8 t0 30 mhz as a broad band RF amplifier
So if you used it as a booster/RF amplifier of your medium wave used it at the High End frequency  the,1,6 mhz.

Since it can be used as a CW transmitter,set it to CW operation as the manual stated.
It only needs a 10 m watt input,So if your Exciter output can be lowered .
Drive the PA amplifier with the lowest drive,than slowly raised the drive, there's a point when the highest output can be reached , higher drive /over drive can not make The PA output more higher.
So the point used just enough drive .to get the Highest power out put.

If we want to modulate the final,that means we only modulate T2,T3 of the PA.
that means we must break  the B+ connection between Dr1 (RF choke)  and pin 2 of Tr2 (RF output Trafo).T1 is still connected to B+ via Dr2 and Dr1

Since it has a low output impedance ,let see if we get a 15 watt output ,by dividing 15 watt by 13.8 v we got about 1 amp current that means we have about 13 ohm impedance.(transmitter impedance)

If you have a 15 0r 25 watt audio amplifier you can used an 16 ohm to 16 ohm audio trafo 1 to 1 ratio as a modulation trafo.
Connect the primary winding in series with the B+ to  pin 2 of Tr2 (RF output trafo)
Connect the Secondary winding to your audio amp.

T1 of your PA is connected from Dr2 to Dr1 to B+
T2,T3 of your Pa is connected from pin2 Of Tr2 (RF output Trafo) to primary modulation trafo ,to Dr1 to B+.

But don't forget about the antenna with You used for your Transmitter,It's a load for your Transmitter.
If its load is not  right ,it can damage your Transmitter ,and it also affect the transmitter range.
Also it affects the power output of your Transmitter.

Gito

[AB2EZ]

Gito is correct (with an assumption, given below as "alternative c"). In any event, I stand corrected on the calculations of the average current and the modulation impedance

The B+ voltage is shown on the schematic, and is 13.8 volts. [I was guessing that it was 20 volts, before I checked the schematic again] The average current flowing through the push pull output stage of the qrp amplifier, at full power output, should be calculated by using the input power to the push pull stage at full output power level (15 watts input is a good assumption, for 10 watts output). This yields an average current of about 1.1 amp with the qrp amplifier running at full output:
 
To use the amplifier as an AM modulated amplifier, one can do either of the following

Alternative a) Reduce the input drive, in order to reduce the output power to around 25% of its 10 watt peak value (2.5 watts out). This will result in a reduction in the average current (in the pair of push pull output transistors) to 0.55 amps, and an efficiency of operation of only 33%. The peak modulated voltage across the output transistors (at 100% modulation) will be twice the B+ = 2 x 13.8 volts = 27.6 volts. One must be concerned as to whether the transistors can handle that high a peak voltage, and whether they will be happy running at 33% efficiency for long periods of "key down" AM operation. If one uses this approach, the modulation impedance will be about 13.8 volts /0.55 amps = 25 ohms, and the modulation transformer must be able the handle the average current of 0.55 amps through its secondary. Result: 2.5 watts output at carrier, and 10 watts peak output, average output transistor dissipation: 67% of input power = 5 watts (at carrier).

Alternative b) Keep the drive level the same as it was with the qrp amplifier running at full power output (10 watts), but reduce the B+ on the output transistors from 13.8 volts to 0.5 x 13.8 volts = 6.9 volts. This will reduce the average output power to 2.5 watts, and the average current to 0.55 amps. The efficiency will remain at ~67%.  The modulation resistance will be 6.9 volts / 0.55 amps = 12.5 ohms. Modulate the qrp amplifier with a modulator that can produce 6.9 volts into a 12.5 ohm load, and whose output transformer can handle the 0.55A average current flowing through its secondary. Result: 2.5 watts output at carrier, and 10 watts peak output, average output transistor dissipation: 33% of input power = 1.25 watts (at carrier) .

