I have some experience with transistor power amps. Unfortunately it's
been with UHF and microwave amplifiers which are a little different to
work with, so I'll answer where I can. See below.
Pat
WB2GBF
AAR2BY/T
P.S., I must ask. Is this a CB amplifier by any chance? If so, you
may want to consider adding filters on the output before putting it
into service.
On Sun, Apr 5, 2009 at 11:53 PM, Dan Sawyer <dansawyer@earthlink.net> wrote:
*** SNIP ***
> 1. What is the normal bias set for this type of amp. Setting bias to the
> current turn on point about .62 V results in a clean signal between
> about 14 MHz and 30 MHz.
The short answer is it depends.
On a BJT design, while the base voltage and current are related, it is
the current and not the voltage you want to record. The setting
depends on class, e.g., A, AB or B, and the output power you are
designing it for. It is my understanding that it is ballparked and
then empirically refined in practice.
> 2. Is it practical to copy the RC values from a similar amp to implement
> a feedback circuit?
You could do that as a starting point, but the actual values depend of
circuit layout. It is my understanding that for HF amplifiers, the
feedback network is empirically determined and optimized for gain,
stability and bandwidth.
> 3. The imput signal seems to be affected by feedback from the amp. Below
> 10 MHz there is a second order harmonic on the input signal. I am not
> sure of the cause of this. At low frequency the output looks clipped. It
> looks as though it is being over driven.
Where are you looking? The devices are non linear and will look
distorted at the base. Also, are you sure that the driver isn't
supplying a distorted signal? I'd check its signal into a dummy load
first. Preferably, it would be with the 50 ohm termination right at
vertical input with the signal going through appropriate power level
attenuator first. Scope probes are notorious for changing the picture
thanks to probe compensation and reflections changing the look of the
signal.
BTW -- I'm assuming your scope is at least 100 MHz bandwidth for this
measurement.
> 4. The board appears to have two grounds, a DC ground and an RF ground.
> What is the purpose of this design approach? The APP note circuits do
> not have this. Is it as simple as connecting the two grounds together?
> Is there a downside to trying this?
This is a SWAG. I'd say the designers were concerned about ground
loops and chose this method. Maybe they found that it solved an
instability they saw in the amplifier. The two should be connected
together at the point where DC is introduced into the board.
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