EMRFD Message Archive 71

Message Date From Subject
71 2006-08-04 09:53:43 Shawn Question about the book
Page 2.25, section 2.7, Feedback Amplifiers: the equations (and related
plots), are they for the general case for the circuit shown in Figure
2.63 or the case where a transformer is used to bump the 50 ohm load up
to 200 ohms? The equations look like they are for the general case
(adjust RL if using a transformer prior to entering the numbers into
the equation), but it doesn't explicitly state what the equations are


Shawn KB1CKT
75 2006-08-05 15:43:59 Wes Hayward Re: Question about the book
Hi Shawn, et al,

You are right -- the equations apply to Fig 2.63, the general case.
So the amplifier that we often see with a 2:1 turns ratio
transformer would use a 200 Ohm load for these calculations.

Be sure to note the info in the caption for Fig 2.69.

The equations were not from a text. Rather, I started with a
simple amplifier assumed to be operating at low frequency, but with
a current gain related to the operating frequency Vs F-t. The
equations were then derived from that simple model.

A more elaborate transistor model (a hybrid-pi small signal model)
was used to write the program FBA. In that one, the transistor F-t
is specified, along with a base spreading resistance and Miller
capacitance. The transducer gain, and the input and output match
were then calculated. I used y-parameters for the calculations,
but that is not important to the external world. The methods are
in Chapter 5 of IRFD.

I hope that this is helpful.

73, Wes
79 2006-08-06 05:28:24 Shawn Upton Re: Question about the book
Just what I thought. Thanks. I'll tack that info
into the margins, along with your recently posted
article on feedback amps and pads (which I haven't
digested yet).


Shawn Upton, KB1CKT

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