Alternative c)  Keep the drive level the same as it was with the qrp amplifier running as a linear amplifier at full power output (10 watts). The efficiency will remain at ~67%.  The modulation resistance will be 13.8 volts / 1.1 amps  = 12.5 ohms. Modulate the qrp amplifier with a modulator that can produce 13.8 volts into a 12.5 ohm load, and whose output transformer can handle the 1.1A average current flowing through its secondary. The amplifier may not be able to deliver the peak power output under 100% modulation (40 watts), and the transistors may not like the peak voltage that will be applied to them or the power they will dissipate at 10 watts output "key down". Result: 10 watts output at carrier, and 40 watts peak output, average output transistor power dissipation: 33% of input power = 5 watts (at carrier)


Stu

 

[DMOD]

I have built these rigs for our physics classes for the EM part of our cirriculum, and the NorthCountryRadio exciter (approx. 100mW input) is capable of some very good AM, but some good adjustments, modifications, and obervations have to happen:

1. Feed it 13.8 volts from a well regulated and clean power supply; DO NOT USE WALWARTS.

Remove D4 and replace it with through-wire ferrite bead for the PS input. Place D4 and a 0.01 ufd ceramic cap across J3 so as to clip reverse voltages and reduce RF feedback, and place a fuse before D4. Change C6 to 1,000 ufd and 25 Volts.

2. Heat sink Q7 and Q9.

3. Make sure the PLL is entirely stable during modulation by scoping Q8's base. If not, Adjust the number of turns of the secondary of L1's windings.

4. For the RF output, use a 1.5 to 3 db 50 ohm resistive Pi-Pad between the pi-L impedance transformation/harmonic network (L7 to L14, C32 to C47) and the input to the MOSFET AMP.

Also make sure the band switches S3A, S3D are switching properly. There has been a QC problem here.


5. Adjust audio input and compression for minimum clipping using R1 and R18.

IF YOU HAVE AN EXTERNAL compressor/limiter, use it and set R18 for no internal compression. 


6. Adjust R16 for best modulated RF as seen on a scope both with a 50 ohm load and then working into the linear AMP

Regards,

Phil - AC0OB

[IW3FZQ]

Enrico

Hi!


I have a very similar amplifier that I am using as a linear amplifier with my Flex radio SDR-1000 (low power version) on AM. Mine is made by a different manufacturer, but it has a very similar design/schematic, and a similar output power. It works very well as a linear amplifier for an AM signal (excellent audio, due to very good linear tracking of output v. input). Therefore, I think your amplifier should also work well as a QRP linear amplifier for AM, if it is working as designed.

Some suggestions

1. Do you know if the QRP transmitter is delivering a good AM signal to the QRP amplfier? I.e. have you listened to the output of the QRP transmitter with an off-air monitor in your shack.

2. Make sure that all three of the transistors in your QRP amplifier a biased properly, as per the instructions that came with the kit. In particular, you want to make sure that the driver transistor's bias is high enough so that the driver is not cut off on negative modulation peaks

3. Check the linearity of the QRP amplifier by modulating the QRP transmitter on AM with a 1 kHz sine wave, and looking at the output of your off-air monitor with a scope. Alternatively, put a 3 dB attenuator between the QRP transceiver and the QRP amplifier, and check to see if the output power of the QRP amplifier drops by 3dB (not more).

If you want to modulate the amplifier directly, then remember that the modulation impedance of the qrp amplifier will be about 20 volts/0.2 amps =  100 ohms.  The transformer that you use as a modulation transformer, between the center tap of the rf output transformer and the 20 volt B+ supply, must be capable of supporting about 0.2 amps of DC without saturating. Make sure that the audio amplifier + modulation transformer that you are using can drive a 100 ohm load, with about 20 volts of output modulation voltage (peak), and that the transformer can handle at least 0.2 amps of DC in its output winding.

Caution: This QRP amplifier does not have a low pass RF filter at its output. If you change the biasing to Class C, it will generate a lot of harmonics of the fundamental r.f. frequency. You will then need to include some kind of an output filter to reduce these harmonics to legal levels. Therefore, it is best to continue to run it in Class B (or AB) push-pull mode.
Good luck
Stu

Hi Stu!

Yes, I'm sure the TX works well because if I use it without amplifier there aren't problems and its modulation sounds well. This afternoon I made some tests and I seen that after a variation of the bias current in the drive of the PA, it works better!! The exciter has an output power between 50 and 200 mWatt and I put the RF into the PA via a 13 dB attenuator. The total result is: 2,5 Watt of carrier without modulation; if I put modulation the peaks I read are about 5 Watts! If I add the drive (3 dB more) and turn the bias again, the RF (carrier) will be about 7 Watts with again positive peak but lots of compression on modulation! It is very strange... Thanks also for your informations about the trafo in PA stage but I haven't them and I tried to use normal trafo (I.E. main AC 230/12V or 230/24 and so...) with absolutely negative results....
With regard to low pass filter, yes I know, I have made a suitable low pass filter but on AB class at present the harmonics are very very low. I think because the LP filter in the exciter is well fitted.

Enrico.

