开云体育

Could someone provide a circuit description for Q202 and Q201 in the Driver_Bias part of the schematic?

I'm also have a problem setting the bias on Q207 (Q208 is OK). I increase the pot to approximately 90 percent, at which point, it suddenly jumps to an excess bias amount. Way too touchy to set it to +125 mA. Could I have a bad pot? This happens regardless whether I set Q207 first, or second.

Thanks a bunch!
- 73, John, N0TA

On 13/07/2024 16:56, John N0TA via groups.io wrote:

a circuit description for Q202 and Q201 in the Driver_Bias part of the schematic

/g/QRPLabs/message/27368

Finally found this /g/softrock40/message/54967 You'll need the Softrock Ensemble schematic for equivalent part numbers.

73 Alan G4ZFQ

On 14/07/2024 07:02, Alan G4ZFQ via groups.io wrote:

Finally found this

Not finally, I now remember where I wrote the full story

The circuit includes the third BS170 in a clever voltage regulator circuit loop so that the supplied bias voltage tracks the BS170’s gate threshold change with temperature

You should be seeing around 120mA going through the drivers with /TX grounded but the PA bias pots anticlockwise

Yes the PA pot adjustment is very sensitive - a fixed resistor in series would have helped but as you’ll note PA bias immediately varies with warm up anyway so get it in the right region and be happy

A bad pot could be the case. Especially true if one varies differently than the other.

I've not found the bias to be "jumpy" or all that sensitive in adjustment for mine
(N=5). The bias does increase with heating but not enough to be a problem.

I assume you have a dummy load at the output and input.

Note Q206/7 are the IRF510 finals. The BS170 bias is independent of the
final bias but share common 12 switching.

--
Allison
------------------
Post online only,?
direct email will go to a bit bucket.

Q202 and Q201 are the bias regulator for the drivers Q203 and Q204.

Q206 is for Transmit enable (enables bias to the amplifier transistors).


--
Allison
------------------
Post online only,?
direct email will go to a bit bucket.

Hi the Bias is a stabilised switch supply, the two IFR FETs will have a different turn on voltage hence two pots. They are on a bit of a knife edge.

These are cheap transistors pressed into service to form a good RF amplifier, however, only a few manufacturers will work correctly as not all IRF 510 have the same spec

They are switching transistors normally switched hard on or off in a switched mode power supply, however, as they have a low capatance they are ideal for a general purpose RF transistor. They are cheap less than ?1.

Most Transmitters have power control which throttlies back the drive when you have a mismatch, to save them from high voltages. The QRP LAB design does not, so they are vulnerable.

You must have a matched load to use them.

You could use a multi turn pot to set the Bias, or a very steady hand, they also will run away with heat as well.

It is a PA on a Budget, having said that mine has been working for 10 years with carful handling.

I build a second unit with a SWR bridge and the reflection port throttlies back the drive, this makes it very robust.

RF power transistors are very expensive ?25 each.

Make sure you are using the original make supplied with the kit. Vishay not sure how it it is spelt.

73

Andrew

Mine including the proto have been in use and testing while working just fine.
Its proven to be very durable despite my abuses. I have not noted any fragility
in the QRPL design.

Part of the reason for that is at 10W the devices are being run at less than half
their capability. That makes for reliable operation.

--
Allison
------------------
Post online only,?
direct email will go to a bit bucket.

Yes Allison, I do agree at 10W theu are fine, and can handle a poor mismatch. The death of these unit is being over driven, the amp has high overall gain as it is part of another project the SSB transceiver, which may never see the light of day, hugely complicated project.

I have seen other designs running these transistors up to 80W class C, they do get killed off with a mismatch.

I rather like this kit and used it in other projects.

KR

Andrew G8UUG

Andrew,

The topic is the 10W QRPL amp not some other trash.
Thanks for letting me know what was not news to me years ago.

FYI most of the amps claiming 80W run IRF530 and can do it. Most
are as I've repeatedly called them copies of bad copies of bad designs.
But we aren't talking about those. Seems you are.

First its hard to overdrive as the driver stage will saturate. The load
line makes it hard.

If you read the manual you know my name.

--
Allison
------------------
Post online only,?
direct email will go to a bit bucket.