wrote:



Tim: Thanks again for your help, and thanks again to all the others
who've replied to my question.

I see what you are saying. I reached back to Terman's description of
these kinds of feedback netwoks in tube circuits. I think he describes
just what you are presenting (but I think your presentation is
clearer!)

Obviously I'm still struggling with some very basic amplifier issues.
Am I correct in thinking that shunt feedback would have a number of
different advantages in a common emitter transistor amp? Looks to me
like the feedback network would give the designer the chance to:
1). Manipulate the input and output impedances
2). Counteract the tendency of the amp to "take off" becasue of the
rising gain characteristic (as frequency is lowered).
3). Reduce any distortion (IMD) generated in the amplifier itself.

I'm most shaky on #3. Am I correct in thinking that if you have some
sort of spur or IMD product generated in the amp itelf (say in the
collector circuit)the negative feedback provided by the shunt tends to
nock some (most? all?) of this distortion down?

Thanks, 73
Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr




Bill,

Negative feedback can be used to help fix input and output impedances. You
have to be careful with shunt type negative feedback to make sure you don't
have any regions where the phase change becomes 0deg. This can happen when
you have both capacitive and inductive reactances in the circuit. That is
one reason parasitic oscillations happen at very high frequencies. Even
connecting wires on resistors can look like inductors at very high
frequencies.

As far as negative feedback having an impact on distortion, you must be very
careful to identify what kinds of distortion you are speaking of and have a
good feel for what the feedback networks are doing. Negative feedback,
improperly designed, can cause audio amplifiers to not be "flat" across the
audio spectrum. This can be considered to be distortion.

Negative feedback, improperly applied can cause gain compression, e.g. in an
RF Linear Amplifier, which distorts the dynamic range of the applied
modulation causing a distortion which impacts intelligibility.

IMD distortion is typically caused by a non-linear device. Since a
transistor is an inherently non-linear device (it is based on "diode"
junctions) at low voltage levels since it acts like an on-off switch, there
will always be some level of IMD that negative feedback won't help.

Where negative feedback *can* help is in keeping a Class A amplifier
operating in the Class A region. This will minimize IMD. If you get an
amplifier out of the Class A region, there is bound to be some IMD. If you
are running a Class B amplifier in push-pull this is a "psuedo" Class A
amplifier and negative feedback that would keep each amplifer right at
cutoff for the negative part of the input would help minimize IMD
distortion. If you are running a Class C amplifier, negative feedback won't
help much because you are running a non-linear amplifer to begin with.

Hope this helps.

tim ab0wr