[IW3FZQ]

Hi Enrico

  It's not clear how to set the operating condition of this PA ,but reading the speck of it ,it works from 1.8 t0 30 mhz as a broad band RF amplifier
So if you used it as a booster/RF amplifier of your medium wave used it at the High End frequency  the,1,6 mhz.

Since it can be used as a CW transmitter,set it to CW operation as the manual stated.
It only needs a 10 m watt input,So if your Exciter output can be lowered .
Drive the PA amplifier with the lowest drive,than slowly raised the drive, there's a point when the highest output can be reached , higher drive /over drive can not make The PA output more higher.
So the point used just enough drive .to get the Highest power out put.

If we want to modulate the final,that means we only modulate T2,T3 of the PA.
that means we must break  the B+ connection between Dr1 (RF choke)  and pin 2 of Tr2 (RF output Trafo).T1 is still connected to B+ via Dr2 and Dr1

Since it has a low output impedance ,let see if we get a 15 watt output ,by dividing 15 watt by 13.8 v we got about 1 amp current that means we have about 13 ohm impedance.(transmitter impedance)

If you have a 15 0r 25 watt audio amplifier you can used an 16 ohm to 16 ohm audio trafo 1 to 1 ratio as a modulation trafo.
Connect the primary winding in series with the B+ to  pin 2 of Tr2 (RF output trafo)
Connect the Secondary winding to your audio amp.

T1 of your PA is connected from Dr2 to Dr1 to B+
T2,T3 of your Pa is connected from pin2 Of Tr2 (RF output Trafo) to primary modulation trafo ,to Dr1 to B+.

But don't forget about the antenna with You used for your Transmitter,It's a load for your Transmitter.
If its load is not  right ,it can damage your Transmitter ,and it also affect the transmitter range.
Also it affects the power output of your Transmitter.

Gito

Hi Gito,

I confirm, it works in the 1,5 to 50 MHz range. As written above, I obtained good out modulation turning the Bias trimmer for a 150 mA in the TR1 device. The both VMOS devices are a current than 100 mA each. I drive the PA with about 5-10 mWatt and I obtain now 2,5 Watt/carrier and about 5 Watts of pek and it is the best result; if I drive the PA with more power (I.E. 15-20 mWatts) I found a very high distorsion in the audio (and power is obout 7 Watts/carrier, 10 with peaks) but I do not see negative peaks! Why ??  I haven't a suitable audio trafo (I.E. 16/16 Ohm) so I can't make any more
However, after a bias regulation and just drive level in the PA, I find the audio works better than before. I also believe if it is a 10 Watt SSB/CW amplifier its max out power in AM mode will be lower respect to previous modes.... or no??

Enrico.

[KD6VXI]

Hi Enrico

  It's not clear how to set the operating condition of this PA ,but reading the speck of it ,it works from 1.8 t0 30 mhz as a broad band RF amplifier
So if you used it as a booster/RF amplifier of your medium wave used it at the High End frequency  the,1,6 mhz.

Since it can be used as a CW transmitter,set it to CW operation as the manual stated.
It only needs a 10 m watt input,So if your Exciter output can be lowered .
Drive the PA amplifier with the lowest drive,than slowly raised the drive, there's a point when the highest output can be reached , higher drive /over drive can not make The PA output more higher.
So the point used just enough drive .to get the Highest power out put.

If we want to modulate the final,that means we only modulate T2,T3 of the PA.
that means we must break  the B+ connection between Dr1 (RF choke)  and pin 2 of Tr2 (RF output Trafo).T1 is still connected to B+ via Dr2 and Dr1

Since it has a low output impedance ,let see if we get a 15 watt output ,by dividing 15 watt by 13.8 v we got about 1 amp current that means we have about 13 ohm impedance.(transmitter impedance)

If you have a 15 0r 25 watt audio amplifier you can used an 16 ohm to 16 ohm audio trafo 1 to 1 ratio as a modulation trafo.
Connect the primary winding in series with the B+ to  pin 2 of Tr2 (RF output trafo)
Connect the Secondary winding to your audio amp.

T1 of your PA is connected from Dr2 to Dr1 to B+
T2,T3 of your Pa is connected from pin2 Of Tr2 (RF output Trafo) to primary modulation trafo ,to Dr1 to B+.

But don't forget about the antenna with You used for your Transmitter,It's a load for your Transmitter.
If its load is not  right ,it can damage your Transmitter ,and it also affect the transmitter range.
Also it affects the power output of your Transmitter.

