EMRFD Message Archive 906

Message	Date	From	Subject
906	2007-07-21 07:01:01	wbdmxbaileybobbut...	power amp
need some help: I am working on a power amp circuit in the DC40A qrp transciver. the output stage is similar to the fig 2.10. the problem I am having is that I have a good display on my scope at the gate showing a good drive signal. but I cannot see any display at the output at all. I also have the same problem with the hands on circuit that was in qst a short time ago that uses a 40 meter xtal and a buffer amp going to a final amp that uses the feadback toriod transformer in a similar circuit, I cannot see the output in that one eather. I feel that if my scope can see the input, it should also be able to see the output, or am I missing something here. thanks bob WB7DMX
907	2007-07-22 08:36:24	Allison Parent	Re: power amp
908	2007-07-22 08:38:38	w4zcb77	Re: power amp
I also have the same problem with the hands on circuit that was in qst > a short time ago that uses a 40 meter xtal and a buffer amp going to a > final amp that uses the feadback toriod transformer in a similar > circuit, I cannot see the output in that one eather. > I feel that if my scope can see the input, it should also be able to > see the output, or am I missing something here. > thanks > > bob > WB7DMX GM Bob. It may well be that there is a DC voltage on your output that offsets the scope trace to a point where the trace cannot be seen. You can determine that. Does your scope have both a DC and an AC input? If so, be sure it's on AC or decouple the output of your amplifier with a series capacitor. Secondly, are you certain that the trace is NOT there but simply of an amplitude that the screen cannot display? Again, the trace being MUCH "taller" than the screen and you're getting a very dim display due to the speed of the trace in amplitude? Is the amplifier well behaved? Do not overlook the possibility that it may be unstable and your scope is telling you that there is a large output signal with or without input. And one last thought, if the scope is a triggered scope, can you obtain an image by changing the trigger control? Regards W4ZCB >
909	2007-07-22 08:57:25	Leon	Re: power amp
----- Original Message -----
910	2007-07-22 14:43:57	wbdmxbaileybobbut...	Re: power amp
912	2007-07-26 22:55:14	Bill Noyce	Re: power amp
When you say > but I cannot see any display at the output at all. I interpret that as an oscilloscope trace that's a straight line, though others seem to interpret it as a blank display. Which do you see? Is there any chance you've confused the pinout of your FET, so you're putting the input on the drain and looking for output on the gate (for example)? What are the actual input and output voltages you're seeing? -- Bill, AB1AV
913	2007-07-29 17:00:08	wbdmxbaileybobbut...	Re: power amp