Matching BS170 for Softrock

THE THEORY

Recently some BS170s supplied by Tony for the Ensemble RXTX have shown rather large variations in standing current when tested according to Robby's instructions. http://www.wb5rvz.com/sdr/ensemble/10_pa.htm 
This current is indicated by measuring the voltages (around 0.05 volts, 50mV) at R48 and R49.

It seems that many BS170s have a similar Gate Threshold Voltage but the device data sheet shows the possibility of a very large range. Such variations will give large variations in the standing current of  the Softrock PA devices . It is said this can affect it's linearity.

A discussion on the Softrock40 Yahoo Group decided that it would be a good idea to check the six BS170s supplied with the kit and select the closest matched pair. This discussion started here http://groups.yahoo.com/group/softrock40/message/55018 but continued with different Subject Headings http://groups.yahoo.com/group/softrock40/message/55292 There are lots of posts, not all relevant. 

This procedure will still not guarantee that they are matched but it is the best so far.
"When you match the Gate Threshold Voltage, this does not guarantee that the transconductances will also match.  In the RXTX circuit, both the Threshold Voltage and the transconductance will affect the current through R48 and R49"

IS IT WORTH MATCHING THEM?

The BS170 matching is to get the voltages on R48/49, at the BS170's sources, similar.  NOTE. It is still not established just how much variation in the R48/49 voltages is acceptable. Robby suggests 10%.  In http://groups.yahoo.com/group/softrock40/message/55570 Mike Collins, K4FBQ, describes the bias circuit. He says even 20% is perfectly acceptable.
"
Q5 is used to establish the operating point and the 2 PA BS170's are slaved to it. The current through Q5 is set by approx 0.7V (Q4 Vbe) across R41 33.2 ohms (assume little base current and high Beta of Q4). The Q4 Vbe drop is compensated for by D3 which makes the 2 PA devices somewhat track Q5. i.e. if Q5 is way off then the bias for the PA devices will be off too."

Another description:- The characteristics of the BS170 in the bias circuit sets the forward bias of Q7, Q8. Q5 is thermally coupled to Q7, Q8 so the constant current in Q5 sets the standing current of Q7, Q8 as the temperature changes. The characteristics of the BS170 allow for a big variation between individual devices. Most times they are not much different but
occasionally a mixture of different batches or makes has produced wide variations of bias current.

Until someone comes up with a definitive answer I suggest that only really large variations should be of concern. I think some are taking this too seriously. Far more important is that these devices should not be overdriven.
The matching described here is done when the transmitter is idle. If they are to be matched it would seem to be best done at full power.

March 2013 Ed GM3SBC, recommends they are checked through their range, a better method than I describe below.
Below, GM3SBC's test setup.See http://groups.yahoo.com/group/softrock40/files/GM3SBC%20QRO%20Modifications/ For more information. He also adds two more BS170s to increase power of his 160m Ensemble.


2017. Automatic component testers obtained cheaply through Ebay and other sources include measurement of Vt, Threshold Voltage that will give similar results as my test below.




Look at the bottom of this page http://sites.google.com/site/g4zfqradio/levels-measured-in-a-softrock-tx for two other ways of testing linearity.

Bob, G8VOI feels that a reasonable balance is desirable.
He also remarks that the guidelines of 1/2 watt SSB and 1 watt for CW and other non-linear modes should be maintained unless you have ways of testing the whole range for possible spurii.
Also bear in mind you need to leave a safety margin, NOT drive to maximum!

TO MATCH THE BS170s

Below is the simple circuit suggested and my notes after I had tested a few.
Here http://groups.yahoo.com/group/softrock40/message/55586 is W1SRB's description of his adjustment of the bias. BUT matching all three may be a better solution.
If you are uncertain please ask in the Yahoo Softrock40 group BEFORE you do anything.



I used a plug I had in my junk box for the BS170. The pink and white wires, Drain and Gate, are connected together and go to 12 volts through a 100R resistor.
The black, Source, wire goes to power negative.
Any suitable "socket" may be used or, with care, clip leads. 
The voltmeter positive goes to the Drain and Gate, the negative to the power negative.

The current through the BS170 will be around 80mA.

My suggested 0.05 volt variation seems to give roughly the same range of variation as Robby's pages specify. You should find two of the six that are much closer. The actual measurements when installed in the Softrock are not too critical, they just need to be close to each other. 
A third BS170 should be selected that is closest to the pair, use that for Q5 in the bias circuit.

Note the BS170s may be damaged by static electricity so the usual ESD precautions are recommended. 

Below, my setup.
The voltages I measured were all just above 3 volts, I do not know what may be considered acceptable limits.

After fitting check the R48/49 voltages, hopefully they will be close.
 


Click on picture for full size.


Ken VA3ABN found some BS170s (Not supplied by Tony) that had a test voltage of around 2.4. These, when fitted, had a high standing current. Q5 should be replaced with a similar tested device and   the bias will adjust so the sources are near 50mV.
All three BS170s need to be matched.

It is likely that if the three BS170s are matched this circuit will not need altering and note +/- at least 20% will make no noticeable difference.







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