**EMRFD Message Archive 3735**

MessageDateFromSubject3735 2009-11-05 09:38:35 David DeMaw Design Equation I have been going through a series of articles by DeMaw on "Learning to work with Semiconductors" in the April 1974 to April 1975 issues of QST.

In several of these articles, April 1975, as an example, he outlines the design equations for an amplifier. I can follow his approach for all except his equation #4

4) iC = Ic x 1.7

What is the factor 1.7 for? My only thoughts are the iC is the instantaneous (AC) Collector current and the Ic is the bias current. Other than that I'm confused. He uses this equation in several articles in the series on different designs so it isn't specific to a particular circuit. I can handle the math and theory in EMFRD and RFD but this simple assumption is really bugging me.

Could someone please enlighten me?

He does mention he covers his assumptions in the April 1974 article by the same name but my editions only go back to May 1974.

What I am surprised about is how many retired RF engineers there are who are getting back to their training after so many years. For me my career path changed when a slick, useless salesman from our company walked into my Lab and mentioned that he made 3 times what I was making. As a slick useless salesman I am now retired and going back to what I really enjoy.3737 2009-11-05 12:18:14 Tim Re: DeMaw Design Equation In April 1975 QST, left bottom side of page 25, he defined Ic = DC collector current and ic = collector signal (AC) current. You see, after you have signal current and signal voltage, then you take voltage divided by current and you get an effective collector impedance and that tells you what sort of impedances to use in the tank.

This is very much in the same spirit as the vacuum tube datasheets that specify plate impedance for different classes of service and operationg points, and I think he was writing to use similar concepts as hollow-staters were dragged into solid-state. If you look at a 70's or earlier ARRL handbook you see all the tank circuit design is parameterized around the ratios of DC plate voltage to DC current without necessarily calling it an impedance (because there's a fudge factor like 1.7 hidden in the chart already).

I think a ratio of 1.7 is the right ballpark for a oscillator or class C amplifier where you want to saturate the transistor. The ratio would be much smaller for class A service... but few transmitter applications in that or previous design eras would be class A, they would usually be class C.

Tim.3738 2009-11-05 12:25:51 Russ Hines Re: DeMaw Design Equation Hello David:

Your "only thoughts" are incorrect in this case. If you reread the

article from April 1975, you'll find your answer. Hint: look on page 25.

BTW, this entire series is available online from the ARRL as PDF files.

One of the perks of membership.

When you have more questions, come on back.

73,

Russ

WB8ZCC

David wrote:

>[Non-text portions of this message have been removed]

>

> I have been going through a series of articles by DeMaw on "Learning

> to work with Semiconductors" in the April 1974 to April 1975 issues of

> QST.

>

> In several of these articles, April 1975, as an example, he outlines

> the design equations for an amplifier. I can follow his approach for

> all except his equation #4

>

> 4) iC = Ic x 1.7

>

> What is the factor 1.7 for? My only thoughts are the iC is the

> instantaneous (AC) Collector current and the Ic is the bias current.

> Other than that I'm confused. He uses this equation in several

> articles in the series on different designs so it isn't specific to a

> particular circuit. I can handle the math and theory in EMFRD and RFD

> but this simple assumption is really bugging me.

>

> Could someone please enlighten me?

>

> He does mention he covers his assumptions in the April 1974 article by

> the same name but my editions only go back to May 1974.

>

> What I am surprised about is how many retired RF engineers there are

> who are getting back to their training after so many years. For me my

> career path changed when a slick, useless salesman from our company

> walked into my Lab and mentioned that he made 3 times what I was

> making. As a slick useless salesman I am now retired and going back to

> what I really enjoy.

>

>

>

>

> __________ NOD32 4576 (20091105) Information __________

>

> This message was checked by NOD32 antivirus system.

> http://www.eset.com

3739 2009-11-05 12:34:11 Russ Hines Re: DeMaw Design Equation Hmm, I guess you don't operate AM or SSB or use audio then. Too bad,

they're fun modes. ;-)

You're right, though, the 1.7 factor is a rule of thumb, mostly for not

letting the smoke out.

73,

Russ

WB8ZCC

Tim wrote:

>[Non-text portions of this message have been removed]

>

>

> I think a ratio of 1.7 is the right ballpark for a oscillator or class

> C amplifier where you want to saturate the transistor. The ratio would

> be much smaller for class A service... but few transmitter

> applications in that or previous design eras would be class A, they

> would usually be class C.

>

> Tim.

>

>