Gito

Hi Gito,

I confirm, it works in the 1,5 to 50 MHz range. As written above, I obtained good out modulation turning the Bias trimmer for a 150 mA in the TR1 device. The both VMOS devices are a current than 100 mA each. I drive the PA with about 5-10 mWatt and I obtain now 2,5 Watt/carrier and about 5 Watts of pek and it is the best result; if I drive the PA with more power (I.E. 15-20 mWatts) I found a very high distorsion in the audio (and power is obout 7 Watts/carrier, 10 with peaks) but I do not see negative peaks! Why ??  I haven't a suitable audio trafo (I.E. 16/16 Ohm) so I can't make any more
However, after a bias regulation and just drive level in the PA, I find the audio works better than before. I also believe if it is a 10 Watt SSB/CW amplifier its max out power in AM mode will be lower respect to previous modes.... or no??

Enrico.

Try to get your carrier power, OUTPUT from the amp, at the 1 watt level.

You need 4 times the carrier power (in PEP) for 100 percent modulation.  If the exciter is capable of doing this, then your overdriving the amplifier still.

1 watt of AM carrier needs to modulate to 4 watts PEP for textbook 100 percent modulation.

--Shane

[IW3FZQ]

I have built these rigs for our physics classes for the EM part of our cirriculum, and the NorthCountryRadio exciter (approx. 100mW input) is capable of some very good AM, but some good adjustments, modifications, and obervations have to happen:

1. Feed it 13.8 volts from a well regulated and clean power supply; DO NOT USE WALWARTS.

Remove D4 and replace it with through-wire ferrite bead for the PS input. Place D4 and a 0.01 ufd ceramic cap across J3 so as to clip reverse voltages and reduce RF feedback, and place a fuse before D4. Change C6 to 1,000 ufd and 25 Volts.

2. Heat sink Q7 and Q9.

3. Make sure the PLL is entirely stable during modulation by scoping Q8's base. If not, Adjust the number of turns of the secondary of L1's windings.

4. For the RF output, use a 1.5 to 3 db 50 ohm resistive Pi-Pad between the pi-L impedance transformation/harmonic network (L7 to L14, C32 to C47) and the input to the MOSFET AMP.

Also make sure the band switches S3A, S3D are switching properly. There has been a QC problem here.


5. Adjust audio input and compression for minimum clipping using R1 and R18.

IF YOU HAVE AN EXTERNAL compressor/limiter, use it and set R18 for no internal compression. 


6. Adjust R16 for best modulated RF as seen on a scope both with a 50 ohm load and then working into the linear AMP

Regards,

Phil - AC0OB

Hi Phil,

yes, we have the same rig. I haven't made the modifications like yours but I do not find problems. Or no, the only problem I found is a very high noise (not hum) by using an internal trafo (AC 230/15 V) near the PCB. Same problem also if I add an iron screen between the trafo and the PCB. Probably the magnetic field enters in the PLL circuit, in fact you can hear a noise like a "vrrrrrrrr" I solved it by using a switching computer PSU inside the box, near the PCB and the noise is.... disappeared!! Its LP filter works very well in the 1200/1800 khz range also if simple mini impedance are used. I tested it below 900 kHz but the filter has some problems.  OK for  heat sink in Q8 and Q9, without them the TRs are very hot! Here you can see how it works!!

http://www.youtube.com/watch?v=qi93V5P6XrE&feature=channel_page

Enrico, IW3FZQ

[Gito]

Hi Enrico

Its true/correct what Stu has wrote,I 'm forgetting the Dissipation  that this Mosfet can Handle.
This Pa used RD16HHF1 Mosfet that has a dissipation/ rating is 15 watt minimal and 50 VDC rating.

Since it used 2 Mosfet it has 30 watt disipation/ rating,So at least we can Get a 6 to 7 watt Carrier for high level modulation.6 watt on the safe side.Wlth 24 watt peak output (modulated)
Also with class C operating we don't need to bias this PA.So no standing current,the PA must be cooler.

The Pa is intended as a linear Amplifier For SSB ,A class B Amplifier
There's a difference using it as SSB linear than AM liniear.
In SSB the Amplifier used to Amplify ,SSB signals from the Exiter. and no carrier when the signals is of
In AM there's always A carrier, with or without modulation,driving this PA.
So maybe that's why we can only get 7 watt output on AM from this linear PA.

looking at T1(Mosfet) it use R4 at the Source (T1) to ground So the voltage at the Source of T1 is depended on the Current flowing from Drain to Source of this PA.
This has an effect of self biasing this Mosfet.
Maybe that's why the compressing effect came.

Gito